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

On 6/20/03 4:28 AM, "John Smith" <[email protected]> wrote:

> 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)

Which next-hop would you forward the packet to, in the case of multiple
next-hops reachable via a multi-access interface (eg ethernet)? Keep in mind
that unlike point-point interfaces you have to do a L2 rewrite that includes
a MAC address on multi-access interfaces.

>>> 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!

I am guessing you will advertise the prefix twice in BGP? BGP's best path
will make sure that only one of the labels will ever get used. (there are
exceptions to the rule in this case, of course).

> 
> Am i missing something?
> 
> Regards,
> John Smith 
> 
> 
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-- 
Aamer Akhter / [email protected]
NSITE - cisco Systems