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Switch Operation for the CCNP BCMSN Exam

Chapter Description

This sample chapter covers the following topics that you need to master for the CCNP BCMSN exam: Layer 2 Switch Operation, Multilayer Switch Operation, Tables Used in Switching, and Troubleshooting Switching Tables.

Tables Used in Switching

Catalyst switches maintain several types of tables to be used in the switching process. The tables are tailored for Layer 2 switching or MLS, and are kept in very fast memory so that many fields within a frame or packet can be compared in parallel.

Content Addressable Memory (CAM)

All Catalyst switch models use a Content Addressable Memory (CAM) table for Layer 2 switching. As frames arrive on switch ports, the source MAC addresses are learned and recorded in the CAM table. The port of arrival and the VLAN are both recorded in the table, along with a timestamp. If a MAC address learned on one switch port has moved to a different port, the MAC address and timestamp are recorded for the most recent arrival port. Then, the previous entry is deleted. If a MAC address is found already present in the table for the correct arrival port, only its timestamp is updated.

Switches generally have large CAM tables so that many addresses can be looked up for frame forwarding. However, there is not enough table space to hold every possible address on large networks. To manage the CAM table space, stale entries (addresses that have not been heard from for a period of time) are aged out. By default, idle CAM table entries are kept for 300 seconds before they are deleted. You can change the default setting using the following configuration command:

Switch(config)# mac address-table aging-time seconds

By default, MAC addresses are learned dynamically from incoming frames. You can also configure static CAM table entries that contain MAC addresses that might not otherwise be learned. To do this, use the following configuration command:

Switch(config)# mac address-table static mac-address vlan vlan-id interface type mod/num

Here, the MAC address (in dotted triplet hex format) is identified with the switch port and VLAN where it appears.


Until Catalyst IOS version 12.1(11)EA1, the syntax for CAM table commands used the keywords mac-address-table. In more recent IOS versions, the syntax has changed to use the keywords mac address-table (first hyphen omitted).

What happens when a host's MAC address is learned on one switch port, and then the host moves so that it appears on a different switch port? Ordinarily, the host's original CAM table entry would have to age out after 300 seconds, while its address was learned on the new port. To avoid having duplicate CAM table entries, a switch purges any existing entries for a MAC address that has just been learned on a different switch port. This is a safe assumption because MAC addresses are unique, and a single host should never be seen on more than one switch port unless problems exist in the network. If a switch notices that a MAC address is being learned on alternating switch ports, it generates an error message that flags the MAC address as "flapping" between interfaces.

Ternary Content Addressable Memory (TCAM)

In traditional routing, ACLs can match, filter, or control specific traffic. Access lists are made up of one or more access control entities (ACEs) or matching statements that are evaluated in sequential order. Evaluating an access list can take up additional time, adding to the latency of forwarding packets.

In multilayer switches, however, all of the matching process that ACLs provide is implemented in hardware. TCAM allows a packet to be evaluated against an entire access list in a single table lookup. Most switches have multiple TCAMs so that both inbound and outbound security and QoS ACLs can be evaluated simultaneously, or entirely in parallel with a Layer 2 or Layer 3 forwarding decision.

The Catalyst IOS Software has two components that are part of the TCAM operation:

  • Feature Manager (FM)—After an access list has been created or configured, the Feature Manager software compiles, or merges, the ACEs into entries in the TCAM table. The TCAM can then be consulted at full frame forwarding speed.

  • Switching Database Manager (SDM)—You can partition the TCAM on Catalyst switches into areas for different functions. The SDM software configures or tunes the TCAM partitions, if needed.

TCAM Structure

The TCAM is an extension of the CAM table concept. Recall that a CAM table takes in an index or key value (usually a MAC address) and looks up the resulting value (usually a switch port or VLAN ID). Table lookup is fast and always based on an exact key match consisting of two input values: 0 and 1 bits.

TCAM also uses a table lookup operation but is greatly enhanced to allow a more abstract operation. For example, binary values (0s and 1s) make up a key into the table, but a mask value is also used to decide which bits of the key are actually relevant. This effectively makes a key consisting of three input values: 0, 1, and X (don't care) bit values—a three-fold or ternary combination.

TCAM entries are composed of Value, Mask, and Result (VMR) combinations. Fields from frame or packet headers are fed into the TCAM, where they are matched against the value and mask pairs to yield a result. As a quick reference, these can be described as follows:

  • Values are always 134-bit quantities, consisting of source and destination addresses and other relevant protocol information—all patterns to be matched. The information concatenated to form the value is dependent upon the type of access list, as shown in Table 3-2. Values in the TCAM come directly from any address, port, or other protocol information given in an ACE.

