From: spduo (frenzeus@streamyx.com)
Date: Thu May 24 2007 - 16:11:32 ART
Hi John,
no, icmp are definitely blocked from the border routers.
HTH.
-K
----- Original Message -----
From: "John Gibson" <johngibson1541@yahoo.com>
To: "spduo" <frenzeus@streamyx.com>; <ccielab@groupstudy.com>
Sent: Friday, May 25, 2007 2:10 AM
Subject: Re: path mtu discovery is considered legacy?
> Hi Kenny, do SPs typically prevent mtu discovery
> from outside ? (some filtering)
>
> I just keep getting the impression that SPs don't
> like ICMP from outside probing the internal.
>
> John
>
> --- spduo <frenzeus@streamyx.com> wrote:
>
>> 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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