two directly connected mulilayer switches and adjacency table.

Answered Question
Dec 15th, 2008
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Hi everybody!

let say we two switches(L3) connected to each other by F0/1 on both switches, assume that cdp is disabled, no ip packet has been sent by either switches.

In such case, would switches have entry (next hop--mac to mac) in their adjacency table for each other.

If yes, is it because of keep alive messages?

thanks a lot!

Correct Answer by Giuseppe Larosa about 8 years 4 months ago

Hello Sarah,

under the conditions you have described the two L3 switches should be unaware of each other:

if CDP is disabled

no ip packets has been exchanged this includes IP routing protocols


unless they are exchanging IS-IS PdUs (that are not IP packets) they don't know of each other


keepalives are L2 only but near to L1 used to understand if the interface can be seen as up/up (line protocol up)

so they cannot carry information like IP to MAC mapping


As soon as you try to ping the other side both switches can build an entry for the other one.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Mon, 12/15/2008 - 10:30
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Hello Sarah,

under the conditions you have described the two L3 switches should be unaware of each other:

if CDP is disabled

no ip packets has been exchanged this includes IP routing protocols


unless they are exchanging IS-IS PdUs (that are not IP packets) they don't know of each other


keepalives are L2 only but near to L1 used to understand if the interface can be seen as up/up (line protocol up)

so they cannot carry information like IP to MAC mapping


As soon as you try to ping the other side both switches can build an entry for the other one.


Hope to help

Giuseppe


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