The MPLS WG Archive

[Anup Anil Cheruvathoor <acheru@ficon-tech.com>]

Hi,

    Typically the aggregation of routes are configured through policies, to
reduce the volume of routing updates or some administrative reasons.
    So the routing protocol which is advertising a agregated route has
information that the it is the point of deaggregation. So when a packet with
FEC say P and it matches to an advertised aggrgated route then the LSR knows
that it has to perform deaggregation.

Regardx,
-Cheru




Abhijit wrote:

> Hi all
>
> In draft-ietf-mpls-arch-06.txt , in section 4.1.3, the example given says
> that 4.1.3. Using the Hop by Hop path as the LSP
>
> Following lines are included from the draft
> ----->
>    If the hop-by-hop path that packet P needs to follow is <R1, ...,
>    Rn>, then <R1, ..., Rn> can be an LSP as long as:
>
>       1. there is a single address prefix X, such that, for all i,
>          1<=i<n, X is the longest match in Ri's routing table for P's
>          destination address;
>
>       2. for all i, 1<i<n, Ri has assigned a label to X and distributed
>          that label to R[i-1].
>
>    Note that a packet's LSP can extend only until it encounters a router
>    whose forwarding tables have a longer best match address prefix for
>    the packet's destination address. At that point, the LSP must end and
>    the best match algorithm must be performed again.
>
>    Suppose, for example, that packet P, with destination address
>    10.2.153.178 needs to go from R1 to R2 to R3.  Suppose also that R2
>    advertises address prefix 10.2/16 to R1, but R3 advertises
>    10.2.153/23, 10.2.154/23, and 10.2/16 to R2.  That is, R2 is
>    advertising an "aggregated route" to R1.  In this situation, packet P
>    can be label Switched until it reaches R2, but since R2 has performed
>    route aggregation, it must execute the best match algorithm to find
>    P's FEC.
> <------------
>
> My question is R3 in this case should not advertise prefix 10.2/16 to R2
> as when the Label Request for FEC 10.2/16 comes to R2, it
> can assign a binding to it and can forward that request to R3,  wherein R3
> can in turn give the binding to R2 again, but when the labelled packet
> arrives at R2 it forwards it to R3 and there is no way for R3 to know now
> where it should forward that packet.
>
> In Other words how can R2 know that it has better prefix match than
> 10.2/16 and thus issue a new request to R3. and stop Label Switching the
> packets there?
>
> Thanks in advance
>
> Abhijit

--

Name : Anup Anil Chervathoor
E-mail : mailto:acheru@ficon-tech.com
Web : www.ficon-tech.com