The MPLS WG Archive

[Jack Brennen <John.Brennen@marconi.com>]

Toni Jackob wrote:
> 1)
> 
> Topology:
> 
> LSR-A If_a1   1.2.1.1 ----------- If_b1 1.2.1.2 LSR-B
>        If_a2  1.2.2.1 -----------  If_b2  1.2.2.2
> 
> ·LDP adjacencies are created over the two parallel links between the two
> LSRs.
> ·One LDP session is established between the LSRs, which work in Global Label
> Space. The session is using the first link.
> ·LSR-B advertises a label for prefix 1.1.1.1/32 to LSR A.
> 
> My question is, does LSR-B should support the advertised label in HW level
> over both interfaces, or only over the interface that the TCP session is
> using?

Global Label Space implies that both interfaces must support the label.
If LSR-B sends Hello messages on multiple interfaces using the same
LDP Identifier, he is telling the world that those interfaces are "equivalent"
with respect to label forwarding.

As a clarification, the TCP session doesn't "use" any particular interface.
The packets which make up a TCP session between two LDP peers are not required
to follow any particular path, and in fact may even traverse interfaces which
are not running LDP.  The fact that the TCP session usually does traverse
an LDP interface which has a hello adjacency to the peer is a happy accident,
but is of no concern to the LDP protocol.

> 
> 2)
> When LSR needs to decide about to next hop of a FEC, does it take into
> account the next hop interface or only the next hop address?
> 
> For example, if the route to prefix 1.1.1.1/32 in LSR-A is:
> 1.1.1.1/32 -> If_a2 , 1.2.2.2
> and there is LDP adjacency only over the first link, does LDP in LSR-A can
> use the advertised label from LSR-B to prefix 1.1.1.1/32?

The next hop interface and the next hop address must both be taken into
account.  To use an LDP-advertised label to forward traffic, a sender must meet
these three requirements:

  * The label has been advertised by a peer on a currently valid session.
       (And has not been withdrawn or released)

  * The next hop address for the traffic has been received on that
    session through the use of an "Address" message.
       (And has not been withdrawn)

  * The next hop interface for the traffic must have a currently valid hello
    adjacency corresponding to that session.

To answer your question...  In the situation described, the third requirement
is not met; there is no hello adjacency on the next hop interface.  So the
traffic should be forwarded unlabeled.