The MPLS-OPS Archive

["Mark Lewis" <markl11@hotmail.com>]

Just to add to what Martin has said (you have to modify the mtu on Ethernet 
i/fs using the tag-switching mtu command (that is if you want support for 
large frames + label stack!)), make sure that the 3com and Cisco switches in 
your environment can support baby giant frames. You can often add this 
support - eg. on the Cisco 6000 platform use the 'mtu' command.

Hope that helps,

Mark Lewis
Frontrunner Solutions

P.S. In a MPLS VPN environment, the mtu on the Ethernet i/f should be 1508 
(1500 + two label stack). In a simple MPLS environment (no VPNs), set it to 
1504 (one label).


>From: "Martin Picard" <mpicard@sinc.ca>
>To: "Venko Moyankov" <venkom@mobikom.com>, <mpls-ops@mplsrc.com>
>Subject: Re: tag-switching and MTU on c7206
>Date: Wed, 25 Apr 2001 13:55:18 -0400
>
>Venko,
>
>If you don't configure the tag mtu on the interfaces, it will default
>to whatever default is on the interface....if this is an Ethernet, F/E
>or G/E...it'll be 1500.....and that's accounting for the labels as well !!!
>Therefore if you want to have 1500-byte packets sent across an MPLS
>network, you have to set the tag mtu to 1500+(4*Nb-labels-expected).
>
>mp
>
>----- Message d'origine -----
>De : "Venko Moyankov" <venkom@mobikom.com>
>A : <mpls-ops@mplsrc.com>
>Envoye : 25 avril, 2001 13:34
>Objet : tag-switching and MTU on c7206
>
>
> > Hi all,
> >
> > Does anyone have the follwing problem with tag-swiching. I couldn't find
> > anything related in mail archieve.
> >
> > I started tag-swiching between two cisco 7206 VXR (IOS 12.1(6)E and
> > 12.1(5)E JS). The routers are connected to each other with FE interfaces
> > via 3Com and cisco switches. It seems that there is problem with MTU
> > path discovery if traffic goes between these two routers and
> > tag-witching is enabled on FE interfaces. Some web servers are
> > inaccessible e.g. mail.yahoo.com. Ping and traceroutes are ok, but http
> > with large pages doesnt work.  When I stop tag-switching on FE
> > interfaces everything is working fine.
> >
> > I know that LDP (TDP) calculates minimum MTU over the LSP and if packet
> > is lager, it is fragmented before entering MPLS domain. So I shoudn't
> > care about actual MTU on interfaces in MPLS core and labels size.
> >
> > Do you know if it is a IOS bug, or I have to configure differen MTU,
> > when take into account interfaces MTU and label stack?
> >
> > And one more thing: when I change tag-switching mtu on FE interfaces to
> > 1540 this problem seems disappear, but I really don't think it is
> > correct
> > to manualy set MTU on LSP.
> >
> > Thanks,
> > Venko Moyankov
> >
> > -------
> > The MPLS-OPS Mailing List
> > Subscribe/Unsubscribe:  http://www.mplsrc.com/mplsops.shtml
> > Archive: http://www.mplsrc.com/mpls-ops_archive.shtml
>
>-------
>The MPLS-OPS Mailing List
>Subscribe/Unsubscribe:  http://www.mplsrc.com/mplsops.shtml
>Archive: http://www.mplsrc.com/mpls-ops_archive.shtml

_________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

-------
The MPLS-OPS Mailing List
Subscribe/Unsubscribe:  http://www.mplsrc.com/mplsops.shtml
Archive: http://www.mplsrc.com/mpls-ops_archive.shtml