QoS With IP Phone in SRST

[tom.gill]

When our IP phones are registered to CallManager, we can see CS3 and EF marked packets going through the network (using access lists).

However, when we force the phone into SRST we only see CS3 traffic - not EF.

At the edge (Catalyst 3560) we are trusting cos (and mapping cos 3 to DSCP 24 and cos 5 to DSCP 46).

Does the IP phone mark packets differently when it is in SRST mode?

Our network:

CallManager 4.1(3)sr4d

Cisco 7941 IP phones

SRST version 3.3

[thisisshanky]

It should mark the same using EF. Remember that unless you place a PSTN call, you should see no EF traffic through the router. EF traffic should be point to point between phones and should traverse through the switch backplane.

Are you using these ACLs in the router or switch ?

[tom.gill]

Our test was placing a call to our cell phone using POTS lines at a REMOTE site. We did consider the path RTP packets would take.

When we say we are looking at an access-list to see what markings are coming through, we are looking at this on the VLAN interface facing the IP phone on a 3750 switch. We have an access-list that permits ef, cs3, and permits any any. We have added the log keyword to the end of each access-list entry (because otherwise we won't see any hits on our access-list, separate issue).

I find it very odd that the phone would stop marking cos 5 or dscp 46, but still mark cs3 - while in SRST.

Help???

[thisisshanky]

Can you use a sniffer instead of the acl and confirm that 46 is indeed used ? You can download Ethereal for free from http://www.wireshark.org. (Note the name change).

[tom.gill]

We just got done sniffing the 3560 switchport that our 7941 IP phone was connected to. We captured packets while the phone was both in and out of SRST mode. The results are pretty interesting.

It appears the phone does in fact only send EF marked RTP packets while NOT in SRST mode.

See for yourself:

-http://www.neybythebay.com/tom/7941_not_in_srst_mode.GIF

-http://www.neybythebay.com/tom/7941_in_srst_mode.GIF

[Christopher Goddard]

I've been working with Tom to resolve this issue, and from the research I've done I can only assume this is function of Call Manager as configured in the Clusterwide Parameters for QOS. If this is the case is there something that needs to be configured in SRST on the router to specify the DSCP value to utilize for audio calls?

[kelvin.blair]

I was really curious about your post, so I took the liberty and tried to lab something up on this. I had two SRST gateways connected over the WAN. Forced both sides to go into SRST mode. Created dial-peers for both ends to call across the WAN. I made two calls across the wan. The first call was using dialpeers with ip qos settings on the calls. The calls was marked correctly. The second calls was done with the qos markings off and it was correctly marked with EF. I'm not sure what is going on with yours, but I wanted to make sure it wasn't SRST. My IOS was 12.4.5b