QoS maximum bandwidth in IOS XR 4.2.0

[Yuyang Jin]

Hi All,

I have two questions regarding the QoS behaviour in IOS XR 4.2.0 (ASR9000)

Q1: Is QoS using 75% link bandwidth like what it is in IOS, and the rest 25% is preserved for internal network control traffic? What is the command to change it to 100%?

Q2: Is there default mapping of network control traffic generated by ASR9000 router (OSPF, RSVP, MPLS, TE, etc) to a DSCP 48 or MPLS EXP 6? If not, what should I do to map them? Like from control panel?

Thanks very much for the answer.

[Alexei Kiritchenko]

Hello Yuyang,

A1. There is no 75% limit. All 100% are available for you. Note: XR already takes into accounting for QoS all L2 frame length without preamble and IFG.

A2. All control traffic originating by ASR9k skips QoS and goes directly to special High Priority queue. There is no need to configure any egress QoS for it. OSPF, BGP, LDP, RSPV uses DSCP 48 and MPLS EXP 6. Again,  no need for any additional config.

Tcpdump capture example

BGP over MPLS

=================================

14:51:22.297801 02:37:c4:7e:8b:7a > 02:1c:1a:ee:58:cb, ethertype MPLS unicast (0x8847), length 58: MPLS (label 16005, exp 6, [S], ttl 255)

       (tos 0xc0, ttl 255, id 30577, offset 0, flags [none], proto: TCP (6), length: 40) 192.168.0.2.bgp > 192.168.0.6.35422: ., cksum 0xca42 (correct), 319306489:319306489(0) ack 710402273 win 32749

       0x0000: 03e8 5dff 45c0 0028 7771 0000 ff06 c245 ..].E..(wq.....E

       0x0010: c0a8 0002 c0a8 0006 00b3 8a5e 1308 3af9 ...........^..:.

       0x0020: 2a57 e0e1 5010 7fed ca42 0000           *W..P....B..

LDP

=================================

15:06:51.115379 02:c4:77:f7:91:d0 > 02:3d:47:4c:87:cc, ethertype IPv4 (0x0800), length 72: (tos 0xc0, ttl 255, id 40354, offset 0, flags [none], proto: TCP (6), length: 58) 192.168.0.6.ldp > 192.168.0.8.43414: P, cksum 0x841d (correct), 2826845698:2826845716(18) ack 4147837509 win 15412

       0x0000: 45c0 003a 9da2 0000 ff06 9bfc c0a8 0006 E..:............

       0x0010: c0a8 0008 0286 a996 a87e 3e02 f73a fa45 .........~>..:.E

       0x0020: 5018 3c34 841d 0000 0001 000e c0a8 0006 P.<4............

       0x0030: 0000 0201 0004 0000 2729                 ........')

Regards,

/A

[Yuyang Jin]

Hi Alexei,

Thanks very much for the answer. However there is still some points unclear to me. It looks like conflict to me that control traffic goes to special High PQ while XR is still able to allocate 100% links available for other traffic in egress QoS. Could you please explain this further?

Regards,

Yuyang

[Alexei Kiritchenko · Accepted solution]

Hello Yuyang,

You may think of it as it is oversubscribed with a local control inject traffic taking priority over configured MQC.  In case the link is 100% utilized by traffic, control packets will be sent on top of it, dropping something else.

Note, we are talking here about locally injected control traffic. Transit traffic  does not take this HP queue and will be classified by your QoS.

Regards,

/A

[Yuyang Jin]

Thanks, Alexei.