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Building Resilient IP Networks: The Access Module

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 9, 2006.

Chapter Description

This chapter focuses on Ethernet switching technology, specifically Layer 2 network resiliency and how it should be built to provide a solid foundation for the Layer 3 network.

Layer 2 Domain

The access and distribution layers form the boundary for the Layer 2 domain of the building block network. This is an area where the Spanning Tree Protocol (STP) plays a critical role. Unfortunately, a common perception of STP is that it is a dated technology and has always been blamed for network disruptions. Whenever there is a network meltdown, chances are fingers will point toward the STP. The fact is, with proper design and configuration, STP is effective and useful.

It is absolutely critical that in the access and distribution layers, a steady state of a Layer 2 network is a prerequisite for a stable IP network. You can have a well-designed IP network, but without a proper underlying Layer 2 network design, you will not be able to achieve your goal. Therefore, besides paying attention to the IP network design, you need to have a Layer 2 network strategy thought out. In addition, having a well-documented Layer 2 network is as essential as that of the Layer 3 network.

Although STP was invented many years ago, it has been constantly improved to make it relevant in today's networks. The next few sections help you understand the original protocol and the subsequent improvements made. These include the following:

  • The Spanning Tree Protocol (IEEE 802.1d)
  • Trunking
  • Per-VLAN Spanning Tree (PVST)
  • Per-VLAN Spanning Tree Plus (PVST+)
  • 802.1w
  • 802.1s
  • Channeling technology
  • Best practices for the Layer 2 domain

The Spanning Tree Protocol: IEEE 802.1d

The purpose of the STP, or IEEE 802.1d specification, is to prevent loops forming within a Layer 2 network. In a bridge network, looping causes problems such as broadcast storms. During broadcast storms, frames circulate in the network endlessly, consuming bandwidth and control plane resources. In a bridge network, there can only be one active path between two end stations that are communicating with one another.

Essentially, the STP uses the spanning-tree algorithm to keep track of redundant paths, and then chooses the best one to forward traffic while blocking the rest to prevent loops. The result of the STP is a tree with a root bridge and a loop-free topology from the root to all other bridges within the network. A blocked path acts as a backup path and is activated in the event that the primary path fails. Each of the ports on a bridge may be assigned a role, depending on the resulted topology:

  • Root— A forwarding port elected for the spanning-tree topology. There is always only one root port per bridge, and it is the port leading to the root bridge.
  • Designated— A forwarding port elected for a LAN segment. The designated port is in charge of forwarding traffic on behalf of the LAN segment, and there is always only one designated port per segment.
  • Alternate— A port that is neither root nor designated.
  • Disabled— A port that has been shut down and has no role.

Depending on the port role, each port on a bridge will either be in forwarding state or blocked. The result is a tree topology that ensures a loop-free environment. Figure 6-4 shows a sample of a STP topology and the resulted role of the ports on the bridges.


Figure 6-4 Port Roles of STP

Every bridge participating in a STP domain is assigned a bridge ID (BID) that is 8 bytes long. The BID is made up of a 2-byte bridge priority and a 6-byte Media Access Control (MAC) address of the switch. In addition, each of the bridge ports on the bridge is assigned a port ID. The port ID is 2 bytes long, with a 6-bit priority setting and a 10-bit port number. Each port also has a path cost that is associated with it. The original default path cost was derived by dividing 1 gigabit by the link speed of the port. However, with the introduction of higher-speed links such as the 10 Gigabit Ethernet, the default cost has been updated, as shown in Table 6-1.

Table 6-1. Default Path Cost for STP

Link Speed

Path Cost

4 Mbps


10 Mbps


16 Mbps


45 Mbps


100 Mbps


155 Mbps


622 Mbps


1 Gbps


10 Gbps


The formation of the spanning tree is determined by the following:

  • The election of a root bridge
  • Calculating the path cost toward the root bridge
  • Determining the port role associated with each of the interfaces on the bridge

The election of the root bridge is an important event in the STP and has a direct bearing on eventual traffic flow. Therefore, it is important for network designers to understand the concept of the election of the root and its impact on the network.

The bridge with the lowest BID is always elected as a root bridge. Remember, BID is made up of the bridge priority and the MAC address. The default value of the bridge priority is 32768. Therefore, by default, the bridge with the lowest MAC address is usually elected as the root bridge. The root bridge acts like the center of a bridge network, and all traffic flows toward it. The paths toward the root that are deemed redundant are put in a blocked state.

The STP is a timer-based protocol. As such, as a participant within the STP, a port goes through a series of states, based on some configured timing, before it starts to forward traffic. The cycle that a port goes through is reflected in Figure 6-5.


Figure 6-5 The STP Cycle

The states shown in Figure 6-5 are as follows:

  • Blocking— No traffic is forwarded.
  • Listening— First state whenever a port is first initialized. In this state, the bridge is trying to determine where the port fits in the STP topology.
  • Learning— The port gets ready to forward traffic. In this state, the port is trying to figure out the MAC address information that is attached to the port.
  • Forwarding— The port is forwarding traffic.
  • Disabled— The port has been disabled.

In the discussion of optimizing a switched network, three parameters are of utmost importance:

  • Hello timer— Determines how often a bridge sends out BPDUs
  • Forward delay timer— Determines how long the listening and learning states must last before a bridge port starts forwarding traffic
  • Maximum age timer— Determines the length of time a bridge stores its information received

As you can see in Figure 6-5, the time taken for transiting from power up to start of transmission for a bridge port is at least 35 seconds. That is, no IP traffic can be transmitted for at least that amount of time. Obviously, if the port goes up and down in a span of 35 seconds, no traffic actually flows through it.

Figure 6-6 shows a simple setup of three bridges connected together. After the STP has converged, the resulted topology is as show,n with bridge B being selected as the root bridge. The link between bridge A and C is blocked to prevent a loop.


