[c-nsp] MTU issue on a GRE tunnel

[Jan Gregor]

Hi,

mss should be mtu - 20 - 20 (IP and TCP headers). So try using "ip tcp
adjust-mss 1436".

Best regards,

Jan

On 5. 5. 2010 1:18, ML wrote:
> I setup a GRE tunnel between two locations served by a Comcast business
> class cable modem.
> 
> Config example. The remote end is the same.
> 
> Tunnel73
>  ip address yy.yy.yy.yy 255.255.255.252
>  ip mtu 1476
>  ip tcp adjust-mss 1460
>  tunnel source xxxxx
>  tunnel destination zzzzzzzzz
>  tunnel path-mtu-discovery
> 
> 
> The two tunnel endpoints are ME3400s.  I expected that this
> configuration would reduce the quantity of fragmentations needed by the
> router but looks like "IP Input" was up to around 20%.  The user
> experience was bad since most web pages would fail to load.  I thought
> "ip tcp adjust-mss" would signal help signal the users OSs to act
> appropriately given the situation.
> 
> Have I missed something obvious?
> 
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