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IP Routing in the LAN

Chapter Description

In this sample chapter from CCNA 200-301 Official Cert Guide, Volume 1, Wendell Odom discusses the configuration and verification steps related to three methods of routing between VLANs with three major sections: VLAN Routing with Router 802.1Q Trunks, VLAN Routing with Layer 3 Switch SVIs, and VLAN Routing with Layer 3 Switch Routed Ports.

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VLAN Routing with Layer 3 Switch SVIs

Using a router with ROAS to route packets makes sense in some cases, particularly at small remote sites. In sites with a larger LAN, network designers choose to use Layer 3 switches for most inter-VLAN routing.

A Layer 3 switch (also called a multilayer switch) is one device, but it executes logic at two layers: Layer 2 LAN switching and Layer 3 IP routing. The Layer 2 switch function forwards frames inside each VLAN, but it will not forward frames between VLANs. The Layer 3 forwarding (routing) logic forwards IP packets between VLANs.

Layer 3 switches typically support two configuration options to enable IPv4 routing inside the switch, specifically to enable IPv4 on switch interfaces. This section explains one option, an option that uses switched virtual interfaces (SVI). The final major section of the chapter deals with the other option for configuring IPv4 addresses on Layer 3 switches: routed interfaces.

Configuring Routing Using Switch SVIs

The configuration of a Layer 3 switch mostly looks like the Layer 2 switching configuration shown back in Parts II and III of this book, with a small bit of configuration added for the Layer 3 functions. The Layer 3 switching function needs a virtual interface connected to each VLAN internal to the switch. These VLAN interfaces act like router interfaces, with an IP address and mask. The Layer 3 switch has an IP routing table, with connected routes off each of these VLAN interfaces. (These interfaces are also referred to as switched virtual interfaces [SVI].)

To show the concept of Layer 3 switching with SVIs, the following example uses the same branch office with two VLANs shown in the earlier examples, but now the design will use Layer 3 switching in the LAN switch. Figure 17-3 shows the design changes and configuration concept for the Layer 3 switch function with a router icon inside the switch, to emphasize that the switch routes the packets.


FIGURE 17-3 Routing on VLAN Interfaces in a Layer 3 Switch

Note that the figure represents the internals of the Layer 3 switch within the box in the middle of the figure. The branch still has two user VLANs (10 and 20), so the Layer 3 switch needs one VLAN interface for each VLAN. The figure shows a router icon inside the gray box to represent the Layer 3 switching function, with two VLAN interfaces on the right side of that icon. In addition, the traffic still needs to get to router B1 (a physical router) to access the WAN, so the switch uses a third VLAN (VLAN 30 in this case) for the link to Router B1. The physical link between the Layer 3 switch and router B1 would not be a trunk, but instead be an access link.

The following steps show how to configure Layer 3 switching using SVIs. Note that on some switches, like the 2960 and 2960-XR switches used for the examples in this book, the ability to route IPv4 packets must be enabled first, with a reload of the switch required to enable the feature. The steps that occur after the reload would apply to all models of Cisco switches that are capable of doing Layer 3 switching.

  • Step 1. Enable IP routing on the switch, as needed:

    1. Use the sdm prefer lanbase-routing command (or similar) in global configuration mode to change the switch forwarding ASIC settings to make space for IPv4 routes at the next reload of the switch.

    2. Use the reload EXEC command in enable mode to reload (reboot) the switch to pick up the new sdm prefer command setting.

    3. Once reloaded, use the ip routing command in global configuration mode to enable the IPv4 routing function in IOS software and to enable key commands like show ip route.

  • Step 2. Configure each SVI interface, one per VLAN for which routing should be done by this Layer 3 switch:

    1. Use the interface vlan vlan_id command in global configuration mode to create a VLAN interface and to give the switch’s routing logic a Layer 3 interface connected into the VLAN of the same number.

    2. Use the ip address address mask command in VLAN interface configuration mode to configure an IP address and mask on the VLAN interface, enabling IPv4 routing on that VLAN interface.

    3. (As needed) Use the no shutdown command in interface configuration mode to enable the VLAN interface (if it is currently in a shutdown state).

Example 17-6 shows the configuration to match Figure 17-3. In this case, switch SW1 has already used the sdm prefer global command to change to a setting that supports IPv4 routing, and the switch has been reloaded. The example shows the related configuration on all three VLAN interfaces.

Example 17-6 VLAN Interface Configuration for Layer 3 Switching

ip routing
interface vlan 10
 ip address
interface vlan 20
 ip address
interface vlan 30
 ip address

Verifying Routing with SVIs

With the VLAN configuration shown in the previous section, the switch is ready to route packets between the VLANs as shown in Figure 17-3. To support the routing of packets, the switch adds connected IP routes as shown in Example 17-7; note that each route is listed as being connected to a different VLAN interface.

Example 17-7 Connected Routes on a Layer 3 Switch

SW1# show ip route
! legend omitted for brevity is variably subnetted, 6 subnets, 2 masks
C is directly connected, Vlan10
L is directly connected, Vlan10
C is directly connected, Vlan20
L is directly connected, Vlan20
C is directly connected, Vlan30
L is directly connected, Vlan30

The switch would also need additional routes to the rest of the network (not shown in the figures in this chapter). The Layer 3 switch could use static routes or a routing protocol, depending on the capabilities of the switch. For instance, if you then enabled OSPF on the Layer 3 switch, the configuration and verification would work the same as it does on a router, as discussed in Chapter 20, “Implementing OSPF.” The routes that IOS adds to the Layer 3 switch’s IP routing table would list the VLAN interfaces as outgoing interfaces.