Figure 6-6 STP in Steady State

The following sections discuss various scenarios that can happen to this steady-state network. The behavior of STP is studied and suggestions are made on how to improve its capability to react to problems. The following features within the Catalyst series of switches are highlighted in the discussion of how to overcome problems:

  • PortFast
  • UplinkFast
  • BackboneFast
  • Unidirectional Link Detection (UDLD)
  • RootGuard
  • LoopGuard
  • BPDUGuard


As described in the preceding section, a port usually takes about 35 seconds before it starts to forward traffic. In some instances, this behavior is undesirable. One example is when an end station is connected to the port. Because the port takes 35 seconds to begin forwarding traffic, the following errors may potentially occur:

  • If the end station is running a Windows client, it may encounter the error "No Domain Controllers Available."
  • If the end station is trying to request an IP address via a Dynamic Host Configuration Protocol (DHCP) server, it may encounter the error "No DHCP Servers Available."
  • If the end station is running the Internetwork Packet Exchange (IPX) protocol, it may encounter no login screen.

The errors may occur for a few reasons, such as a link-speed negotiation problem or duplex problems. However, the way the STP port behaves may have contributed to the problem. One of the reasons errors occur is that the port is still transiting to the forwarding state even after those protocol requests have been sent out by the end station. This is not uncommon, because advancement in end station technology has resulted in extremely fast boot time. The booting of the operating systems is so fast now that the system sends out requests even before the port goes into a forwarding state.

As shown in the STP topology illustrated earlier in Figure 6-6, user-facing ports are connected to end stations only. These ports do not connect to other bridges and are called leaf nodes. The use of the STP is to prevent loops between bridges. Therefore, there is no reason why a leaf node needs to toggle through the different states, because it will not be connecting to another bridge. It might be possible to bypass those transition states and put the port into forwarding mode immediately upon detection of a connection.

The portfast command in the Catalyst series switches is a feature that enables a leaf node in the STP topology to have such bypass capability. It can be configured at the global level, in which all ports are configured with the PortFast feature by default. It can also be configured at the individual port level.

It is critical that a port with the PortFast feature enabled should only be used for connection to end devices. It should never be connected to other Layer 2 devices such as a hub, switch, or bridge. For this reason, configuring the PortFast feature at port level is always preferred. Example 6-1 demonstrates how to configure the PortFast feature at the port level.

Example 6-1. Configuring PortFast

Cat3750#configure terminal
Cat3750(config)#interface GigabitEthernet 1/0/1
Cat3750(config-if)#spanning-tree portfast


Figure 6-6 illustrated a simple Layer 2 network that is in steady state, with bridge A forwarding traffic toward bridge B. The link that is connected to bridge C is blocked and acts as a redundant link. By default behavior of STP, when the link between bridge A and B fails, the STP needs about 30 seconds before diverting traffic to the redundant link. Before the redundant link takes over, all user traffic from bridge A is not forwarded, as shown in Figure 6-7.


Figure 6-7 Default STP Action on an Uplink Failure

Because the backup link takes such a long time to forward traffic, one area of focus is how to accelerate the takeover process so that traffic is diverted to the redundant link immediately after the active link has failed.

The feature that makes this possible is delivered through the UplinkFast feature. Essentially, UplinkFast moves the original blocked port on bridge A to the forwarding state without going through the listening and learning states, as shown in Figure 6-8.


Figure 6-8 STP Action with UplinkFast

When the UplinkFast feature is enabled, the bridge priority of bridge A is set to 49152. If you change the path cost to a value less than 3000 and you enable UplinkFast or UplinkFast is already enabled, the path cost of all interfaces and VLAN trunks is increased by 3000. (If you change the path cost to 3000 or above, the path cost is not altered.) The changes to the bridge priority and the path cost reduce the chance that bridge A will become the root bridge.

It is important to note that the UplinkFast feature is most applicable to access switches only and may not be relevant in other parts of the network. In addition, it is a feature that is most useful in a triangular topology, such as the one previously shown. Its behavior may not be so predictable in other complex topologies, as discussed later in the section "Layer 2 Best Practices."

The uplinkfast command is issued at the global level. Example 6-2 shows an example configuration of the UplinkFast feature.

Example 6-2. Configuring UplinkFast

Cat3750#configure terminal
Cat3750(config)#spanning-tree uplinkfast


In the steady-state network, sometimes links other than those connected directly to bridge A will fail, as shown in Figure 6-9.


Figure 6-9 Default STP Action Without BackboneFast

An indirectly connected failure causes bridge A to receive a type of BPDU called an inferior BPDU. An inferior BPDU indicates that a link that is not directly connected to this bridge has failed. Under STP default configuration, bridge A ignores the inferior BPDU for as long as the maximum aging timer; therefore, the STP topology is broken for as long as 50 seconds. That means no traffic flow from bridge A for 50 seconds.

The BackboneFast feature on bridge A circumvents this problem by initiating a state transition on the blocked port from blocked to listening state. This is done without waiting for the maximum aging time to expire and, thus, speeds up topology reconstruction to about 30 seconds, as shown in Figure 6-10.


Figure 6-10 STP Action with BackboneFast

The backbonefast command is issued at the global level. Example 6-3 shows an example configuration of the BackboneFast feature.

Example 6-3. Configuring BackboneFast

Cat3750#configure terminal
Cat3750(config)#spanning-tree backbonefast

Unidirectional Link Detection (UDLD)

Up to this point, all the features discussed deal with a link or box failure scenario. However, there is still a possibility of a physical error that may wreak havoc in the access module and that is a unidirectional link error. A unidirectional link occurs whenever traffic sent by a device is received by its neighbor, but traffic from the neighbor is not received by the device. This happens, especially with fiber cabling, when only part of the fiber pair is working. A unidirectional link error can cause unpredictable behavior from the network, especially with the STP topology.

