QoS on 3550
Below is from pramodctl ,#14864,www.routerie.com
Since VoIP traffic requires special treatment throughout the network, a carefully designed end-to-end QoS policy is required in a network utilizing voice over data. In order to facilitate in creating this policy, QoS must extend down to the access layer. Traffic marking at the access layer is supported through layer 2 Class of Service (CoS) values.
On the assumption that ur using 3550 series switch:-
By default the 3550 does not process CoS values, and rewrites all frames with a CoS value of zero. To enable the processing of CoS, QoS must be enabled globally by issuing the mls qos global configuration command. Once QoS is enabled you must decide how the switch will process frames that already have a CoS value set. Typically you would want to set the switch to trust the CoS value that is coming from the IP phone. This is accomplished by issuing the "mls qos trust cos" interface level command.
In order to prevent the device attached to the phone from getting better service throughout the network and interfering with VoIP traffic, the Cisco IP phone by default will re-tag all frames received from its extension with a CoS value of zero. To tag them with a different value or to leave them untagged, use the interface level command
"switchport priority extend [ trust | cos value) ].
In the above case all traffic received from the PC attached to the IP phone is remarked with a CoS value of one.
Since ports on the 3550 series switch default to dynamic mode, installing Cisco IP Phones into the network is a very straightforward process. When a phone is connected the 3550 will automatically form an 802.1q trunk to the phone. Traffic destined for the PC attached to the IP phone will be carried in the access VLAN. Voice traffic destined for the phone itself will be carried in the voice VLAN. These VLANs are defined with the "switchport access vlan" and "switchport voice vlan" command respectively.
Note:-
Unlike a data VLAN a voice VLAN will not automatically be created when it is assigned. Be sure to create the voice VLAN in the VLAN
database before assigning it.
Sorry for the lengthy reply but I hope that u will get a very good idea from my explanation.
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below is from ChrisL ,# 18270,www.routerie.com
A minor correction (I'm sure you meant it this way anyways). The default configuration does not have mls qos globally enabled. In this configuration CoS and DSCP are left untouched. It is only when MLS QoS is globally enabled that the switch (in the absence of any trust state configuration) will re-mark all CoS and DSCP to zero.
Since VoIP traffic requires special treatment throughout the network, a carefully designed end-to-end QoS policy is required in a network utilizing voice over data. In order to facilitate in creating this policy, QoS must extend down to the access layer. Traffic marking at the access layer is supported through layer 2 Class of Service (CoS) values.
On the assumption that ur using 3550 series switch:-
By default the 3550 does not process CoS values, and rewrites all frames with a CoS value of zero. To enable the processing of CoS, QoS must be enabled globally by issuing the mls qos global configuration command. Once QoS is enabled you must decide how the switch will process frames that already have a CoS value set. Typically you would want to set the switch to trust the CoS value that is coming from the IP phone. This is accomplished by issuing the "mls qos trust cos" interface level command.
In order to prevent the device attached to the phone from getting better service throughout the network and interfering with VoIP traffic, the Cisco IP phone by default will re-tag all frames received from its extension with a CoS value of zero. To tag them with a different value or to leave them untagged, use the interface level command
"switchport priority extend [ trust | cos value) ].
In the above case all traffic received from the PC attached to the IP phone is remarked with a CoS value of one.
Since ports on the 3550 series switch default to dynamic mode, installing Cisco IP Phones into the network is a very straightforward process. When a phone is connected the 3550 will automatically form an 802.1q trunk to the phone. Traffic destined for the PC attached to the IP phone will be carried in the access VLAN. Voice traffic destined for the phone itself will be carried in the voice VLAN. These VLANs are defined with the "switchport access vlan" and "switchport voice vlan" command respectively.
Note:-
Unlike a data VLAN a voice VLAN will not automatically be created when it is assigned. Be sure to create the voice VLAN in the VLAN
database before assigning it.
Sorry for the lengthy reply but I hope that u will get a very good idea from my explanation.
--------------------------------------------------------------------------------
below is from ChrisL ,# 18270,www.routerie.com
A minor correction (I'm sure you meant it this way anyways). The default configuration does not have mls qos globally enabled. In this configuration CoS and DSCP are left untouched. It is only when MLS QoS is globally enabled that the switch (in the absence of any trust state configuration) will re-mark all CoS and DSCP to zero.
posted by Jun Wen at 4:54 PM
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