Labels not being popped in a basic MPLS topology.

[Mitrixsen]

Hello, everyone!

I have this topology here, everyone is running OSPF. I am studying basic MPLS so I am still figuring out how things works.

Only the routers on the right (P2 and PE2) are running MPLS and LDP.

My question is, why is P2 telling PE2 that in order to reach

10.0.0.0/30 and 192.168.1.0/24

the packet must be labeled?

If P2 is at the end of the MPLS domain and P1/PE1 are not running MPLS, shouldn't P2 tell PE2 to pop the label instead?

Thank you in advance.

David

[Harold Ritter · Accepted solution]

Hi @Mitrixsen ,

P2 does not send an implicit null and therefore does not tell PE2 to pop the label, as it is not itself the router terminating the LSP for

10.0.0.0/30 and 192.168.1.0/24

The reason why P2 allocates a label for both

10.0.0.0/30 and 192.168.1.0/24

even though it has not received a label for these prefixes from the downstream router is that Cisco IOS uses the independent allocation mode rather than the ordered mode.

For more information about the independent vs ordered mode, please refer to RFC3031, section 3.19:

https://datatracker.ietf.org/doc/html/rfc3031#section-3.19

Regards,

[M02@rt37]

Thanks @Harold Ritter for providing that clarification on this behavior. 

->https://www.thenetworkdna.com/2012/01/mpls-control-mode-ordered-lsp-control.html

[Harold Ritter]

You are very welcome M02@rt37