Devices on same VLAN lost connectivity

Unanswered Question

We have a network with four 3750's at the core doing HSRP for several VLANS and running OSPF. There are trunk ports from these devices connecting other swithes in various buildings on the campus. All of our servers and some outlying users are on VLAN5. Periodically our outlying users on this VLAN will lose connectivity to the servers. There are no obvious log entries to indicate any problems with OSPF (the routes to the gateway are in the routing tables), HSRP or the trunks. This morning when this happened, we did a shut/no shut the user port and connectivity came back up.

Does any body have an indication of what could be causing this temporary loss of connectivity? It seems to only happen with end devices in the same VLAN as the servers going across the trunk ports - users in other VLANS don't seem to have this issue.

Thank you,


I have this problem too.
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jasonmcl Tue, 05/27/2008 - 12:20


If your users are in the same vlan as the server then you never go to layer three. your trunks it would seem are probably forwarding all vlans so it may be an arp issue. How are the hosts/servers assigned IP's. It could be that there is a duplicate IP on the network. Can the host ping the gateway for its network (Vlan5) when it can no longer talk to the servers? Can it ping all the hsrp ip's. I doubt it is layer three but pinging gateways will tell you that. Let me know what you find.

jasonmcl Tue, 05/27/2008 - 13:17

Humm.. If the hosts can ping their gateway and they are on the same subnet/vlan as the servers then I would guess it is a cam/mac-address table issue. How are the hosts/servers assigned IP's. a careful inspection of the mac and arp tables will show you why you are losing connectivity. Instead of doing a shut/no shut on the hosts interface try a "clear mac-address dy" then try your pins again. if still not working try your "clear arp" and see if that helps let me know what happens.


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