The situation depicted in Figure 6-11 does happen with fiber cabling systems that use a patch panel. In the figure, the switch ports are patched incorrectly at the patch panel, which may be located far away from the switches. The consequence of the erroneous cabling is unpredictable and may be difficult to troubleshoot just by issuing commands on the switch console. Even though the port status displays as up and working, errors will occur at the higher-layer protocol. In certain cases, the STP may still consider the link valid and move the effected port into a forwarding state. The result can be disastrous because it will cause loops to occur in the Layer 2 network.


Figure 6-11 Possible Causes of Unidirectional Link Error

The UDLD feature provides a way to prevent errors such as the one depicted in Figure 6-11. It works with the Layer 1 mechanism to prevent one-way traffic at both physical and logical connections. The Layer 1 mechanism works by using the autonegotiation feature for signaling and basic fault detection. UDLD operates by having the devices exchange Layer 2 messages and detect errors that autonegotiation cannot. A device will send out a UDLD message to its neighbor with its own device/port ID and its neighbor's device/port ID. A neighboring port should see its own ID in the message; failure to do so in a configurable time means that the link is considered unidirectional and is shut down.

UDLD can be configured in two modes:

  • Normal— If the physical link is perceived to be up but UDLD does not receive the message, the logical link is considered undetermined, and UDLD does not disable the port.
  • Aggressive— If the physical link is perceived to be up but UDLD does not receive the message, UDLD will try to reestablish the state of the port. If the problem still persists, UDLD shuts down the port. Aggressive mode also supports detection of one-way traffic on twisted-pair cable.

You can configure UDLD at both the global level and at the port level. Example 6-4 shows how to configure UDLD at the global level.

Example 6-4. Configuring UDLD Globally

Cat3750#configure terminal
Cat3750(config)#udld enable

Example 6-5 shows how to configure UDLD at the port level.

Example 6-5. Configuring UDLD at the Port Level

Cat3750#configure terminal
Cat3750(config)#interface GigabitEthernet 1/0/1
Cat3750(config-if)#udld port


If you are running STP, the resulting topology always centers on the location of the root bridge. Whether a particular physical link is used or not depends on the port state of either forward or blocked. And this, in turn, depends on the location of the root bridge. It is possible that a certain topology was envisioned during the design phase so that traffic flows in a certain direction. However, the introduction of an additional bridge may turn things around and have an unexpected result, as shown in Figure 6-12.


Figure 6-12 Result of Illegal Root Bridge Introduction

Before the introduction of the new bridge D, the topology of the network was intended as that shown earlier in Figure 6-6. However, the topology changes when bridge D is introduced. Because bridge D has a lower bridge priority, it begins to announce that it is the new root bridge for the Layer 2 topology. As a result, a new topology is formed to account for this new root bridge. As shown in Figure 6-12, the link between bridge B and bridge C has now been blocked. In other words, the communication path between the two bridges has to be via bridge A. If both links from bridge A are slow links, say 10 Mbps, this might have a performance impact between the two bridges B and C.

The RootGuard feature prevents the preceding scenario from happening. It is configured on a per-port basis and disallows the port from going into root port state. Example 6-6 shows how to configure the RootGuard feature.

Example 6-6. Configuring RootGuard

Cat3750#configure terminal
Cat3750(config)#interface fastethernet 1/0/24
Cat3750(config-if)#spanning-tree guard root

In Example 6-6, when a BPDU that is trying to claim root is received on the port, it goes into an error state called root inconsistent. The new state is essentially the same as the listening state and will not forward traffic. When the offending device has stopped sending BPDU, or the misconfiguration has been rectified, it will go into a forwarding state again. This way, a new bridge cannot "rob" the root away from the Layer 2 network. The following shows the error message that appears if RootGuard is put into action:

%SPANTREE-2-ROOTGUARDBLOCK: Port 1/0/24 tried to become non-designated in VLAN 10.
Moved to root-inconsistent state

To prevent the situation illustrated in Figure 6-12 from happening, you should introduce the RootGuard feature in bridge A on the port facing bridge D.


Even if STP is deployed, loops might still occur. Loops happen when a blocked port in a redundant topology erroneously transits to a forwarding state, which usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) stopped receiving BPDUs. As a timer-based protocol, STP relies on continuous reception or transmission of BPDUs, depending on the port role (designated port transmits, nondesignated port receives BPDUs). So when one of the ports in a physically redundant topology stops receiving BPDUs, the STP conceives that nothing is connected to this port. Eventually, the alternate port becomes designated and moves to a forwarding state, thus creating a loop.

The LoopGuard feature was introduced to provide additional protection against failure, such as the one just described. With the LoopGuard feature, if BPDUs are not received any more on a nondesignated port, the port is moved into a new state, called the loop-inconsistent state. The aim is to provide a sanity check on the change in topology. Example 6-7 shows how to configure LoopGuard on all ports at the global level.

Example 6-7. Configuring LoopGuard

Cat3750#configure terminal
Cat3750(config)#spanning-tree loopguard default

The following shows the error message that appears on the console when an error is detected by LoopGuard:

SPANTREE-2-LOOPGUARDBLOCK: No BPDUs were received on port 3/4 in vlan 8. Moved to
  loop-inconsistent state.


BPDUGuard is a feature that is recommended to be run together with the PortFast feature. Remember that the PortFast feature allows a port that has end systems connected to it to skip various STP states and go directly into a forward state. The port is not supposed to be connected to other bridges and, therefore, should not be receiving BPDUs. Any detection of a BPDU coming from the port is an indication that an invalid bridge has been introduced into the Layer 2 network. Upon detection of a BPDU, the BPDUGuard feature sends the port immediately into an error-disabled state, which enables you to investigate the problem, and rectify the situation before disaster strikes.

BPDUGuard can be enabled at the global command level as a global PortFast option. It can be also configured at the individual port level for granular control. Example 6-8 shows how to configure BPDUGuard.

