The MPLS-OPS Archive

["Martin Picard" <mpicard@sinc.ca>]

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
> 
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