[nsp] checkpoint & gre tunnel

Streiner, Justin streiner at stargate.net
Tue Aug 5 09:33:25 EDT 2003

On Tue, 5 Aug 2003, Ali Kemal Yurtseven wrote:

> 	Hi,
> 	I have a problem with vpn over gre tunnel. Topology is given
> below:
> 	Checkpoint-> Router ------- GRE TUNNEL ------- Router ->
> Checkpoint
> 	VPN is configured between the firewalls. Ping tests are fine,
> but when a user tries to open a web page , problem occurs. Has anybody
> experienced a problem like this ?

Sounds like an MTU/fragmentation issue.  I ran into this with a product I
was testing recently.  I was able to fix it by using the "ip tcp
adjust-mss" interface command on the client-side router, like this:

interface Serial0/0
 description T1 to ISP POP
 ip address
interface Tunnel0
 description GRE Tunnel to ISP FW appliance
 ip address
 ip mtu 1476
 ip route-cache flow
 no ip split-horizon
 ip tcp adjust-mss 1436
 keepalive 10 2
 tunnel source Ethernet0/0
 tunnel destination
ip route

I tried other methods of resolving the issue like writing a route-map to
forcibly clear the DF bit on outgoing packets on both sides, with mixed
results.  Adjusting the TCP maximum segment size (MSS) has been problem
free for me for several months.

Note that the "ip tcp adjust-mss" and "keepalive" commands require fairly
new IOS builds to work, like later 12.2S and 12.2T releases.


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