Layer 3 Switching

1. What might you need to implement interVLAN routing?

** One or more Layer 3 interfaces

One or more SVIs

Static routes

A dynamic routing protocol

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

** Yes. Packets can be forwarded between the VLANs carried over the trunk.

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

** First, make sure the VLAN is defined on the switch.

  • interface vlan vlan-id
  • ip address ip-address mask
  • no shutdown

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

** show interface type mod/num switchport


show interface status

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
What is necessary for packets to get from the FastEthernet interface to the

Answer: Nothing. Both are assigned to VLAN 5, so normal Layer 2 transparent
bridging will take care of all forwarding between the two.

6. What is the source of FIB information?

** The routing table, as computed by the Layer 3 engine portion of a switch.

7. How often is the FIB updated?

** As needed. It is downloaded or updated dynamically by the Layer 3 engine
whenever the routing topology changes or an ARP entry changes.

┬á8. What is meant by the term “CEF punt”?

** A packet can’t be forwarded or switched by CEF directly because it needs
further processing. The packet is “punted” to the Layer 3 engine, effectively

bypassing CEF for a more involved resolution.

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

** It is simply replicated to each of the independent CEF engines. The FIB itself
remains intact so that each engine receives a duplicate copy.

10. What happens during a “CEF glean” process?

** The MAC address (ARP reply) for a next-hop FIB entry is not yet known. The
Layer 3 engine must generate an ARP request and wait for a reply before CEF
forwarding can continue to that destination.

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

** The TTL is decremented by one, as if a router had forwarded the packet.

12. What is fallback bridging?

** On switch platforms that cannot multilayer-switch (route) all routable protocols,
those protocols can be bridged transparently between VLANs instead.

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

** Yes. The SVI can be shut down administratively with the shutdown command,
as with any other interface. Also, if the VLAN associated with the SVI is not
defined or active, the SVI will appear to be down


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