[c-nsp] Sup2T MPLS-TE - Strange issue with MTU selection

[Mack McBride]

Sounds like there is a link somewhere in the path with 1500 MTU.
I would check everything in the path.
And verify the path is what you think it is.
Remember tunnel paths are not necessarily symmetrical.

Mack McBride | Network Architect | ViaWest, Inc.
O: 720.891.2502 | mack.mcbride at viawest.com | www.viawest.com | LinkedIn | Twitter | YouTube



-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of James Jun
Sent: Sunday, May 17, 2015 5:50 PM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] Sup2T MPLS-TE - Strange issue with MTU selection

Greetings!

I'm having a really strange problem with Sup2T running RSVP-TE over MPLS that's making me scratch my head.  The problem is, Sup2T on this box thinks that underlying transport MTU for the LSP TE tunnels is 1500, even though it's 9216 all the way across everywhere (wtf!).  It used to work perfectly fine until recently, about a day after we inserted a WS-X6704-10GE card with 6700 CFC.  Anybody had similar problems before?

The underlying transport (core-facing) interfaces are Ten1/4 & Ten2/1, each set with MTU=9216.  But, when TE tunnels come up, the output of 'show mpls interface detail' lists the tunnel as having only 1500 bytes MTU.  However, other PE routers pointing back at this 6500 properly show MTU=9216 on their LSPs, as they should.

This problem is causing LSPs to fail at times and the overlayed LDP-over-TE sessions to bounce repeatedly due to hold-timer expiration.  Additionally, I can't even ping the remote side PE's loopback at any size greater than 1500 w/ df-bit set.

And yes, I've verified that jumboframe actually works -- not only could I ping adjacent neighbor IPs at full 9100+ size with df set, OSPF also works without any retransmissions.  Likewise, if I take the remote side PE's loopback IP and static-route it (so it goes over native IP instead of MPLS), it will ping at size 9000.  But anytime it encapsulates over the TE tunnel, it can't transmit at sizes bigger than 1500 bytes.

Any ideas on why Sup2T is fooled into thinking that underlying transport tunnel MTU is only 1500 for the MPLS-TE tunnels?


The box is running s2t54-adventk9-m 15.1(2)SY4, with following:

Slot 1:  VS-S2T-10G
Slot 2:  WS-X6704-10GE w/ F6700-CFC version 4.1 Slot 3:  WS-X6724-SFP w/ F6700-CFC version 3.0 System PFC operating mode is: PFC4 (both legacy X6700 LCs have CFC)

This is a standalone E chassis, without VSS.

Many thanks in advance.

Best,
james

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