Example 6-8. Configuring BPDUGuard

Cat3750#configure terminal
Cat3750(config)#spanning-tree portfast bpduguard default

VLANs and Trunking

Thus far, the discussion has focused on a common bridged network. The advent of Layer 2 switching technology introduces many new concepts that require further understanding and treatment for STP. For example, with Layer 2 switching comes the concept of VLANs and trunking.

A VLAN is basically a collection of network nodes that share the same broadcast domain. In a switched network design, an IP subnet is usually mapped to a VLAN, although there may be a rare exception. Because of their close association, the health of a VLAN directly impacts the functioning of the associated IP subnet.

The introduction of a VLAN brings about the concept of trunking, when many VLANs have to be transported across a common physical link. Figure 6-13 shows the trunking process.


Figure 6-13 Nontrunking Versus Trunking Mode

The reason for using the trunking mode feature varies, but the most obvious one is to save on ports that would have otherwise been spent on carrying each individual VLAN. This is especially true when connecting an access switch to the distribution layer. To carry multiple VLANs across the single physical link, a tagging mechanism or encapsulation has to be introduced to differentiate the traffic from the different VLANs. There are two ways of achieving this:

  • Inter-Switch Link (ISL)
  • IEEE 802.1q

ISL is a Cisco proprietary way of carrying different VLANs across a trunk. It is an encapsulation method whereby the payloads are wrapped with a header before transmission across the trunk. Figure 6-14 shows the format of an ISL frame.


Figure 6-14 ISL Header

The fields in the ISL header are as follows:

  • DA— Destination address
  • Type— Frame type
  • User— User-defined bits
  • SA— Source address
  • Len— Length
  • SNAP— Subnetwork Access Protocol
  • HAS— High bits of source address
  • VLAN— Destination virtual LAN ID
  • BPDU— Bridge protocol data unit
  • Indx— Index
  • Res— Reserved

ISL encapsulation adds 30 bytes to the original data frame. The implication of this is that you have to be careful of maximum transmission unit (MTU) size support on the various transmission interfaces. If the entire ISL-encapsulated frame size is bigger than the MTU size of the interface, fragmentation may occur. The largest Ethernet frame size with ISL tagging is 1548 bytes. Another point worth noting is that an ISL frame contains two Frame Check Sequence (FCS) fields: one for the original data frame, which is kept intact during encapsulation; and another one is for the ISL frame itself. Therefore, any corruption within the original data frame will not be detected until the ISL frame is de-encapsulated, and the original frame is sent to the receiver. ISL can carry up to 1000 VLANs per trunk.

Example 6-9 shows how to turn on trunking on a switch port using ISL.

Example 6-9. Configuring ISL Trunking

Cat3750#configure terminal
Cat3750(config)#interface fastethernet 1/0/24
Cat3750(config-in)#switchport mode trunk
Cat3750(config-in)#switchport trunk encapsulation isl

The other trunking mechanism is the IEEE 802.1q standard. It uses an internal tagging method that inserts a 4-byte Tag field into the original Ethernet frame. The Tag field is located between the Source Address and Type/Length fields, as shown in Figure 6-15.


Figure 6-15 IEEE 802.1q Tag Field

The Tag field is as follows:

  • TPID— Tag protocol identifier
  • Priority— User priority
  • CFI— Canonical format indicator
  • VID— VLAN identifier

For IEEE 802.1q tagging, there is only one FCS field. Because the frame is altered during the tagging, the FCS is recomputed on the modified frame. Because the tag is 4 bytes long, the resulting Ethernet frame size can be 1522 bytes. The IEEE 802.1q can carry 4096 VLANs.

Example 6-10 shows how to turn on trunking on a switch port using 802.1q.

Example 6-10. Configuring 802.1q Trunking

Cat3750#configure terminal
Cat3750(config)#interface fastethernet 1/0/24
Cat3750(config-in)#switchport mode trunk
Cat3750(config-in)#switchport trunk encapsulation dot1q

The 802.1q is the preferred method for implementing trunking because it is an open standard. In addition, 802.1q can carry more VLANs, a feature that is good to have in a metro Ethernet implementation.

The implication of VLANs and trunking is not so much having the concept of multiple broadcast domains within a single device, or the ability to carry multiple VLANs across a single physical link. It is the concept of multiple instances of STP that comes with it that is crucial in resilient network design. The following sections show how to deal with this challenge.

Common Spanning Tree (CST)

As discussed previously, the purpose of STP is to prevent loops from forming in a Layer 2 network. The introduction of the switch gives rise to many broadcast domains, called the VLANs. The IEEE 802.1d, as a protocol, does not have the concept of VLAN. The IEEE 802.q introduces the concept of VLANs, and it subjects the multiple VLANs to the control of a single STP instance by introducing a native VLAN that is not tagged during transmission. The STP state of the native VLAN governs the topology and the operation of the rest of the tagged VLANs. This mode of operation is commonly known as Common Spanning Tree (CST).

Figure 6-16 illustrates the concept of a CST. Because there is only one instance of STP, all the VLANs share the same topology, with common forwarding ports and blocked ports, depending on the final STP state. In this case, no load balancing of traffic is possible and the redundant path does not carry any user traffic during normal operation.


Figure 6-16 Common Spanning Tree Topology

Per-VLAN Spanning Tree (PVST)

In contrast to CST, the Per-VLAN Spanning Tree (PVST) is a Cisco proprietary extension of IEEE 802.1d. It introduces an instance of STP process for each VLAN being configured in the Layer 2 networks. In this manner, each VLAN may have a different topology, depending on individual STP configuration. PVST works only with ISL trunking. Figure 6-17 illustrates the concept of a PVST implementation.


Figure 6-17 PVST Topology

Because there is one instance of STP per VLAN, each VLAN can have its own topology, independent of the others. Figure 6-18 shows the topology of the voice VLAN.


