Cisco HWIC EtherSwitch (ESW) Module - QoS Support

HWIC ESW can be connected to IP Phones when it’s configured for untagged, dot1p, or none mode for voice vlans.

HWIC module has limited support to QoS features (the new hardware NM-ESW has much powerful QoS capabilities).

In HWIC-ESW, MLS QoS is enabled globally and can’t be disabled. Also, MLS QoS trust/untrust interface commands aren’t functional although they are configurable (the commands can be applied per interface to trust dscp or cos but they aren’t effective and this can be confirmed using ‘show mls qos interface ’ command).

interface FastEthernet0/3/2

 switchport access vlan 20

 switchport stacking-partner interface FastEthernet0/3/2

 mls qos trust cos

end

RTR#sh mls qos int fa0/3/2

FastEthernet0/3/2

trust state: not trusted

trust mode: trust cos

COS override: dis

default COS: 0

pass-through: none

But then if trust/untrust aren’t functional, how HWIC QoS works??

Each interface in HWIC module has two egress queues which are high and low priority queues. Incoming packets with dot1q tag will be mapped to egress queues (low queue or high queue) based on their CoS values as follow:

-          Packets are buffered in a high-priority queue for CoS values 4 to 7 and scheduled for expedited forwarding.

-          The low-priority queue is used for packets with CoS values 0 to 3

For untagged frames, they will be assigned the default CoS value configured per-port. Based on the CoS value configured on the port, ingress frames through that port will be assigned to egress queues (low and high).

interface FastEthernet0/3/2

 mls qos cos 4

VERY IMPORTANT: Note that the frame never gets tagged during this process. They will be only assigned to queues based on CoS without tagging.

Also, HWIC support CoS override feature to rest the CoS values of tagged frames to default one. Accordingly all frames entering that port will be assigned to same queue based on default CoS value.

interface FastEthernet0/3/2

 switchport access vlan 20

 switchport stacking-partner interface FastEthernet0/3/2

 mls qos cos 4

 mls qos cos override

end

RTR#sh mls qos int fa0/3/2

FastEthernet0/3/2

trust state: not trusted

trust mode: not trusted

COS override: ena

default COS: 4

pass-through: none

HWIC module doesn’t have ingress queues similar to catalyst switches. Traffic entering the ingress port is sent directly to the shared memory or CPU, depending on the priority.