prioritize BFD packets

[jamesma01]

Hi, Expert,

Is there a way to mark BFD control packets in Cisco routers (such as ASR903) with high priority, so it gets treated with priority in swithcing network between two routers, in turn not suffers from packet loss and delay   ? 

if it can be done, how to configure it?

Thanks,

James             

[Joseph W. Doherty]

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Posting

First you might want to check if BFD packets have any special marking now as often control packets get special markings.

If not, or if not the markings you want, if the ASR supports egress CBWFQ polices that can both classify and mark, that might allow you to mark the BFD packets as desired.

[jamesma01]

I did some investigation and found the followings,

BFD opackets, like most of control packets, has CS6 DSCP marking.

Next step is to map DSCP to COS marking. This can only happens on ethernet subinterface of a router, because you need COS field in VLAN tag.

the CLI to map DSCP to COS is "set cos dscp" under policy-map, then this policy map has to be applied outbound on every sub-interface of an Etehrnet port.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

BTW, I've not familiar with ASR 9000 IOS, which is why I didn't know if BFD packets were already marked as "special" and/or how well the ASR 9000 supports CBWFQ.

However, since you say BFD packets are marked with DSCP CS6, then if the ASR does support CBWFQ, you could define an output service policy to specialy treat DSCP CS6 marked packets.

BTW, some Cisco device also give special priority to control packets even without an explicit QoS policy; I recall seeing on some platforms this referred to as "PAK" priority.

Now as far as QoS across any separate network switches between the two routers, many of the later Cisco switches, even those "just L2", support enhanced examination of the inbound frame, i.e. they often can examine L3 ToS and use it to treat such packets differently.  I.e. with such support, you would not need VLAN tagged frames to use L2 CoS for special treatment of the DSCP CS6 marked BFD packets.