Troubleshooting Routing with SVIs

There are two big topics to investigate when troubleshooting routing over LANs with SVIs. First, you have to make sure the switch has been enabled to support IP routing. Second, the VLAN associated with each VLAN interface must be known and active on the local switch; otherwise, the VLAN interfaces do not come up.

First, about enabling IP routing, note that some models of Cisco switches default to enable Layer 3 switching, and some do not. So, to make sure your switch supports Layer 3 routing, look to those first few configuration commands listed in the configuration checklist found in the earlier section “Configuring Routing Using Switch SVIs.” Those commands are sdm prefer (followed by a reload) and then ip routing (after the reload).

The sdm prefer command changes how the switch forwarding chips allocate memory for different forwarding tables, and changes to those tables require a reload of the switch. By default, many access switches that support Layer 3 switching still have an SDM default that does not allocate space for an IP routing table. Once changed and reloaded, the ip routing command then enables IPv4 routing in IOS software. Both are necessary before some Cisco switches will act as a Layer 3 switch.

Example 17-8 shows some symptoms on a router for which Layer 3 switching had not yet been enabled by the sdm prefer command. As you can see, both the show ip route EXEC command and the ip routing config command are rejected because they do not exist to IOS until the sdm prefer command has been used (followed by a reload of the switch).

Example 17-8 Evidence That a Switch Has Not Yet Enabled IPv4 Routing

SW1# show ip route

% Invalid input detected at '^' marker.

SW3# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW3(config)# ip routing

% Invalid input detected at '^' marker.

The second big area to investigate when troubleshooting SVIs relates to the SVI state, a state that ties to the state of the associated VLANs. Each VLAN interface has a matching VLAN of the same number, and the VLAN interface’s state is tied to the state of the VLAN in certain ways. In particular, for a VLAN interface to be in an up/up state:

  • Step 1. The VLAN must be defined on the local switch (either explicitly or learned with VTP).

  • Step 2. The switch must have at least one up/up interface using the VLAN, either/both:

    1. An up/up access interface assigned to that VLAN

    2. A trunk interface for which the VLAN is in the allowed list, is STP forwarding, and is not VTP pruned

  • Step 3. The VLAN (not the VLAN interface) must be administratively enabled (that is, not shutdown).

  • Step 4. The VLAN interface (not the VLAN) must be administratively enabled (that is, not shutdown).

When working through the steps in the list, keep in mind that the VLAN and the VLAN interface are related but separate ideas, and the configuration items are separate in the CLI. The VLAN interface is a switch’s Layer 3 interface connected to the VLAN. If you want to route packets for the subnets on VLANs 11, 12, and 13, the matching VLAN interfaces must be numbered 11, 12, and 13. And both the VLANs and the VLAN interfaces can be disabled and enabled with the shutdown and no shutdown commands (as mentioned in Steps 3 and 4 in the previous list), so you have to check for both.

Example 17-9 shows three scenarios, each of which leads to one of the VLAN interfaces in the previous configuration example (Figure 17-3, Example 17-6) to fail. At the beginning of the example, all three VLAN interfaces are up/up. VLANs 10, 20, and 30 each have at least one access interface up and working. The example works through three scenarios:

  • Scenario 1: The last access interface in VLAN 10 is shut down (F0/1), so IOS shuts down the VLAN 10 interface.

  • Scenario 2: VLAN 20 (not VLAN interface 20, but VLAN 20) is deleted, which results in IOS then bringing down (not shutting down) the VLAN 20 interface.

  • Scenario 3: VLAN 30 (not VLAN interface 30, but VLAN 30) is shut down, which results in IOS then bringing down (not shutting down) the VLAN 30 interface.

Example 17-9 Three Examples That Cause VLAN Interfaces to Fail

SW1# show interfaces status
! Only ports related to the example are shown
Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/1                        connected    10         a-full  a-100 10/100BaseTX
Fa0/2                        notconnect   10           auto   auto 10/100BaseTX
Fa0/3                        connected    20         a-full  a-100 10/100BaseTX
Fa0/4                        connected    20         a-full  a-100 10/100BaseTX
Gi0/1                        connected    30         a-full a-1000 10/100/1000BaseTX
SW1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.

! Case 1: Interface F0/1, the last up/up access interface in VLAN 10, is shutdown
SW1(config)# interface fastEthernet 0/1
SW1(config-if)# shutdown
*Apr 2 19:54:08.784: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan10, changed
state to down
*Apr 2 19:54:10.772: %LINK-5-CHANGED: Interface FastEthernet0/1, changed state to
administratively down
*Apr 2 19:54:11.779: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1,
changed state to down

! Case 2: VLAN 20 is deleted
SW1(config)# no vlan 20
*Apr 2 19:54:39.688: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan20, changed
state to down

! Case 3: VLAN 30, the VLAN from the switch to the router, is shutdown
SW1(config)# vlan 30
SW1(config-vlan)# shutdown
SW1(config-vlan)# exit
*Apr 2 19:55:25.204: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan30, changed
state to down

! Final status of all three VLAN interfaces are below
SW1# show ip interface brief | include Vlan
Vlan1                  unassigned      YES manual administratively down down
Vlan10              YES manual up                    down
Vlan20              YES manual up                    down
Vlan30              YES manual up                    down

Note that the example ends with the three VLAN interfaces in an up/down state per the show ip interface brief command.

4. VLAN Routing with Layer 3 Switch Routed Ports | Next Section Previous Section

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