08-02-2007 08:34 AM - edited 03-09-2019 06:31 PM
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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08-02-2007 09:38 PM
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
08-02-2007 09:59 AM
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.
08-02-2007 06:56 PM
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?
08-02-2007 09:38 PM
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
08-02-2007 09:49 PM
Your message is helpful.
Thanks very much!
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