question about penultimate hop popping...

[ashok_boin]

Hi,

The penultimate hop popping says that if the route is directly connected or summary route, an additional L-3 lookup is needed and a rotuer assigns a NULL label to that prefix.

Why it is for directly connected network and summary networks only?

CEF FIB will have entries for directly connected as well as summary networks also after recursive lookup right.

Thanks in advance...

Regards...

-Ashok.

[Harold Ritter]

The reason why a router advertises implicit null labels (lavel value 3) only for directly connected networks is because this router becomes an egress router for these networks.

Hope this helps,

[ashok_boin]

Hi,

Thank you for the response. I am not still clear about null label assignment to summary prefixes according to PHP concept. Because PHP says that summary routes needs another L-3 lookup so that it assigns NULL label value to that one.

But, CEF already does the recursive lookup and MPLS needs CEF, then why PHP concept?

Thanks in advance..

Regards...

-Ashok.

[Harold Ritter]

The router that does the route aggregation becomes the egress router (LSP termination point) and advertise an implicit null since more specific prefixes might have to be forwarded via different egress interfaces.

If the penultimate hop didn't do the PHP, the summarizing router would receive a labeled packet, remove the label and lookup the CEF table to find out where to forward the packet based on the more specific route.

Hope this helps,

[Harold Ritter · Accepted solution]

I forgot to mention that because of the PHP, the summarizing router receives an IP packet therefore doesn't need to do the label lookup and only need to do a CEF lookup.

Hope this helps,

[ashok_boin]

Hi,

I am clarified.

Thank you very much for the information.

Regards...

-Ashok.