Re: mpls doubts

Marko Milivojevic @ 20/05/2011 14:11 -0300 dixit:
>> 1) what is ment by ldp - igp synchornisation . ? by default its disabled
>> ....what benifits i will get if i enable it ?
>
> I think it may be beneficial for you to take a look at this document here:
> http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/fsldpsyn.html
>
> If you have any doubts left after that, let's follow it up here.

A small description of a situation leading to the problem may help.
If you have an MPLS vpn, say CE1-PE1-P-PE2-CE2, and there is a
problem with the P-PE2 LDP session, it will not impact by default
the routing between CEs (i.e. routes are going to be exchanged).
But the PE1-PE2 LSP is broken, and all traffic from CE1 to CE2
will be blackholed.

If there were an alternative path, it might not get used either!
Synchronization avoids this.

>
>> 2) in the follwoing output outgoing actiion is Aggregate ...what does it
>> imply ? i have not done summarization any where ....
>
> It's an Aggregate label. In almost all cases, it indicates an MPLS
> VPN, but it doesn't have to be (in your case it is).
>
>> 3) why is it required that two PE must use loopback interface for BGP
>> sessions ? why it is required that these loopback must be /32 route ?
>
> It isn't required. However, if you are not using Loopbacks for
> peering, or you are not using /32 masks, you need to understand the
> implications. Not using /32 Loopback mask is kind of an issue with
> OSPF that you may need to address. If you're not using Loopbacks, you
> can use whatever interface you like, except the downstream interface
> to your PE peer. Take a look at this:
> http://blog.ipexpert.com/2010/05/31/next-hop-in-mpls-vpns/

That is a great example of what goes bad.
I whould just add the primary reason, in my perspective: PHP.
The way it works is that a router sends "pop tag" when it is connected
to a route/prefix.
The evil here is any P router being "connected" to the next hop address.
That's why /32 next hops drive you safe, IMHO.

-Carlos
>
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-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net