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

[James Jun]

On Mon, May 18, 2015 at 03:07:18PM +0000, Adam Vitkovsky wrote:

[ snip ]
 
> That's strange indeed I guess the card was added while the box was running right?
> I'm thinking whether restart of the chases would clear the problem.
> I'm also worried that once the card is removed the MTU will remain messed up.
> 

Yea, so it looks like MTU=1500 showing on control side of things is normal behavior to SY code on Sup2T (observed on 15.1(2)SY4 and 15.1(1)SY1) and appears to be cosmetic in nature.

Just tried to replicate this on another Cat6500 with Sup2T by configuring TE tunnels there and that too is showing MTU 1500 on any LSPs originated by it.  So it wasn't the line card insertion.

However, a workaround found was to simply configure 'ip mtu 9216' on the affected TE tunnel interface originated by the 6500 (despite warning about fragmentation because the box falsely thinks underlying transport MTU is 1500 when it clearly isn't).  After doing that, I was able to ping the other PE across the TE tunnel at size 9200 w/ df-bit set; and miraulously, overlayed LDP-over-TE sessions stopped flapping.  Been stable for 7 hours so far with traffic load (no signs of the workaround breaking cef forwarding either).


Thanks for the help!

James