[c-nsp] xconnect on ASR9k - Type 5 PW, what happens with VLAN tag?

Marcin Kurek notify at marcinkurek.com
Mon Jun 8 02:11:16 EDT 2015

W dniu 2015-06-05 o 15:36, Adam Vitkovsky pisze:
> Hello Marcin,
Hi Adam,

> Yes unfortunately the PW MTU on XR changes automatically depending on the rewrite operation.
> But obviously the system can account for only what it sees so if I know there are double tagged frames coming through e.g. an Ethernet NNI and I only pop the service tag on ingress I have to account for the additional c-tag when configuring the interface MTU.
> Basically always configure the MTU manually yourself on both ends and account for the rewrite operations performed at each end so that the resulting PW MTU is the same on both ends.
> adam

Thank you for your reply. Most of the things are clear, however I still 
find the signalled AC MTU 1504 (when not performing rewrite operation) a 
bit confusing.
When I ping between 4948s, packet size 1500 bytes goes through without 
any problems, but 1501 doesn't make it to the other end.

4948-LAB#ping size 1500
Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
4948-LAB#ping size 1501
Type escape sequence to abort.
Sending 5, 1501-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 0 percent (0/5)

Of course 1501 bytes means frame size of 1519 ( 1501 + 14 + 4) and 
because this exceeds the MTU of ingress interface it is dropped as giant 

So my understanding of the 1504 signalled via PW is that it includes 
possible dot1q tags. The router knows that it isn't doing any rewrite 
operations on a frame, it knows its own MTU, so it can tell the other 
side how many bytes it can accept from its point of view.
If I add rewrite operation it drops to 1500, because the router will 
have to add another dot1q tag on top of what it receives from the far end.


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