[j-nsp] MPLS-in-MPLS mtu

[Amos Rosenboim]

I believe that the problem for this dilemma is fairly simple - the  
use of path-mtu-discovery allows you to use the maximum mtu size  
supported on the network.

Amos




On Apr 16, 2007, at 3:21 PM, Jared Mauch wrote:

> On Mon, Apr 16, 2007 at 06:26:38AM -0400, Jeff S Wheeler wrote:
>> On Mon, 2007-04-16 at 10:32 +0100, Alex wrote:
>>> Jared,
>>> Max BGP message size is 4096 bytes anyway and I cannot possibly  
>>> see how 9K
>>> MTU can further increase tcp performance in case of iBGP in  
>>> comparison with
>>> MTU of 4470.
>>> Rgds
>>> Alex
>>
>> It's sometimes foolish to run iBGP sessions with a TCP MSS > 536  
>> bytes.
>> This is the minimum allowed for IP networks, and there should  
>> never be a
>> router transporting your iBGP session which cannot forward TCP  
>> segments
>> conforming to this minimum MSS value.
>>
>> A topology change might happen and cause your iBGP session, which  
>> under
>> nominal conditions is transported over a 4470 MTU path, to pass  
>> across a
>> 1500 MTU link.  Now your TCP stack is sending packets larger than the
>> path MTU, which will either be fragmented or discarded.
>>
>> The minimum values are the safest ones for iBGP, in my opinion;  
>> and the
>> benefit to using any larger TCP MSS for iBGP is poor given the CPU  
>> time
>> your router spends in its TCP/IP stack vs other tasks like  
>> calculating
>> routes.  You could argue that converge time is improved if the  
>> iBGP TCP
>> session is more efficient, but that benefit goes away the first time
>> your operators page you at 4am to find out why an iBGP session flaps
>> when a topology change favoring a lower-MTU core link happens.  :-)
>
> 	If you don't have sufficent control over your network
> to know the underlying mtu of the links (be they l2vpn-type or
> otherwise carrier provided) you have some serious issues.
>
> 	There was a presentation years ago at IETF where Cisco showed
> the performance increases of enabling path mtu discovery and how this
> took those bgp message sizes grew from the cisco default to match your
> link mtu.
>
> 	I'm advocating a consistent internal MTU for your network, be that
> 1500, 4470, or something larger.  If the underlying transport does not
> support it and you are dealing with broken host stacks from your  
> vendors
> then you should discontinue using their equipment until they
> repair these critical defects.
>
> 	Scaling your bgp update messages to something larger than 500 bytes
> can have a significant win in route convergence as we're all carrying
> voip and other similar sensitive traffic on our networks (even if  
> we don't
> know what all that sensitive traffic is).
>
> 	- jared
>
> -- 
> Jared Mauch  | pgp key available via finger from jared at puck.nether.net
> clue++;      | http://puck.nether.net/~jared/  My statements are  
> only mine.
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