Date: Fri, 13 Oct 2000 13:53:43 -0400 (EDT) From: Larry Lile <lile@stdio.com> To: Nicolai Petri <nppsecure@swamp.dk> Cc: freebsd-tokenring@FreeBSD.ORG Subject: Re: MTU size! Message-ID: <Pine.BSF.4.05.10010131345320.91216-100000@heathers.stdio.com> In-Reply-To: <008301c0352d$2d471ed0$6732a8c0@atomic.dk>
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On Fri, 13 Oct 2000, Nicolai Petri wrote:
> Was is the correct MTU size for tokenring ??
>
> Cisco says :
>
> Default Media MTU Values
> Media Type Default MTU
> Ethernet 1500
> Serial 1500
> Token Ring 4464
I have never found a hard and fast default value, so I punted and
used 1500. It could have been worse, 576 seems to be very poplular
due to bridging.
I think that Cisco is probably following RFC 1042:
Given a token-holding time of 9 milliseconds and a 4
megabit/second ring, the maximum packet size possible is 4508
octets including all octets between the access control and the FCS
inclusive.
This allows 4508 - 36 (MAC header+trailer with 18 octet RIF) - 8
(LLC+SNAP header) = 4464 for the IP datagram (including the IP
header).
However, some current implementations are known to limit packets
to 2046 octets (allowing 2002 octets for IP). It is recommended
that all implementations support IP packets of at least 2002
octets.
> So should I change my MTU to 4464 on my TR interfaces ??
Give it a try and let us know how it works out.
--
Larry Lile
lile@stdio.com
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