[afnog] Problem bringing up MPLS on disimilar interfaces

[Mark Tinka]

On Saturday 20 September 2008 04:31:06 Scott Weeks wrote:

> Have you checked the MTU on the router interfaces
> themselves?  It sounds like a possible MTU mismatch.

Be careful when adjusting interface MTU parametres, as link 
state routing protocols such as OSPF and IS-IS require a 
common MTU value across links before adjacencies can be 
successfully established.

Not sure which routers you're using, but if it's Cisco, 
typically IOS requires that you flap the interface 
(shut/unshut) for any interface MTU changes to take effect 
(my experience in later code).

If it's Juniper, JunOS would implement the new MTU upon 
a 'commit' of the candidate configuration.

Keep this in mind to avoid melting your IGP domain.

side note: It is possible to implement an MTU value at the
           protocol level that would affect only said
	   protocol, while leaving any other protocols to
	   infer their working MTU value from the main interface
	   MTU setting.

  	   However, I'd recommend this be the exception than the
	   rule, depending on the existing complications of your
	   network that you're trying to get around. In otherwords,
	   it may add unnecessary complexity where it's not really
	   needed.

Cheers,

Mark.
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