The MPLS WG Archive

[Bob Thomas <rhthomas@cisco.com>]

Jim,

> My understanding is that the list of addresses messages given to me by a
> peer will allow me to answer the question if this peer "is the nexthop" for
> a particular route.  This allow me to know whether or not I should install
> a mapping the peer has given me.  What if ... the address that I match
> (which was given to me by the peer) is from a differnt labelspace then the
> label he has allocated to me?  Maybe a picture will help.
> 
> 
>    ---------                 ---------
>   |         |A11---------A21|         |
>   |   R1    |               |    R2   |------ FEC
>   |         |A12---------A22|         |
>    ---------                 ---------
> 
> -A21 means R2's 1st address.
> -each interface has a labelspace that matches the address number
>  (thus R2 has 2 labelspaces it allocates from 21,22)
> -R1 and R2 have two LDP sessions
> -R1 session with R2:21 has A22,A21 for an address list
> -R1 session with R2:22 has A21,A22 for an address list
> 
> -R1 has a routing table of:
>  Prefix     outif      nexthop
>  FEC        A11        A21
> 
> R2:22 allocates a label(L1) in labelspace 22 and send the mapping to R1.
> R1 gets the mapping and searches the address list for R2:22 and finds that
> the next hop of A21 is in R2:22 address list, so it installs the label out
> A12 for FEC.
> 
> -R1 now has a FTN that looks like:
>  Prefix     outif      nexthop   label
>  FEC        A12        A22       L1
> 
> I can see avoiding this problem if the check for "is the nexthop" also takes
> into consideration the outgoing if.  How would that work in targeted mode?
> 
> I think this can be solved for both modes by limiting the address messages 
> R2:22 gave to R1. Why not only give out the addresses of the interfaces that
> match the labelspace you are advertising?

I think you are making this more complicated that it need be.

The purpose of the Address Message is to make it possible for an LSR
to establish a mapping between the first 4 octets of a peer's LDP Id
and addresses bound to the peer.

This mapping is useful in determining whether the peer is the next
hop for a particular prefix.

Bob