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arp

When "show arp" in cisco switch, the arp table only contain the information about neighboring switch but without any hosts, why?

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Hall of Fame Super Blue

Re: arp

Eric

Assuming HostA and HostB are in the same subnet.

Because hostA will arp out for the mac-address of hostB. So hostA broadcasts onto the local subnet syaing who owns this IP address ie. the IP address of host B.

Host B repsonds with it's mac-address to HostA. The switch if it is a pure layer 2 switch only cares about the mac-address and the port it came in on it and it should record that it in it's mac-address table.

Are you saying that you don't see the mac-address of HostB in the mac address table ?

Bear in mind that mac address entries do get timed out of the switch cache.

Jon

4 REPLIES

Re: arp

Because a L2 switch does not have IP connectivity with all hosts attached to the switch, only a L3 switch (a router) has all hosts in its arp table. A L2 switch normally only does an arp lookup of its own default gateway.

Use "show mac-address-table" to see all CAM entries of attached hosts.

"show arp" is an IP (layer 3) command.

New Member

Re: arp

Thanks for your reply.

I still have one concern.

If hostA of L2 switch want to send to another hostB of same L2 switch. How can hostA know mac-address of hostB which does not appear in mac-add-table?

Hall of Fame Super Blue

Re: arp

Eric

Assuming HostA and HostB are in the same subnet.

Because hostA will arp out for the mac-address of hostB. So hostA broadcasts onto the local subnet syaing who owns this IP address ie. the IP address of host B.

Host B repsonds with it's mac-address to HostA. The switch if it is a pure layer 2 switch only cares about the mac-address and the port it came in on it and it should record that it in it's mac-address table.

Are you saying that you don't see the mac-address of HostB in the mac address table ?

Bear in mind that mac address entries do get timed out of the switch cache.

Jon

New Member

Re: arp

Your message is helpful.

Thanks very much!

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