ASR 920 - BFD echo - far end not matching QoS policy-map

Benjamin Bartsch · ‎11-07-2016

Topology is simple - two ASR 920 routers directly connected with a 1 Gbps copper cable. Interfaces configured as a regular /30 routed interface on each side. IGP = IS-IS. We use 10ms x 3 detections for BFD with echo (we have to use a bfd-template to accomplish aggressive timers <50ms). We are on latest code 3.16.4.

The problem is as follows - we see no packets match our QoS policy map applied egress on either side. We know that the echo packet is originated by router A and put on the wire as PAK priority. We see the packets leave router A and hit the far end router B marked CS6. We see the packets turning around and leaving router B heading back to router A still marked CS6. We do not see any packets matching the egress QoS policy-map in any classes (CS6, class-default, etc).

I know the ASR 9k requires far end egress QoS to protect BFD under congestion. But this ASR 920 seems to behave differently.

I am having problems finding supporting documentation. TAC is having problems getting this to work for us. SR 681187159

Benjamin Bartsch · ‎11-08-2016

I removed all QoS policy-maps and oversubscribed the backbone BFD interfaces. BFD remained stable in a 10 ms by 3 echo configuration. I'm not sure if I'm just lucky, or if the echo packet on the remote router is somehow being given PAK priority and it's just not documented.

tsarles · ‎09-08-2017

I was searching for the same answer, and it does appear that BFD automatically recieves priority.

From the following article:

https://www.cisco.com/c/en/us/td/docs/routers/crs/software/crs_r4-2/qos/configuration/guide/b_qos_cg421crs/b_qos_cg421crs_chapter_0101.html

If no QoS policy is attached to the interface on which BFD echo packets are received and switched back, then the BFD echo packets are marked as vital packets (when received) and are sent to the high priority queue in the ingressq ASIC reserved for transit control traffic.

If ingress QOS policy is present on the interface on which BFD echo packets are received and switched back, then the BFD echo packets are marked as vital packets (when received) and all QOS actions of the matching class except for taildrop and WRED are performed on the packets. The packets are then sent to the high priority queue in the ingressq ASIC reserved for transit control traffic, overriding the queue selected by the ingress QOS policy.

In the egress direction, the BFD echo packets are treated like other vital packets (locally originated control packets) and are sent to the high priority queue of the interface in the egressq ASIC.