Weird QoS Problem, Cisco network, Cisco Phones, Cisco Callmanager :)

[petterjoh]

Hello!, Thanks for reading.

I Have a really weird QoS problem.
If i plug a Cisco 7911 Phone into my switch and trust it, i can see that the packets get a DSCP value when they enter that interface, and I can see them leaving the external interface.

I also see them returning, so everything works.

NOW, when i plug in a 7941 Phone, its the same.
EXCEPT that they dont return.
I see the packets leaving the phone port, and leaving the switch, but they never return..

I got completly stuck here, would really appriciate it if someone could help

-Petter Johnsen

 

[ADB100]

What do you mean 'They don't return'? Are you talking about the return traffic from the other IP Phone or Gateway the 7941 has an open converstation with? If this is the case then it isn't hte 7941 that generates that traffic, it is the remote end (plus the SCCP traffic to/from your CCM, assuming it is CCM and not SIP?).

Andy

 

[petterjoh]

---- ------ ------ ----
|7941|--|Switch|--internet--|Switch|--|7911|
---- ------ ------ ----

Maybe this explains it better.
Between the phones and the switches we trust COS
(use Auto QoS, trust for switches and device cisco-phone for phones
)
over the WAN link we trust DSCP
(since thats the only thin out ISP supports)

If i use two 7911 phones, the dscp/QoS marking is OK.
when i call from a 7941 to a 7911 the packets have DSCP46
from the 7941 to the 7911, but not back.
On the interface that faces towards the 7911 phone i can see incoming DSCP/COS markings, but they just dissapear.

Now, if i change the trust between the phones and the switches to DSCP, the marking works both ways between a 7941 and a 7911.

The problem about doing that is that not all of our switches support DSCP (like the Cisco 2940)

This is a really weird problem since we have a COS-DSCP map on all of the switches so that the values should be translated.

What it seems like to me is that when a 7911 phone starts talking to a 7941 it suddenly switches from COS marking to DSCP (which is really weird) and when we trust the COS value on the ports the switch strips off the DSCP value

 

[ADB100]

This might the be the DSCP pass through 'feature' on the 2950/2940:

interface fastethernet0/1
mls qos trust cos pass-through dscp


Both phones should write both CoS & DSCP values in to the packets, assuming you are running Tagged Voice VLAN's on your access ports:

Signalling
CoS 3 & DSCP 24 or 26 depending on CCM version

RTP
CoS 5 & DSCP 46


Do you have any other layer-2 switches involved with VLAN trunks? I have seen issues where CoS is trusted on a trunk but traffic was arriving on the Native (untagged) VLAN. This results in the CoS being stripped off and then the switch re-writing the packets IP header with DSCP 0.

Andy

 

[petterjoh]

we use Vlan1 for administrative purposes only, no user traffic on it, then we have 1 vlan pr. purpose (like voice got its own vlan).

We use 2940,2950,2970,3550,3560,3750 switches & some different cisco routers wich depend on the size of the location. small locations uses 1712's those that have a 3560 uses that as a router.

some of the locations may have up to 10-12 L2 switches but theres VLANs for all user traffic so this shouldnt be a problem.

What really stumbles me is that when we are using 7911 phones everything works (but these phones doesnt support headset's and 2-way speakers) so therefore we have put in some 7941s. This was done after the network was tested OK and everything was found working optimaly.

We use Cisco 1040 boxes to test the quality of our IPT communication.

i see both incoming COS and DSCP in the switchport where the 7911 phone is connected when it speaks to a 7941 but the switch doesnt forward these packets marked.

phoneports has this conf.
switchport mode access
switchport access vlan xxx
switchport voice vlan xxx
spanning-tree portfast
srr-queue bandwidth share 10 10 60 20
srr-queue bandwidth shape 10 0 0 0
mls qos trust device cisco-phone
mls qos trust cos
auto qos voip cisco-phone

trunk ports have.
switchport trunk encap dot1q
switcport trunk allowed vlan 1,xxx-xxx,1002-1005
switcport mode trunk
speed 1000
duplex full
srr-queue bandwidth share 10 10 60 20
srr-queue bandwidth shape 10 0 0 0
priority-queue out
mls qos trust cos
auto qos voip trust

router ports has:
no switchport
bandwidth 100000
ip address xxx.xxx.xxx.xx xxx.xxx.xxx.xxx
ip pim dense-mode
ip mroute-cache distributed
srr-queue bandwidth share 10 10 60 20
srr-queue bandwidth shape 10 0 0 0
priority-queue out
mls qos trust dscp
auto qos voip trust

 

[petterjoh]

Hello.

I figured it out.
One of the phone ports i used in the test had
switchport mode access
switchport access vlan xxx (the voice vlan)
and no switchport voice vlan.

When i changed this it worked..
Still abit weird that this worked if i had 7911 phones in both ends ^^.

But thank for the help!

 

[ADB100]

The switch will reset the DSCP values back to 0 if QoS is enabled and you are not trusting DSCP. Since you are trusting CoS I assume the switch is working as designed. What CoS to DSCP Maps have you got configured?

Andy