Figure 6-18 STP Instances for Voice VLAN

Figure 6-19 shows the topology of the wireless LAN virtual LAN (WLAN VLAN), which has the same uplink as that of the voice VLAN.


Figure 6-19 STP Instances for WLAN VLAN

Unlike the voice and WLAN VLANs, the data VLAN has been configured to use the other uplink. This design allows for load balancing of traffic between the VLANs. Whereas the voice and WLAN VLANs use the uplink on the left, the data VLAN uses the uplink on the right. Figure 6-20 shows the data VLAN topology.


Figure 6-20 STP Instance of Data VLAN

From an IP network perspective, regardless of the topology of the Layer 2 network, the logical IP networks of the three VLANs look exactly the same. The load balancing of traffic is achieved at a Layer 2 level and is transparent from an IP network perspective. The implication of this is that we have improved traffic-forwarding capability for all VLANs from switch A, utilizing both uplinks toward the distribution switches. In the event of a failure on any of the uplinks, all VLANs will be carried on the single remaining uplink.

Because PVST has each VLAN governed by its own STP instance, it still operates within the confines of STP and may exhibit problems that are typical of a STP implementation. Therefore, the following features are available with the implementation of PVST:

  • PortFast
  • UplinkFast
  • BackboneFast
  • BPDUGuard
  • RootGuard
  • LoopGuard

Per-VLAN Spanning Tree Plus (PVST+)

As mentioned in the preceding section, PVST gives you flexibility in creating a separate Layer 2 topology for each VLAN, thereby achieving Layer 2 load balancing toward the distribution layer. However, it runs only with the ISL trunking feature, not IEEE 802.1q.

Per-VLAN Spanning Tree Plus (PVST+) was introduced so that a network with PVST can interoperate with a network that runs IEEE 802.1q. When you connect a Cisco switch to other vendors' devices through an IEEE 802.1q trunk, PVST+ is deployed to provide compatibility.

The following features are available with the implementation of PVST+:

  • PortFast
  • UplinkFast
  • BackboneFast
  • BPDUGuard
  • RootGuard
  • LoopGuard

Although PVST and PVST+ give flexibility in terms of traffic distribution on the trunks, they tax the control planes of the switches implementing them. In both PVST and PVST+, each STP instance is a separate process and requires independent control a plane resources. Imagine if there are tens or hundreds of VLANs defined; there will be the same number of STP instances running in the switch. In the event of a topology change, these STP instances will have to go through the same steps to process the change. When this happens, the performance of the switch will be affected, and in some cases, no traffic can be forwarded at all because the control plane is overloaded. One solution to this is to implement 802.1s, which you learn about in the "IEEE 802.1s" section.

IEEE 802.1w

The IEEE 802.1d protocol was invented at a time when network downtime was not stringent at all. With the increase in networking speed and expectation of the network resiliency, the IEEE 802.1d protocol is finding it hard to keep up with the demand of the modern IP network. The enhancements discussed thus far have been Cisco proprietary enhancements, and interoperability with other vendors' implementation may not be possible.

IEEE 802.1w, or better known as Rapid Spanning Tree Protocol (RSTP), is the standard's answer to all the enhancements discussed so far. The motivations for the development of 802.1w include the following:

  • Backward compatibility with the existing 802.1d standard
  • Doing away with the problem with respect to long restoration time in the event of a failure
  • Improvement in the way the protocol behaves, especially with respect to modern networking design

Because 802.1w is an enhancement of the 802.1d standard, most of the concepts and terminology of 802.1d still apply. In addition, because it is backward compatible with the IEEE 802.1d standard, it is possible to interoperate with older networks that are still running the original protocol.

As its name implies, the most important feature of 802.1w is its capability to execute transition rapidly. Whereas a typical convergence time for 802.1d is between 35 and 60 seconds, the 802.1w can achieve it in 1 to 3 seconds. In some cases, a subsecond convergence is even possible. The 802.1d protocol was slow in its operation because ports take a long time to move into a forwarding state. In 802.1d, the only way to achieve faster convergence is to change the value of the Forward Delay and Maximum Age timers. Even then, it is still a timer-based protocol. The 802.1w can achieve faster convergence because it can move a port to a forwarding state without waiting for any timer to expire. It does so via a few new features:

  • New port roles and states
  • New BPDU format and handshake mechanism
  • New edge port concept
  • New link type concept

In the original 802.1d standard, a bridge port may be in one of the five possible states: disabled, blocked, listening, learning, or forwarding. 802.1w reduces the port states to three: discarding, learning, and forwarding. Table 6-2 compares the port states of the two standards.

Table 6-2. Comparison of 802.1d and 802.1w Port States

802.1d Port States

802.1w Port State

Port Included in Active Topology?

Port Learning MAC Addresses?





















In the original 802.1d standard, a port can take on the role of a root, designated, or alternate. In the 802.1w, a port can take on a root, designated, alternate, or backup role. Table 6-3 compares the port role of the two standards.

Table 6-3. Comparison of 802.1d and 802.1w Port Roles

802.1d Port Role

802.1w Port Role











In 802.1w, an alternate port is one that receives preferred BPDUs from other bridges, whereas a port is designated as backup if it has received preferred BPDUs from the same bridge that it is on. Figure 6-21 illustrates the concept of the new port roles.


Figure 6-21 Port Roles of IEEE 802.1w

The format of the 802.1w BPDU differs from that in 802.1d. In 802.1d, only the Topology Change (TC) and Topology Change Acknowledgement (TCA) were defined. In the new standard, the rest of the 6 bits are used to include the following flags:

  • Proposal
  • Port Role (2 bits)
  • Learning
  • Forwarding
  • Agreement

The inclusion of the new flags enables the bridges to communicate with each other in a new manner. In the 802.1d standard, BPDUs are propagated via a relay function. In other words, the root bridge sends out BPDUs and the rest will relay the message to the downstream.

