how MPLS router know it is penultimate hop and do the PHP

[ronald.su]

hey guys, how MPLS router know it is penultimate hop and do the PHP, I mean the label tagging is base on route table, right ? from the route table, the router should know "what is tthe next hop" only, I think the oruter doesn't know the full topology, so why the router will know he is the penultimate hop ??

please correct me ..thanks

[rais]

Router behaves PHP when the label received from adjacent router is a value 3 - the Implicit Null Label. 

Label value 3 is a reserved value and never appears in encapsulation because it specifies that the top label should be removed [POPed] and remaining packet should be forwarded to next-hop. 

HTH.

[luis_cordova]

Hi @ronald.su,

Check this discussion:

https://community.cisco.com/t5/mpls/mpls-php/td-p/826983

Regards

[Vishnu Vardhan S · Accepted solution]

That was a nice question.

It's very simple that every prefix in the MPLS cloud is bind to a label value which is unique to the router. 

For example see the below setup

RouterA ------MPLS--------- Router B-----MPLS------Router C----IPV4---Router D. Loopback 3.3.3.3

        10.0.0.1                   10.0.0.2     12.0.0.1           12.0.0.2      13.0.0.1     13.0.0.2

once I enable MPLS on all the router Router D will start assigning a label to the prefixes to which it was the reachability and directly connected its going to assign a value of 3 and will send it to Router C

Now 3.3.3.3 got a value of 3 and will be sent to Router C which will assign another label instead of 3 and will send to Router B similarly Router B will assign and send to Router A. 

For 3.3.3.3

Routers          Label Assigned   Label sent              Label received

D                      3                       3 sent to C                Non

C                    194                    194 sent to B               3

B                     135                   135 sent to A               194

A                                 ------------- Similar pattern---------

The thumb rule for the MPLS router is whenever they receive a label value of 3 for a prefix then it need to open the MPLS packet remove the MPLS label and send it as IPv4 packet