Chapter 12: Multilayer Switching

Cisco Press

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  • Forwarding Information Base (FIB)—Contains routing and next-hop information. Lookups are performed according to longest-match IP prefix.

  • Adjacency table—Contains Layer 2 address information for next-hop FIB entries that are one hop away.

  • Packet rewrite—Hardware dedicated to rewriting the Layer 2 and Layer 3 header information of outbound packets after the forwarding decisions have been made.

Table 12-3 Fallback Bridging Configuration Commands


Command Syntax

Define a fallback bridge group

Switch(config)# bridge-group bridge-group protocol vlan-bridge

Assign an interface to a bridge group

Switch(config)# interface vlan vlan-id

Switch(config-if)# bridge-group bridge-group

Table 12-4 Multilayer Switching Verification Commands


Command Syntax

Show a Layer 2 port status

Switch# show interface type mod/num switchport

Show a Layer 3 port status

Switch# show interface type mod/num

Show an SVI status

Switch# show interface vlan vlan-id

View the FIB contents

Switch# show ip cef

View FIB information for an interface

Switch# show ip cef [type mod/num | vlan vlan-id] [detail]

View FIB information for an IP prefix

Switch# show ip cef [prefix-ip prefix-mask] [longer-prefixes] [detail]

View FIB adjacency information

Switch# show adjacency [type mod/num | vlan vlan-id] [summary | detail]

View counters for packets not switched by CEF

Switch# show cef not-cef-switched

Show fallback bridge group status

Switch# show bridge group

Show fallback bridging table contents

Switch# show bridge bridge-group


The questions and scenarios in this book are more difficult than what you should experience on the actual exam. The questions do not attempt to cover more breadth or depth than the exam; however, they are designed to make sure that you know the answers. Rather than allowing you to derive the answers from clues hidden inside the questions themselves, the questions challenge your understanding and recall of the subject. Hopefully, these questions will help limit the number of exam questions on which you narrow your choices to two options and then guess.

You can find the answers to these questions in Appendix A.

  1. What might you need to implement interVLAN routing?

  2. Can interVLAN routing be performed over a single trunk link?

  3. To configure an SVI, what commands are needed?

  4. What command can verify the VLAN assignments on a Layer 2 port?

  5. A switch has the following interface configurations in its running configuration:

    interface fastethernet 0/1
     switchport access vlan 5
    interface vlan 5
     ip address
     no shutdown
  6. What is necessary for packets to get from the FastEthernet interface to the VLAN 5 SVI?

  7. What is the source of FIB information?

  8. How often is the FIB updated?

  9. What is meant by the term CEF punt?

  10. What happens to the FIB when distributed CEF (dCEF) is used?

  11. What happens during a CEF glean process?

  12. What does a multilayer switch do to the IP TTL value just before a packet is forwarded?

  13. What is fallback bridging?

  14. Is it possible for an SVI to go down? If so, for what reasons?

Copyright © 2007 Pearson Education. All rights reserved.

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