Unlike 802.1d, 802.1w uses Hello packets between bridges to maintain link states and does not rely on the root bridge. In 802.1w, bridges send out BPDUs to their neighbors at fixed times, as specified by the Hello timer. When a bridge has not received a BPDU from its neighbor, it can assume that its neighbor is down. This reliance on Hello packets, rather than the Forward Delay and Maximum Age timers in the original 802.1d, enables the bridges to perform notification and react to topology change in a much more efficient manner.

In 802.1w, the edge port refers to a port that is directly connected to an end station. Because an end station is not a bridge and will not cause loops, the port can be put into a forwarding state immediately. This is exactly the same concept as that of the PortFast feature in the Cisco implementation. An edge port does not cause topology change, regardless of its link state. However, if it starts to receive BPDUs, it will lose its edge port status and become a normal STP port.

The advent of switching technology has resulted in changes to network design. Nowadays, switches are connected more in a point-to-point manner than through shared media. By taking advantage of the point-to-point link's characteristics, 802.1w can move a port into forwarding state much faster than before. The link type is derived from the duplex mode of the port. A link connected via a full-duplex port is considered to be point to point, whereas one that is connected via a half duplex is considered to be shared. The link type can also be configured manually in 802.1w. Therefore, a simple configuration of a port mode now significantly affects the behavior of the STP.

The 802.1w is made available on the Catalyst switches through the implementation of Rapid-PVST+. Rapid-PVST+ enables you to load balance VLAN traffic and at the same time provide rapid convergence in the event of a failure. 802.1w is also incorporated into the Cisco implementation of IEEE 802.1s. IEEE 802.1s is discussed in the following section.

Example 6-11 shows how to turn on Rapid-PVST+ in the global configuration mode.

Example 6-11. Configuring Rapid-PVST+

3750-A#configure terminal
3750-A(config)#spanning-tree mode rapid-pvst

Because 802.1w has introduced a few enhancements to the original 802.1d, not all features discussed in the section "The Spanning Tree Protocol: IEEE 802.1d" are needed. The following features are available in conjunction with Rapid-PVST+:

  • PortFast
  • BPDUGuard
  • RootGuard
  • LoopGuard

As mentioned earlier, the 802.1w protocol can work with the 802.1d protocol to provide interoperability. However, all the 802.1w enhancements are lost, and the behavior will be that of the original standard.

IEEE 802.1s

The IEEE 802.1s protocol is another standard that deals with mapping VLAN topology with an independent instance of STP. To have a good understanding of IEEE 802.1s, you have to appreciate the differences between the CST model and PVST model.

In the deployment of CST, only one instance of STP is running in the switches, and as a result, all VLANs share the same Layer 2 topology. The characteristics of a CST implementation are as follows:

  • Only one instance of STP is running for all VLANs; therefore, the CPU load on the switches is minimal.
  • Because all VLAN topologies are governed by the same STP process, they all share the same forwarding path and blocked path.
  • No load balancing of traffic is permitted.

By adopting a PVST strategy, you can achieve a different implementation, such as that shown earlier in Figure 6-17, with the following characteristics:

  • Each VLAN is governed by a separate STP instance. Because of this, the CPU load on the switch has increased. With a network of 1000 VLANs, there are actually 1000 instances of STP running.
  • Because each VLAN has its own STP instance, they can be configured differently to achieve different Layer 2 topology.
  • Load balancing of traffic is achieved via configuration.
  • There will be many VLANs sharing the same Layer 2 topology, and each VLAN will have its own STP instance.

Both of the preceding implementations have their pros and cons. CST achieves saving on CPU resources but sacrifices on topology flexibility and load-balancing capability. PVST sacrifices CPU resources to achieve better control of the topology and uplink capacity. The IEEE 802.1s standard aims to combine the benefits of the two implementations.

The idea behind the IEEE 802.1s standard is to map those VLANs that share the same topology to a common instance of STP. This way, you can reduce the number of instances of STP running, thus reducing the load on the CPU. At the same time, other VLANs may be mapped to another instance of STP for a different topology. This way, the flexibility of topology is made possible and load balancing can be achieved.

In Figure 6-22, both voice and data VLANs are mapped to STP instance 1, where the uplink on the right is blocked. On the other hand, as illustrated in Figure 6-23, the WLAN VLAN is mapped to STP instance 2, where the uplink on the left is blocked.


Figure 6-22 STP Instances of IEEE 802.1s for Voice and Data VLANs


Figure 6-23 STP Instances of IEEE 802.1s for WLAN VLANs

All in all, there are two instances of STP governing the topology of the three VLANs. With this implantation, it is possible to have, for example, 500 VLANs mapped to STP instance 1 and another 500 VLANs mapped to STP instance 2. Without 802.1s running, you need 1000 instances of STP running in the switch.

The benefit of the IEEE 802.1s is in saving CPU resources from managing too many STP instances. However, the drawback is that it adds complexity to the network design. Keeping track of which VLAN is mapped to which STP instance may be an administrative burden.

Example 6-12 shows an 802.1s configuration. The Catalyst series switches support a maximum of 16 instances of RSTP. Note that RSTP is enabled by default when you configure 802.1s.

Example 6-12. Configuring 802.1s

3750-A#configure terminal
3750-A(config)#spanning-tree mst configuration
3750-A(config-mst)#instance 1 vlan 5-10
3750-A(config-mst)#name HRServers
3750-A(config-mst)#revision 1

The following error-prevention features are still made available to the instances of STP within 802.1s:

  • PortFast
  • BPDUGuard
  • RootGuard
  • LoopGuard

In the Catalyst series of switches, the 802.1s feature is implemented together with the 802.1w. Therefore, its implementation is not only capable of saving control-plane resources but also converging in subseconds in some cases.

Channeling Technology

The channeling technology enables you to use multiple Ethernet links combined into one logical link. This configuration allows for increments in transmission capability between devices and provides link redundancy. It can be used to interconnect LAN switches or routers and even improves the resiliency of a server connection to a switch.

