LLQ
LLQ addresses the needs of real-time traffic for low delay and guaranteed
bandwidth. It creates one priority queue (in addition to the CBWFQs) with
bandwidth that is both guaranteed and policed. This is a strict priority
queue—traffic is sent from it before any other queues. However, when the
interface is congested, the priority queue is not allowed to exceed its configured
bandwidth to avoid starving the other queues. Voice traffic is typically
enqueued into the LLQ. You can place more than one class of traffic in the
LLQ. If so, the router still creates just one priority queue but allocates bandwidth
to each class, and meters the traffic so that it does not exceed the
bandwidth assigned to that class.
Configure LLQ under the class statement in the policy map:
(config-pmap-c)#priority {bandwidth [burst] | percent percentage
[burst]}
Bandwidth is configured in kilobits per second, and burst is configured in
bytes. These bandwidth amounts include the layer 2 headers.
Example 4-2 shows RTP voice traffic put into an LLQ, then guaranteed and
policed to 256K of bandwidth. Traffic bound to URLs that include the string
“ccnp” are placed in another queue, guaranteed 128K of bandwidth, and
congestion avoidance is applied via WRED. Traffic bound for eDonkey
applications is dropped. All other traffic falls into the default class, is placed
in its own queue, and WRED is applied. The policy is applied outbound on
the serial interface.
Example 4-2 Configuring LLQ and CBQFQ
Router(config)#class-map VOIP-Out
Router(config-cmap)#match ip dscp ef
Router(config-cmap)#!
Router(config-cmap)#class-map Exams-Out
Router(config-cmap)#match ip dscp 31
Router(config-cmap)#!
Router(config-cmap)#class-map eDonkey-Out
Router(config-cmap)#match ip dscp 13
Router(config-cmap)#!
Router(config-cmap)#policy-map QOS
Router(config-pmap)#class VOIP-Out
Router(config-pmap-c)#priority 256
Router(config-pmap-c)#class Exams
Router(config-pmap-c)#bandwidth 128
Router(config-pmap-c)#random-detect dscp-based
Router(config-pmap-c)#class eDonkey
Router(config-pmap-c)#drop
Router(config-pmap-c)#class class-default
Router(config-pmap-c)#fair-queue
Router(config-pmap-c)#random-detect dscp-based
!
Router(config)#int s1/0/0:1
Router(config-if)#service-policy output QOS
Use the show policy-map interface interface command to see the service
policy configuration and also the effect it has had, as shown in Example 4-3.