The MPLS WG Archive

["LIAN Franklin FTLD/IAP" <franklin.lian@francetelecom.com>]

Hello, Wu Min

I am sure someone can correct me if I am wrong.  Please see my 
comments interleaved below.

wu min wrote:
> 
> Hi,
> 
> I have questions regarding longest prefix address.
> 
> The following is from section 4.1.3. of RFC 3031:
> ". .
> 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."
> 
> Questions:
> ----------
> 1. Does it mean R2 has to terminate LSP for FEC with 10.2/16 address prefix
> by means of stripping the top label (the only label) and look into IP header
> to determine its finer FEC and then label the packet again with the
> appropriate outgoing label. Is this true?

My understanding of the RFC is the LSP for 10.2/16 stops at R2.  The
the packet may or may not be label switched after R2.

> 
> 2. Suppose the LSP is being used as a tunnel that ends after R2 and if the
> answer for my question (1) is true, then stripping the top label will never
> reveal the IP header yet since there will still be 1 or more labels. Is this
> situation valid? If so, can R2 still manage to terminate the LSP and perform
> best match algorithm? If yes, then how?

I don't think you can build an LSP beyond R2 in this case, in another
word, I don't know how you can have a tunnel that ends after R2.

> 
> 3. Since R3 also advertised a coarser granularity prefix address, 10.2/16,
> to R2; can R2 safely maps its own 10.2/16 prefix with R3's equivalent prefix
> regardless of the existent of the finer granularity prefixes? What are the
> consequences if it does not follow the rules.

I think the reason for not using the aggregate route is trying to avoid
routing loop.  Building the LSP by ignoring the longest prefix route
is conflict with the original intention.

> 
> _________________________________________________________________
> Join the world’s largest e-mail service with MSN Hotmail.
> http://www.hotmail.com