The Cisco implementation of channeling technology is the EtherChannel. Depending on the connection speed, EtherChannel is referred to as Fast EtherChannel (FEC) for 100-Mbps links or Gigabit EtherChannel (GEC) for 1-Gbps links. The EtherChannel for 10-Gbps links is also available today. Figure 6-24 shows the basic concept of EtherChannel.


Figure 6-24 EtherChannel

EtherChannel operation comprises two parts:

  • The distribution of data packets on the multiple physical links
  • A control portion that governs the working of the technology

The distribution of the packets is based on a hashing algorithm that derives a numeric value to decide which links to send the packet to. The algorithm may decide on the distribution based on the following:

  • Source MAC address
  • Destination MAC address
  • Source/destination MAC address
  • Source IP
  • Destination IP
  • Source/destination IP
  • Source TCP port
  • Destination TCP port
  • Source IP/destination IP/TCP port

Based on the distribution criteria, the hashing is deterministic, and all packets that share the same characteristics are always transmitted via the same physical link to avoid sequencing errors. Depending on the hardware architecture, not all the Cisco devices support the algorithm.

The control portion of the EtherChannel governs the working of the EtherChannel. There are three ways to bring up EtherChannel:

  • Manual configuration
  • Port Aggregation Control Protocol (PAgP)
  • IEEE 802.1ad, also known as Link Aggregation Control Protocol (LACP)

Before participating in the channeling operation, a port can be in one of the EtherChannel modes listed in Table 6-4.

Table 6-4. Different Modes of EtherChannel




Forces the port to channel unconditionally. Ports configured in the on mode do not negotiate, and they only operate with a partnering port that has been configured to on mode.


PAgP mode that places a port into a passive negotiating state. It receives PAgP requests but does not initiate PAgP negotiation. This is the default.


PAgP mode that places a port into an active negotiating state. The port sends out PAgP packets to initiate negotiations.


LACP mode that places a port into a passive negotiating state. It receives LACP requests but does not initiate LACP negotiation.


LACP mode that places a port into an active negotiating state. The port sends out LACP packets to initiate negotiations.

The manual way to configure EtherChannel dictates that both ports must be set to on to form the EtherChannel.

The PAgP supports the autocreation of EtherChannel by the exchange of the PAgP packets between ports. This happens only when the ports are in auto and desirable modes. The basic rules for PAgP are as follows:

  • A port in desirable mode can form an EtherChannel with another port in desirable mode.
  • A port in desirable mode can form an EtherChannel with another in auto mode.
  • A port in auto mode cannot form an EtherChannel with another also in auto mode, because neither one will initiate negotiation.

Likewise, the LACP supports the autocreation of EtherChannel by the exchange of the LACP packets between ports. This happens only when the ports are in active and passive modes. The basic rules for LACP are as follows:

  • A port in active mode can form an EtherChannel with another port in active mode.
  • A port in active mode can form an EtherChannel with another in passive mode.
  • A port in passive mode cannot form an EtherChannel with another also in passive mode, because neither one will initiate negotiation.

Channeling technology is a good way to increase link capacity, provide link redundancy, and at the same time maintain the logical design of the IP network. However, care should be taken when deploying channeling technology, as discussed in the following section.

Layer 2 Best Practices

The health of the Layer 2 network plays an important role in maintaining the stability of the IP network within the access module. However, it is a common source of network error, with problems ranging from broadcast storms to complete network meltdowns. However, by following some simple principles, you can avoid these problems.

Simple Is Better

It cannot be stressed enough that in the Layer 2 network design, simple is more, and more may not be beautiful. What this means is that you have to keep the design as simple as possible, with easy-to-remember configuration settings. The following are some examples:

  • If there is no uplink congestion problem, there is no need to load balance traffic.
  • In the event that you load balance traffic, choose a simple formula. For example, the human resources (HR) department uses the left uplink as the primary path, and the finance department uses the right uplink as the primary path. Never try to formulate a complex web of tables just to keep track of which VLAN has the left uplink as primary and which has the right as primary. A common approach is to have VLANs with even VLAN IDs going to the left and VLANs with odd VLAN IDs going to the right.
  • If changing the bridge priority is enough, leave the path cost alone.

By keeping the Layer 2 design simple, you can actually improve the resiliency and convergence of the network. Another benefit is that it will make troubleshooting a lot easier in the event of a problem.

Limit the Span of VLANs

If possible, confine the span of the VLAN to a minimum. This means you should avoid building a VLAN that spans multiple access modules. Provided you are dealing with some legacy application that needs all hosts to be on the same subnet, there is no reason why you should build a campus-wide VLAN. Such a VLAN not only takes up too many resources to maintain topology, you will probably end up with a nonoptimized traffic path that uses bandwidth unnecessarily.

In addition, when there is an STP problem within the VLAN, too many switches will be involved in the convergence exercise. This increases the chance of a network-wide outage and should be avoided.

Build Triangles, Not Squares

Features such as UplinkFast and BackboneFast work best in a triangle topology—that is, a single access switch having dual uplinks to a pair of distribution switches. In some situations, access switches are daisy-chained via physical connections. With two access switches, you end up with a square topology. With more than two access switches, you end up with a ring architecture. Both the square and ring designs suffer from poorer convergence time in the event of a failure. Although RSTP may still achieve the desired convergence time, the resulting topology may be too difficult to manage and troubleshoot.


Figure 6-25 Triangle and Square Topologies

If you have to daisy chain the access switches, you should consider the StackWise tech-nology available on the Catalyst 3750. With StackWise technology, you can stack up to nine access switches and yet they are seen as one device from a Layer 2 and 3 perspective. From an STP perspective, they are viewed as one bridge, and from an IP network perspective, they can be managed via a single IP address. Figure 6-26 shows three Catalyst 3750s connected to each other via the StackWise technology.


