problem 2950 arp address not mapped to a destination port

[ppaki]

I have a problem loosing connectivity to some switch connected directly to my 2950.

I have the mac address in the arp table but when not working the mac-address is not linked to any port in the show mac-address table.

Please help

[Roberto Salazar]

mac-address-table is built by the switch when that host sends a packet to through the switch. So, when that host stops sending packet the switch purges the mac-address-table after 300 secs. The question will be why is the host stops sending packet or stop responding, so that host needs to be troubleshot. Is this happening on a single host or all the host connected to the switch? Maybe a WOL issue with the NIC? The switch cannot learn a mac address unless that host sends a packet.

Please rate all posts.

[Kevin Dorrell]

I think this could be simply a question of aging. The ARP tables are aged out after 4 hours. By contrast, the entries in the MAC forwarding tables are aged out after 5 minutes. All it means is that the host has not sent any frames to or through this particular switch in the last 5 minutes.

There can be other explanations, but they are rare. For example, if your host is actually a service address in a WLBS cluster configured in unicast mode, then the host will source its frames from a different address from the address it gives in response to an ARP request.

Another is if you have port security configured with a timeout shorter than your ARP table. You may have reached the maximum permitted MAC addresses. That would also account for your intermittent connectivity.

But, as I said, these are very particular cases.

Kevin Dorrell

Luxembourg

[Roberto Salazar]

the mac-address-table should get re-populated even after the 300 sec agingtimeout if there is a packet destined to that host and that host responds. the key is the host sending packet towards the switch in order for the switch to populate/re-populate the mac-address-table. No traffic incoming, no mac-address learned.