we are using IPSec/GRE tunnels in a hub and spoke topology. As we have some problems related to fragmentation because of a small MTU we need to use the "ip tcp adjust-mss" command. We are a bit unsure where to use that command. We have found some documents that propose to use it on the external or the tunnel interface. For me it seems to be "natural" to configure it on the LAN interface. Who is correct ? Another question : Do we only need to configure that on routers connected to DSL lines ? In our main (hub) location our internet line has MTU 1500 Bytes. Do we need adjust-mss here as well, because all of that TCP connections end in that location ?
I had similar issues when using PPPoE on ADSL or wireless as the internet connection. I have resolved the issues by using a combination of the below commands on the tunnel interface.
ip tcp adjust-mss
NOTE: In my experince it is hard to tell an exact value as it seems it varies depending on the media and the extra headers been added as the packet gets encrypted, encapsulated ..etc ..etc but I started by changing the mtu size to small value ( i.e 1360 ) and then once the tunnel was up ..increasing it until the tunnel was not able to be establish .. the I took the last good one as its MTU limit. the mss size needed to be 40 bytes smaller that the MTU (i.e 1320 )
In my experience with over 200 implementation of IP GRE Tunnel over IPSEC or without IPSEC through multiple Telco services (DSL, MPLS, ATM, MetroE, etc..), I never use this command as I only use "ip mtu 1500" in the GRE Tunnel. I think its because when I first use GRE Tunnel in 2003, this command is not available or the IOS that I have doesn't have this command feature.
I'm amaze that a lot of people always recommend this command whenever someone complains of slow network throughput or some services unreachable, it is even recommend to put it in virtually any interface (LAN/WAN) and very low value at that.
I only change the GRE Tunnel mtu. I use route it in routers ranging from 800 to 7500 series with IOS 12.2 to 12.4
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