switch arp tables

Unanswered Question
Apr 13th, 2007

Hi all, can anyone tell me, when a pc makes an arp reqauest and the device is not on the local switch, does the switch cache every mac address thats not local on its uplink port ?

I have this problem too.
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m.sir Fri, 04/13/2007 - 03:31

Yes you are right

the switch is listening to incoming packets and cache MAC addresses of hosts behind the port so if you have uplink to another switch where are 24 hosts active you have 24 entries in mac-address-table for this port

Check output show mac-address-table of my 2950 IC Fa0/2

xx 0013.21f5.9bcd DYNAM for port where i have another switch connected (with few host active)

2950#sh mac-address-table dynamic interface fastEthernet 0/2

Mac Address Table


Vlan Mac Address Type Ports

---- ----------- -------- -----

xx 000d.9dd0.119e DYNAMIC Fa0/2

xx 000f.2039.0225 DYNAMIC Fa0/2

xx 000f.2039.7724 DYNAMIC Fa0/2

xx 000f.2039.e349 DYNAMIC Fa0/2

xx 000f.203a.a1b0 DYNAMIC Fa0/2

xx 000f.206e.7ab4 DYNAMIC Fa0/2

xx 0011.85f4.1972 DYNAMIC Fa0/2

xx 0013.21f5.9b7d DYNAMIC Fa0/2

xx 0013.21f7.4504 DYNAMIC Fa0/2

xx 0013.7f66.5f32 DYNAMIC Fa0/2


hope that helps rate if it does

Newport_s Tue, 04/17/2007 - 13:51

I think you're talking ARP here, not CAM right? If that's the case the switch only arp's addresses local to the subnet (or VLAN) of it's management interface. If you're PC is on the same subnet (or VLAN) as the switch's management interface the MAC address of the destination will be in the switche's arp table. If the PC's on a different subnet the destination MAC will not appear in the arp table.

dteichmer Tue, 04/17/2007 - 19:42

Actually, if the device is in the same layer 2 domain (read as VLAN) then the switch will record the MAC address of the device once the device sends any frame accross a switch port, uplink or not. Most switches will cache an MAC entry in the CAM as a learn event from the source MAC address of the frame. It stores it in the CAM as an entry for the VLAN and port it was learned on. In your scenario the ARP reply would come into the switch on the uplink port sourced from the replying device. The source MAC address of the frame (from the device replying) is stored as learned on port x in VLAN y.

As a side note, if the ARP request goes through a router providing proxy ARP service then the MAC of the router will be learned on the port since it will use its MAC address as the source in the ARP reply.


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