entry in fib table

Answered Question
Dec 7th, 2008
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hi everybody!


I have few questions about the following:

switch# show ip cef vlan101

prefix next hop interface

10.1.1.0/24 attached vlan101

10.1.1.2/32 10.1.1.2 vlan 101

10.1.1.3/32 10.1.1.3 vlan 101

2.2.2.0/24 3.3..3.3 f0/1


My understanding is 3.3.3.3 is ip address of neighbor that can be reached via f0/1.

But when I look at entries:

10.1.1.2/32 10.1.1.2 vlan 101

10.1.1.3/32 10.1.1.3 vlan 101

I get confused, My question is vlan 101 is virtual interface ,thus no real device can be connected to virtual interface as it only exists in software inside the switch, how could two neighbors ,10.1.1.2,and 10.1.1.3, connect to svi vlan 101?


Usually there is one- to- one correspondence between physical interface and neighbor( adjacent switch/router).

thanks a lot and have a nice weekend!

Correct Answer by Giuseppe Larosa about 8 years 4 months ago

Hello Sarah,

the output is clearly of a multilayer switch that can have a mix of L3 routed physical ports and SVI like vlan101.

The sh ip cef here is telling you:

10.1.1.0/24 is the IP subnet associated to SVI Vlan 101

There are two neighbors 10.1.1.2 and 10.1.1.3 that can be reached out of L3 interface SVI vlan101.

To know what ports are the devices connected you need to usual a multi step approach


a) sh ip arp 10.1.1.2

take not of the MAC address associated


b) show mac-address-table address mac-address


in this way you have moved from L3 OSI information (neighbor is out vlan101 ) to L2 OSI information ( neighbor mac-address has been learned out L2 port x/y that is member of L2 broadcast domain with vlan-id 101)


When dealing with multilayer switches you need to move between SVIs at L3 and L2 switched ports.

A confirmation of this is provided by the output of

sh ip arp

the interface listed is always a L3 interface either an SVI or a routed port


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Mon, 12/08/2008 - 08:07
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Hello Sarah,

the output is clearly of a multilayer switch that can have a mix of L3 routed physical ports and SVI like vlan101.

The sh ip cef here is telling you:

10.1.1.0/24 is the IP subnet associated to SVI Vlan 101

There are two neighbors 10.1.1.2 and 10.1.1.3 that can be reached out of L3 interface SVI vlan101.

To know what ports are the devices connected you need to usual a multi step approach


a) sh ip arp 10.1.1.2

take not of the MAC address associated


b) show mac-address-table address mac-address


in this way you have moved from L3 OSI information (neighbor is out vlan101 ) to L2 OSI information ( neighbor mac-address has been learned out L2 port x/y that is member of L2 broadcast domain with vlan-id 101)


When dealing with multilayer switches you need to move between SVIs at L3 and L2 switched ports.

A confirmation of this is provided by the output of

sh ip arp

the interface listed is always a L3 interface either an SVI or a routed port


Hope to help

Giuseppe


sarahr202 Mon, 12/08/2008 - 10:09
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Thanks alot Giuseppe!

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