[c-nsp] Layer 2 VLAN advice..

Mark Tinka mtinka at globaltransit.net
Wed Feb 10 09:45:52 EST 2010

On Thursday 04 February 2010 05:11:49 am Peter Kranz wrote:

> So in terms of enabling MPLS on a fully meshed set of
>  routers running BGP and OSPF..
> Here are the general steps I believe;
> #conf t
> Tag-switching advertise-tags
> !
> Int g0/0
> 	Mtu 9216
> 	Tag-switching ip
> !

Be very careful here - changing the interface MTU would 
bring down OSPF as adjacencies with other OSPF speakers 
depend on the link MTU being the same for both sides.

However, yes, MPLS needs larger-than-default Ethernet MTU's 
to work, so 9,000 bytes is good. To guard against dropping 
your OSPF adjacencies, set 'ip mtu 15000' so that OSFP can 
continue to use 1,500 bytes while all other protocols 
(including MPLS) use 9,000.

You can then regularize this setup once your MPLS turn-up is 


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