Friday, 9 December 2011

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.

2 comments:

  1. hi Mohammad, need your help here. I have a cisco 2900 router with HWIC ESW and I am not even getting the "mls" option under the interface. did you turn on anything globally to use this option?

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