We are experiencing problems with a 3750 switch in our test environment. The switch is a WS-C3750G-24TS-E1U and is supposed to forward multicast streams to the multicast routers through a trunk optical port. The streams consume a total of about 100Mbps and are received through a GigE electical interface that operates in access mode.
We were supposed to mark the incoming multicast traffic with the appropriate CoS values. We tried to implement this in the simplest way possible:
switchport access vlan 100
switchport mode access
mls qos cos 5
mls qos override
When we added the "mls qos override" command, immediately the streams began experiencing heavy packet loss. When we left that command out, the stream were forwarded without lost frames (but of cource no marking took place then).
We are using the "c3750-ipservices-mz.122-25.SEE2" image.
Dscp rewrite is enabled and sdm settings are the default (access-desktop).
I am afraid that if such basic functionality does not function correctly then I must doing something horribly wrong. Can anyone provide any suggestions about possible ways to overcome this problem.
1) You dont need the "mls qos cos x override" command. This is required to replace the existing cos value preset into Dot1Q tagged frames.
2) Since you have a access port for the multicast source/headend, you can simply use the "mls qos cos" command to set the default port cos value into the Dot1Q Tags for the Access Vlan defined on the access port.
DSCP rewrite enabled will reqrite the IP header of the packets using the default COS-DSCP map. This is also ok if you want to further categorize the downstream towards the destination.
With 100 MB of total stream traffic ideally you shouldnt be seeing drops, even if you enable the command "mls qos cos override", which is not required in your scenario.
TO further answer your question, why you see drops, when you enable the override command, you topology and the complete config done on this switch needs to be understood, as for all you know it may be a bug as well.
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