The MPLS WG Archive

[Bob Thomas <rhthomas@cisco.com>]

Santosh,

> Hello Bob
>     I have gone thru the section but still have a doubt. Even in DownStream
> unsolicited Mode of label distribution, according to the draft each LSR
> keeps a list of address prefix and their associated  (LDP Identifier, label)
> 
> LDP Identifier = Interface IP Address : Label Space.

There is no guarantee/requirement that the first 4 octects of the
LDP Id is the IP address of the "Interface".


> Actually the Next hop address in the routing table will exactly match the
> "Interface IP Address" specified in the LDP Identifier. This "Interface IP

I think you cannot guarantee that this is always true.


> Address" is the one directly connected to the peer.
> This is depicted below.
> 
>                                             C
>                                       /
>                                 A   -----  | B |
>                                       \
>                                             D
> 
> If B,C,D are directly connected to A then the "Interface IP Address" that
> will be specified in the Address Message will be the same as the Interface
> which A concludes from the HELLO message exchange. So what is the need for a
> seperate "Address Message" altogether ?

Please see comments above.


Bob


> One more thing, what is the use of specifying some other "Interace IP
> Address" to A by B (say the interface of B with C ) if that is not the
> directly connected interface to A?
> 
> 
> > Santosh,
> >
> > >     In draft-ietf-mpls-ldp-06.txt, there is an Address Message and =
> > > Address Withdraw Message. I am not able to figure out what purpose does
> =
> > > it serve. In the Hello packet that an LSR sends, it can send the =
> > > Interface IP Address optionally. If it doesnt send then the receiving =
> > > LSR can make out when it receives from the particular socket. And what =
> > > do we gain if an LSR advertizes IP Addresses of other interfaces also ?
> =
> > > The connection between 2 LSRs is anyway thru some particular interface =
> > > and knowing the IP Address of other interfaces will be of no use.
> >
> > See Section "LDP Identifiers and Next Hop Addresses".
> >
> > Bob
> >
> 
>