  • Masks are also 134-bit quantities, in exactly the same format, or bit order, as the values. Masks select only the value bits of interest; a mask bit is set to exactly match a value bit, or not set for value bits that don't matter. The masks used in the TCAM stem from address or bit masks in ACEs.

  • Results are numerical values that represent what action to take after the TCAM lookup occurrs. Where traditional access lists offer only a permit or deny result, TCAM lookups offer a number of possible results or actions. For example, the result can be a permit or deny decision, an index value to a QoS policer, a pointer to a next-hop routing table, and so on.

Table 3-2 TCAM Value Pattern Components

Access List Type

Value and Mask Components, 134 Bits Wide (Number of Bits)


Source MAC (48), destination MAC (48), Ethertype (16)


Source IP (32), destination IP (32), protocol (16), ICMP code (8), ICMP type (4), IP type of service (ToS) (8)

Extended IP using TCP/UDP

Source IP (32), destination IP (32), protocol (16), IP ToS (8), source port (16), source operator (4), destination port (16), destination operator (4)

Other IP

Source IP (32), destination IP (32), protocol (16), IP ToS (8)


Source IP (32), destination IP (32), protocol (16), IP ToS (8), IGMP message type (8)


Source IPX network (32), destination IPX network (32), destination node (48), IPX packet type (16)

The TCAM is always organized by masks, where each unique mask has eight value patterns associated with it. For example, the Catalyst 6500 TCAM (one for security ACLs and one for QoS ACLs) holds up to 4096 masks and 32,768 value patterns. The trick is that each of the mask-value pairs is evaluated simultaneously, or in parallel, revealing the best or longest match in a single table lookup.

TCAM Example

Figure 3-5 shows how the TCAM is built and used. This is a simple example, and might or might not be identical to the results that the Feature Manager produces. This is because the ACEs might need to be optimized or rewritten to achieve certain TCAM algorithm requirements.

Figure 5Figure 3-5 How an Access List Is Merged into TCAM

The example access list 100 (extended IP) is configured and merged into TCAM entries. First, the mask values must be identified in the access list. When an address value and a corresponding address mask are specified in an ACE, those mask bits must be set for matching. All other mask bits can remain in the "don't care" state. The access list contains only three unique masks: one that matches all 32 bits of the source IP address (found with an address mask of or the keyword host), one that matches 16 bits of the destination address (found with an address mask of, and one that matches only 24 bits of the destination address (found with an address mask of The keyword any in the ACEs means match anything or "don't care."

The unique masks are placed into the TCAM. Then, for each mask, all possible value patterns are identified. For example, a 32-bit source IP mask (Mask 1) can be found only in ACEs with a source IP address of and a destination of (The rest of Mask 1 is the destination address mask Those address values are placed into the first value pattern slot associated with Mask 1. Mask 2 has three value patterns: destination addresses,, and Each of these is placed in the three pattern positions of Mask 2. This process continues until all ACEs have been merged.

When a mask's eighth pattern position has been filled, the next pattern with the same mask must be placed under a new mask. A bit of a balancing act occurs to try and fit all ACEs into the available mask and pattern entries without an overflow.

Port Operations in TCAM

You might have noticed that matching strictly based on values and masks only covers ACE statements that involve exact matches (either the eq port operation keyword or no Layer 4 port operations). For example, ACEs like the following involve specific address values, address masks, and port numbers:

access-list test permit ip any
access-list test permit tcp any host eq www

What about ACEs that use port operators, where a comparison must be made? Consider the following:

access-list test permit udp any host gt 1024
access-list test permit tcp any any range 2000 2002

A simple logical operation between a mask and a pattern cannot generate the desired result. The TCAM also provides a mechanism for performing a Layer 4 operation or comparison, also done during the single table lookup. If an ACE has a port operator, such as gt, lt, neq, or range, the Feature Manager software compiles the TCAM entry to include the use of the operator and the operand in a Logical Operation Unit (LOU) register. Only a limited number of LOUs are available in the TCAM. If there are more ACEs with comparison operators than there are LOUs, the Feature Manager must break the ACEs up into multiple ACEs with only regular matching (using the eq operator).

In Figure 3-5, two ACEs require a Layer 4 operation:

  • One that checks for UDP destination ports greater than 1024

  • One that looks for the UDP destination port range 1024 to 2047

The Feature Manager checks all ACEs for Layer 4 operation, and places these into Logical Operation Unit (LOU) register pairs. These can be loaded with operations, independent of any other ACE parameters. The LOU contents can be reused if other ACEs need the same comparisons and values. After the LOUs are loaded, they are referenced in the TCAM entries that need them. This is shown by LOUs "A1" and the "B1:2" pair. A finite number (actually a rather small number) of LOUs are available in the TCAM, so the Feature Manager software must use them carefully.

5. Troubleshooting Switching Tables | Next Section Previous Section

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