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MPLS & Path MTU problems

  • From: Donn Lasher <dlasher@clearskynet.net>
  • Date: Thu, 21 Feb 2002 11:26:23 -0800
  • Resent-Date: Thu, 21 Feb 2002 15:27:51 -0500
  • To: mpls-ops@mplsrc.com
  • X-Sender: dlasher2@mail.clearskynet.net


I've read all the MPLS-OPS archives on this issue, and tried the fixes, and 
I'm still seeing a problem.

two 3620's running 12.2.6, hooked to a 3524 switch. Multiple MPLS/Dot1Q 
vlan's off both routers. When traffic passing through these routers from a 
customer to a site blocking PMTU discovery(nice long list any more), it all 
the traffice appears to the customer to be blackholed.

ip icmp debug shows:

Feb 20 22:39:11 X.X.X.X 5813: 3w5d: ICMP: dst (X.X.X.X) frag. needed and DF 
set unreachable sent to 216.221.138.92
Feb 20 22:39:12 X.X.X.X 5815: 3w5d: ICMP: dst (X.X.X.X) frag. needed and DF 
set unreachable sent to 216.221.138.92

At the moment I'm applying a route-map to clear df bit on my NSP connected 
router, but that's a hack. I'd really like to find and solve the real issue.

Ethernet MTU is 1500. tag-switching MTU unset (defaulting to 1500)
tag-switching ip set on all interfaces

I've tried setting the tag-switching mtu to 1540, making sure the switch 
was set mtu 2018, didn't make any difference.

Thanks in advance for any help.



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