Below is a topology for RSVP LSP. As you can see MTU is varied across the path.




-> Possible Black-hole when we receive high traffic Egress on R7 towards R1 with higher MTU on what link supports.


How do we avoid it, if it is RSVP Setup LSP

To understand better,  first we need to understand what RSVP uses to communicate MTU value then decide upon what is the lowest MTU value.

Two main objects would be

ADSPEC  – Relate to advertisement – Hence associate with PATH message

FLOWSPEC – Relate to Confirmation – Hence associate with RESV message


Digging Deeper , let see a screen shot on how it looks like, one simple thing, even the smallest value has to be advertised nothing is Magical per se 🙂


Lets see the flow-spec return


Let have a look at the command line at each and every hop, Its interesting to see how Ingress/Transit/Egress routers interpret the Spec


At Ingress

We have sent Adspec requesting 1015, we have received Path MTU 1015 – so far so good


At transit – Received 1015 / sent back 1015


At Egress – Adspec Received is 1015



I am changing mtu to 1001 in between the path, D1 should have this recognized , this is the last step. Fantastic we see that we have received path MTU to be 1001 , which is the lowest through out the path.




Rakesh M