[j-nsp] path-mtu-discovery

Harry Reynolds harry at juniper.net
Thu Oct 8 12:40:52 EDT 2009

You are correct. Its called mss (max segment size) in the show system connections, and it does not include tcp/ip OH. Note that we have some tcp noop and time-stamp options, so the tcp header tends to be > 20 bytes.

When pmtu is off you tend to see a mss of ~ 530 for off net, else it's the smallest value discovered in that direction. IIRC, bgp pmtu is off by default and has to be enabled. The default internet PMTU (internet-options path-mtu-discovery) is enabled by default. Docs indicated otherwise, and doc pr 75430 was opened to get that corrected.

Also, once we discover a reduced pmtu we do not ramp back up. The connection has to be cleared for ~7 minutes to completely age out previous pmtu state, and then we can discover a larger pmtu if things have changed.



-----Original Message-----
From: juniper-nsp-bounces at puck.nether.net [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Bit Gossip
Sent: Thursday, October 08, 2009 5:08 AM
To: juniper-nsp
Subject: [j-nsp] path-mtu-discovery

I guess that the effect of this command is to maintain a cache of all the active connection and for each of them assign the discovered value of the max mtu allowed accross the path.
At least the output of 'show system connections inet extensive'
doesn't show any trace of PMTU;
Anyidea of where I can find this information?

BTW: on a linux box I found that with the command:
'ip route show table cache'

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