MTU issues accross a Cisco 4000

[KY]

Hi,
I'm having problems getting a location to connect via vpn due to mtu problems through a Cisco 4000 core switch they are running. The location run some apps that work fine over the existing serial connection, but won't run over a gre vpn we are trying to install. The vpn portion of the config is fine & connected, and the mtu for the tunnel is set to 1400, so plenty of room for the GRE and ipsec headers. IP routing checks out as well. This is a config I have used many times with success, but never with a Cat4000 involved and I am convinced this is the issue. Does anyone have experience of this problem?

 

[BuckWeet]

It could be possible. I have heard of another application recently with the 4000's that doesn't work..

I can't for the life of me remember what that application was though..


BuckWeet

 

[KY]

I'm 100% it's mtu related, i've seen a few hints on the web about mtu related issues with the cat4000. I've just enabled path-mtu-discovery on the routing card, not sure if this will fix the problem though... any thoughts anyone???

 

[Cotton9]

Check out thread: Adding new GRE tunnel broke all others

Good information and resource link near end. We were having problems with Windows clinets logging in and Outlook over a tunnel. After using a sniffer found wondows was not responding to icmp reply to decrease packet size. Our quick work around was to increase mtu size on the routers. Will try the SYNC workaround in the above thread.

 

[ADB100]

Have you turned off ICMP unreachables on any interfaces? This is usually the cause as it prevents the routers informing the clients the packets are too big.

interface Ethernet0
ip unreachables

HTH

Andy