The MPLS WG Archive

[Pedro Roque Marques <roque@juniper.net>]

Roger Pottier writes:

> Hi, If i have "send label" configured for a BGP peer, do the peers
> need to be on directly connected interfaces?

While details may differ between implementations, BGP SAFI=4, which is
what i assume you are refering to, does not require a directly
connected interface.

It can be used over a directly connected interface, in case in which
it is the top label, or over a non-directly connected session, case in
which the intra-area signaling protocol manages the top.

> If it is not, how would
> the MPLS switches in between know about the labels?

      link
[AS1]      [AS2			      ]
ASBR1 ---- ASBR2 --- P1 --- P2 --- PE1

===> direction of routing advertisements

asbr1 advertises 10.0.1.1/32	label 10000	via directly connected
						ebgp session to asbr2

asbr2 advertises 10.0.1.1/32	label 20000	via ibgp safi=4
						session to PE1

PE1 needs LDP/RSVP path to asbr2, w/ hop-by-hop labels.
assume P2 advertises label 30000 for the ASBR2 FEC.

PE1 forwarding state:
	10.1.1.1/32	push	(20000, 30000)....

>  And if it is on
> directly connected interfaces, does it allocate a label from the
> interface label spaceof that interface?

implementation dependent.

I'm not sure that this is the question that you want to ask also...

Some implementations use only a platform space but restrict allocated
labels so that they can be used only in a subset of
interfaces (where they are announced).

You can consider this to be 'interface label space' or not. The
important property is that a CoC customer interface cannot use labels
which have not been assigned to him.
 
>  I guess, for carrier's Carrier case, the CEs need to be on directly
> connected interfaces with the PE.
 
That is the most common model. However the statement "need to be" is
not acurate. One could configure it as a multihop session... i'm sure
somebody will find a good reason why one of this days. Lots of
creative people out there ;-)

  Pedro.