[c-nsp] transport path-mtu-discovery - ME3600....too unpredictable to use?

[CiscoNSP List]

Cheers Nic - Yes, it is an annoyance (With the third party/carriers variable MTU sizes)...I like to run biggest mtu I can across all the links we "own"...i.e. typically 9100, but then on the carrier/interpop links, have to hard set MTU to what they "say" they guarantee....we have been bitten on more than one occasion by this guarantee.


________________________________________
From: Nick Hilliard <nick at foobar.org>
Sent: Friday, 26 February 2016 9:55 AM
To: CiscoNSP List
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] transport path-mtu-discovery - ME3600....too unpredictable to use?

Your safest approach is:

> Your transport providers need to guarantee you a specific MTU and you
> need to configure your router L3 interfaces with that number.

If you only set your router tcp mss, that will work fine for BGP
sessions but will not fragment transport data packets properly.  You
have a hard requirement here to set the interface MTU to whatever the
minimum is that the provider can guarantee.

Nick

CiscoNSP List wrote:
> Thanks for all the replies - Given our circumstances, I think the "safest/sanest" approach would be to disable PMTUD, and enable "ip tcp mss xxx" (Maybe 1300 or 1400).
>
> Cheers
>
> ________________________________________
> From: Nick Hilliard <nick at foobar.org>
> Sent: Thursday, 25 February 2016 6:35 AM
> To: CiscoNSP List
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] transport path-mtu-discovery - ME3600....too unpredictable to use?
>
> CiscoNSP List wrote:
>> Quick synopsis of our network, multiple pops, all connected via
>> various 3rd party carriers, who all use differing MTUs, that can also
>> "change" unexpectedly(Unavoidable unfortunately!)
>
> MTU means _maximum_ transport unit, so if your underlying transport
> network unexpectedly changes this so that the max transport packet size
> decreases below the configured MTU on the layer 3 side, then you are
> going to drop packets, end of story.
>
> Your transport providers need to guarantee you a specific MTU and you
> need to configure your router L3 interfaces with that number.
>
> Nick
>
>