From: spduo (frenzeus@streamyx.com)
Date: Thu May 24 2007 - 12:26:33 ART
Hi John,
it is still very much used in bgp environment AFAIK, especially in SP
environment. As BGP uses TCP, and by default every TCP session has a limit
in terms of how much data it can transport in a single packet, whilst this
limit also known as the Maximum Segment Size (MSS) on the IOS routers is 536
bytes only. When using such a small value, it creates a large amount of
tcp/ip overhead, and especially when tcp carries a lot of data when bgp is
using it! tcp will take all the big chunks of data and break it up into 536
bytes chunks, by default.
With tcp path mtu discovery, it allows tcp to determine the smallest mtu
size among all links between the ends of a tcp session, i.e. ur ibgp
neighbor peering btw PE-PE in MPLSVPN. Usually there are multiple hops (P
routers) in between the 2 PE routers, and hence the need to enable "ip tcp
path-mtu-discovery".
do a "show ip bgp nei | i max data" before and after you configure "ip tcp
path-mtu-discovery" and u'll see wut i mean. the values displayed, after u
configure this.. is actually ur smallest mtu along the path of ur ibgp
session, minus of ip and tcp headers.
HTH.
Kenny
----- Original Message -----
From: <johngibson1541@yahoo.com>
To: <ccielab@groupstudy.com>
Sent: Thursday, May 24, 2007 6:04 AM
Subject: path mtu discovery is considered legacy?
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