[afnog] MTU Size for transit links for ISPs

[Perreau, Luc]

Dear all,

Many thanks for the enrichment on MTU knowledge :)

I have asked my upstream and they use 1514 on my Ethernet links and I was asked to set 4500 on POS links.

As you all rightly pointed out, MTU will not cause this issue.

After intense troubleshooting over a few long days, we have found that our MUX though having 4 Ethernet ports, two of them have larger buffer sizes than the other two.

We connected one link on a large buffer size port, and the other on a smaller buffer size link. Using EIGRP with a variance of 2, we got very unstable performance.

The minute we corrected that, the links started performing as expected!

Thanks again.

Kind regards,

Luc Perreau



-----Original Message-----
From: Randy Bush [mailto:randy at psg.com] 
Sent: Sunday, September 22, 2013 3:03 AM
To: Mark Tinka
Cc: Perreau, Luc; Nishal Goburdhan; afnog at afnog.org
Subject: Re: [afnog] MTU Size for transit links for ISPs

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> In general, the MTU across IP Transit links (i.e., between your and 
> your upstream's border routers, or between your peering router and 
> those of other networks) is about 1,500 bytes.
> 
> The MTU within your core (or core of the network your connected to) 
> would, these days (although not always), be larger than what is 
> supported across the peering link.

if we assume s/peering/external/.  and for most networks, is not the vast majority of the traffic external.  thus, will the above advice not cause your border routers to have to fragment all outbound traffic?

it might be good to negotiate higher mtus with external links.

randy
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