Subject: Prefix Mismatch

From: "wu min" <[email-masked]>
Date: Thu, 18 Apr 2002 15:56:16 +0000

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?

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?

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.

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From: "LIAN Franklin FTLD/IAP" <[email-masked]>
Date: Thu, 18 Apr 2002 15:26:35 -0400

Hello, Wu Min

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

> algorithm to find P's FEC."

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.

> 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.

> 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.