GRE Tunnel MTU

[gerardor884]

hello 

what would be the equivalent to this ip tcp adjust-mss 1360 ??? i cannot put in this command and the closest  to it are these options

SLC_SW_3850_Stack(config-if)#ip tcp ?
compression-connections Maximum number of compressed connections
header-compression Enable TCP header compression

[Georg Pauwen]

Hello,

I assume this is a tunnel interface you are trying to configure ? I don't think that command is available on the 3850...

[gaston.benitez]

Hi Can you share the show version?

BR

Gaston

[Joseph W. Doherty]

As Georg notes, the TCP adjust command might not be provided on a 3850 (if the GRE tunnel is hosted on the 3850, that's a recent feature to the Catalyst 3K series, as I recall earlier 3Ks (e.g. 3650/3750) didn't support them).

BTW, for just a GRE tunnel, you don't need to set the MSS to 1360, you only need to set the MSS to 28 bytes less.

[Richard Burts]

I read something recently that indicates that 3850 has added support for GRE tunnels. I can not confirm whether that is accurate, but believe that it well may be.

The original post asks about equivalent commands. As far as I know there is not really an equivalent command. The neat thing about adjust-mss is that the router has an opportunity during the negotiation between the remote end points to affect their choice of mss. And this eliminates issues with fragmentation. Probably the closest to an equivalent is ip mtu applied to the tunnel interface. This would help if devices did Path MTU Discovery. But it still leaves open the possibility that the end points will send packets that are large enough to require fragmentation.

HTH

Rick

[Joseph W. Doherty]

An additional note to Rick's mention of setting IP MTU on tunnel interface, for two way traffic, this command needs to be applied to both ends of the tunnel, but the IP TCP MSS-ADJUST command, I recall, only needs be applied anywhere in-line of the two way traffic.

[Richard Burts]

Joseph makes a good point about needing to configure adjust-mss on interfaces at both ends of the tunnel while adjust-mss only needs to be configured on a single side. The difference reflects the fact that adjust-mss affects the negotiation between end points of the tcp session and means that neither side will send a tcp packet that requires fragmentation. But setting mtu affects individual packets as they enter the tunnel and so needs to be implemented on both ends of the tunnel.

HTH

Rick

[Joseph W. Doherty]

". . . makes a good point about needing to configure adjust-mss on interfaces at both ends of the tunnel while adjust-mss only needs to be configured on a single side. "

Rick, I beleive you meant to write: ". . . makes a good point about needing to configure IP MTU on interfaces at both ends of the tunnel while adjust-mss only needs to be configured on a single side. "

Also, as Rick notes, the IP TCP MSS-ADJUST needs to "see" the TCP startup, so it doesn't work on existing flows that switch into a path that has this command.  (This is worth understanding, especially in situations where the tunnel is a backup path, that might accept existing flows that "fail-over" to it.)

[Richard Burts]

Joseph

Thank you for spotting my mis-statement. Indeed I did intend to say IP mtu but somehow typed adjust-mss. 

HTH

Rick