Figure 6-26 Connecting Catalyst 3750s via StackWise Technology

Protect the Network from Users

The access module is where users have a direct connection to the network. You can never trust that only end stations such as PCs and laptops are connected to the access switches. The combination of users connecting a switch to the network and turning on the PortFast feature on that port could spell disaster for the STP topology. Therefore, the STP network has to be protected from all users via features such as BPDUGuard and RootGuard. You have to be cautious with the user-side connections to the network. Always imagine that users are going to put in technologies that are detrimental to the health of the network.

Selecting Root Bridges

The root bridge is the most important component in the Layer 2 network. Its location within the Layer 2 topology and the selection of hardware impact the health of the Layer 2 network.

Because all traffic in the Layer 2 network flows toward the root, the distribution layer switches are always good candidates. This corresponds to the way the IP traffic flows in the Layer 3 network. For redundancy purposes, there are usually two distribution switches per block, and they should be selected as the primary and secondary root bridge. Remember that in STP the bridge with the lowest BID is always elected as a root bridge. Therefore, you should not leave default values when configuring the BID of the switches. For this reason, part of the network design should include a BID assignment strategy.

One common mistake that network managers make is to leave the operation of STP to the default values and behavior. The problem with this is that the selection of the root is not deterministic, and the resulting Layer 2 topology might not be optimized. Worse still, in the event of a problem, you have to determine the location of the root bridge. The most difficult problem to troubleshoot is a floating network design, where it is difficult to determine how it should work in the first place. You should decide on a root bridge and a backup root bridge so that the Layer 2 topology is fixed and easily understood. With proper configuration and documentation, traffic has to flow in a certain manner. This way, any anomaly can be easily identified and the problem diagnosed. The root bridge has to be protected via features such as RootGuard, and if it is possible, no access switch should be elected as a root bridge.

Remember that the distribution layer is the most complex part of the access module, because it runs both Layer 2 and Layer 3 protocols. Therefore, the load on its control plane will be relatively higher than the rest of the switches in the network. For this reason, careful selection of hardware is important, and switches with higher-performance CPUs should be preferred.

Use Value-Added Features

You can also use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal Hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated Hello time. Example 6-13 shows a configuration sample of limiting the diameter to four switches for VLAN 100.

Example 6-13. Setting the Diameter of a Layer 2 Network

3750#configure terminal
3750(config)#spanning-tree vlan 100 root primary diameter 4

EtherChannel Deployment

A common place to deploy EtherChannel is the connection between the distribution switches. Such a connection is usually configured as a switching trunk port, and, therefore, the bundling is called a Layer 2 EtherChannel. On the other hand, an EtherChannel may be deployed as a routed interface, such as that between a distribution layer switch and a core layer switch. Such an implementation is called a Layer 3 EtherChannel.

Extra care has to be taken with the implementation of a Layer 3 EtherChannel because many advanced IP features, such as Multiprotocol Label Switching-Traffic Engineering (MLPS-TE), Reservation Protocol (RSVP), and QoS, with complex configurations may not be supported in conjunction with a Layer 3 EtherChannel. Different linecards on different platforms may have an individual caveat with respect to this scenario, and it is important that you are familiar with these caveats. Because of the potential complexity that Layer 3 EtherChannel may introduce in such a scenario, link capacity scaling and resiliency in the core module require a different approach than that provided by EtherChannel.

EtherChannel Load Balancing

The default hashing algorithm for EtherChannel is based on the source/destination IP pair. This mode is adequate for most user access implementations. However, sometimes this might yield unexpected results. One good example is when a pair of servers is performing application synchronization. Because the source/destination IP pair remains constant throughout the transmission, you end up with only one of the links of an EtherChannel being utilized. This may have a performance impact if the transmission exceeds the transmission capacity of the single link.

One good way to overcome this problem is to adopt the source IP/destination IP/TCP port hashing algorithm for the EtherChannel. Including Layer 4 information allows for a more balanced use of the available links within the EtherChannel bundle.

Consistent EtherChannel Port Settings

When configuring EtherChannel, you have to ensure that all members of the bundle have the same port setting. This includes settings such as the link speed and duplex mode. If you have ports with different settings, you may face unexpected results and find troubleshooting more difficult. For example, ports belonging to the same EtherChannel configured with different trunking modes may produce undesirable STP behavior. All ports need to be in enabled mode, too. Having some ports in disabled mode will cause traffic to be diverted to the rest.

Layer 2 Setting for EtherChannel

When setting up the Layer 2 EtherChannel, take care with respect to the Layer 2 setting. For example, if the EtherChannel is to act as a logical access port for VLAN 10, all the switch ports that make up the EtherChannel have to be set to the same VLAN. If the EtherChannel is to act as a logical trunk port, however, all the switch ports that make up the EtherChannel have to be in trunk mode. In addition, the allowed range of VLANs has to be the same for all the ports. Failing to ensure port setting consistencies means that the EtherChannel will never be formed.

Turning Off Autonegotiation

Although the switches can autonegotiate for the formation of EtherChannel via protocols such as PAgP, it is better to configure them manually. For example, PAgP is on by default for switches running CatOS, whereas it is off by default for those running IOS. A mismatch PAgP setting can incur unnecessary negotiation time and may take a long time to restore connection.

Turning off autonegotiation on ports between servers and switches is also a good idea to improve resiliency of the network. Some server network interfaces might not negotiate the link properly and cause duplex mismatch. The server network interface card may think that the switch port is running as a half duplex, when in fact the switch port is a full duplex. Therefore, it is always recommended to explicitly set the duplex settings on both the server interface and the switch port.

So far, you have learned about all the Layer 2 building blocks that enable you to build a stable Layer 2 network. After the Layer 2 network is robust and has high resiliency, you can then build the corresponding IP subnets to be always reachable from other parts of the network.

4. Layer 3 Domain | Next Section Previous Section

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