Connection stops working - clear arp fixes it

Unanswered Question


I have a strange problem:

I have a connecton between 2 routers; Our router (a C1721) and an ISP (a C1812). The ISP router is at our location, and they use it to establish VPN connections to third party service providers.

Currently, we have 2 third party service providers through that link. They have one VLAN each, connected through a Trunk to the ISP router.

Following ip addresses has been assigned (in our router) to communicate over the trunk:

Our router (C1721):

interface Vlan910

ip address

interface Vlan911

ip address

At start, I can ping their counterparts on the ISP router; and, and the connections to the third party SP works fine.

But then, after some random time, I cannot ping and 5 anymore, and the connection to the SP's (of course) goes down.

But if I do a "clear arp" on our router, it starts to work again.. for a while.

I have checked the arp tables on both routers.

They look the same when it works and when it does not work:

Here are the tables:

Our router (C1721): - 000d.bd64.a8cf ARPA Vlan910 7 0023.5e80.c1c4 ARPA Vlan910 - 000d.bd64.a8cf ARPA Vlan911 0 0023.5e80.c1c4 ARPA Vlan911

ISP router (C1812) 0 000d.bd64.a8cf Vlan911 - 0023.5e80.c1c4 Vlan911 9 000d.bd64.a8cf Vlan910 - 0023.5e80.c1c4 Vlan910

I notice the MAC addresses are the same on both VLANS and both adresses.. can this be correct?

What could cause this error?

I found a workaround for it, by setting the arp timeout to 120 seconds on the interfaces, but that does of course not take away the underlying error.

Thank you.

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Giuseppe Larosa Thu, 10/08/2009 - 03:49

Hello Oystein,

>>I notice the MAC addresses are the same on both VLANS and both adresses.. can this be correct?

this is correct if the router uses two Vlan subinterfaces taken from the same physical interface like

int f0/0.900

enc dot1q 900

ip address ...

int f0/0.910

enc dot1q 910

ip address ...

So this shouldn't be the cause of your problem.

Verify if you are using IP next-hops with your static routes or not.

If you are not using them this can cause the router to build a very big ARP table that you clear.

Hope to help


Thanks for answering.

I'm not sure what you mean, but I don't use IP next-hop, just "regular" route entries. This is for VLAN 911:

ip route

ip route

This is a router with multiple IP NAT OUTSIDE interfaces, so I use route-maps to direct traffic properly.

Should I put an IP NEXT-HOP in there instead?

Shouldn't think routes has anything to do with it, since the traffic stop already at next hop, which is directly connected. That should never be unreachable, unless physically disconnected, right?

Anyway, the arp table on the router is not big at any time.

Right now there are only 26 entries in it.

Giuseppe Larosa Thu, 10/08/2009 - 05:07

Hello Oystein,

an IP nexthop is like in your static routes:

ip route >>>

using an interface as outgoing interface is like

ip route fas0/0.900

What you have done is good and as you say ARP table is never big.

You should see if ARP entries change over time when you have the issue, you have written that they look like the same.

Before doing clear arp take the arp table.

Hope to help



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