Cannot remark DSCP 0 on Catalyst 3750

Franjo Pongrac · ‎04-14-2010

I’ve got Catalyst 3750 switch. On the ingress port (Gi1/0/4) I am receiving lot of traffic from some Voice equipment. This is the configuration:

interface GigabitEthernet1/0/4
description "media"
switchport access vlan 113
switchport mode access
service-policy input MEDIA

policy-map MEDIA
class MEDIA-SPC
set dscp cs2
class MEDIA-RTP
set dscp cs5

class-map match-all MEDIA-SPC
match ip dscp af11
class-map match-all MEDIA-RTP
match ip dscp default

I want to remark AF11 to CS2. And this works just fine. ( I can see the packets correctly marked when leaving the switch (uplink monitored with Wireshark).

I would like also to remark DSCP 0 to DSCP 40 (CS50) but the switch doesn’t remark that as expected.

Does anyone know why I can remark AF11 and I cannot do the same for DSCP0?

Regards,

Franjo

andrew.prince · ‎04-14-2010

Show the outout of

show mls qos

show mls qos maps ip-prec-dscp

Franjo Pongrac · ‎04-16-2010

CRI31#show mls qos
QoS is enabled
QoS ip packet dscp rewrite is enabled

CRI31#show mls qos maps ip-prec-dscp
   IpPrecedence-dscp map:
     ipprec:   0 1 2 3 4 5 6 7
     --------------------------------
       dscp:   0 8 16 24 32 40 48 56

CRI32#
CRI32#show mls qos maps ip-prec-dscp
   IpPrecedence-dscp map:
     ipprec:   0 1 2 3 4 5 6 7
     --------------------------------
       dscp:   0 8 16 24 32 40 48 56

andrew.prince · ‎04-16-2010

change your config to:-

policy-map MEDIA
class MEDIA-RTP
set dscp 40

Franjo Pongrac · ‎04-16-2010

I actually configured it this way but in sh running-configuration you see it as CS5. So it is only how IOS prefers to display binary value 101 000.

mlund · ‎04-16-2010

Hi Franjo

The "match ip dscp default" actally means "match ip dscp 0"

There are a very high chance that the rtp-streams already are marked with some dscp-value (normally 40 or 46), and therefore are not matched.

Try to match something else in the "class-map match-all MEDIA-RTP", "match dscp 40" or "match dscp 46" or maybe "match any" , if the stream already are marked with dscp 40 you don't need to rewrite it.

/Mikael