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tag-switching and MTU on c7206

  • From: Venko Moyankov <venkom@mobikom.com>
  • Date: Wed, 25 Apr 2001 20:34:31 +0300
  • Organization: MOBIKOM
  • Resent-Date: Wed, 25 Apr 2001 15:02:22 -0400
  • To: mpls-ops@mplsrc.com

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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