RE: [MPLS-OPS]: Nos of Labels in CE

John,

If both the next hops are available on the same interface (ethernet as you
mentioned), then you need to have a distinct mac address for each of the
next hop.

Either you have one label per route and associate the next hop L2
information (usually the destination mac address) with the label and save it
in the lookup table OR you pop the label and do a lookup on the IP DA to
figure out the next hop that you can ARP (assuming it is still ethernet) to
get the next hop L2 information.

However, if you have point to point link (say PPP) as the outgoing
interface, then it might make some sort of sense to associate label binding
for IP routes based on outgoing links. But if you do that you lost your
ability to load balance unless you can figure out a way to associate
multiple label bindings for the same IP routes emanating from different
interfaces.

Vinay Bannai
Luminous Networks

-----Original Message-----
From: John Smith [mailto:[email protected]]
Sent: Friday, June 20, 2003 1:29 AM
To: [email protected]
Cc: [email protected]
Subject: Re: [MPLS-OPS]: Nos of Labels in CE


Aamer,

> > WHat is that implementations generally do when they have multiple
prefixes
> > associated
> > with a single interface.
>
> I think you want to say same next-hop rather than interface.

No, I mean the same interface. What difference does it make to me if i have
a prefix
which has different next-hops and both the next-hops are reachable to me
thru the same
interface/sub-interface (e.g. ethernet)

>
> >DO they advertise a single label for all these routes
> > OR does it
> > advertise multiple unique labels per route?
> >
> > I think it should be the former.
> >
> > Is there any case when one might want to do the latter?
>
> In general, cases where it might be useful to be able to distinguish
between
> prefixes.
>
> One specific case is in mpls-vpn if the prefix is reachable via two
> different  nexthop's on the same PE. This allows the PE to loadshare
between
> the two next-hops.

PE can only load share if both the next-hops are accessable through
different interfaces.
Thus in this case too, the PE will be advertisiing differnet labels per
interface. IMHO
the label should be associated with an interface. So that when this PE
receives a MPLS
packet with this label, it does nothing but pushes the native IPv4 packet
out on the
associated interface!

Am i missing something?

Regards,
John Smith


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