All Cisco-Network Study Notes: Low-Latency Queuing

Low-Latency Queuing

Low-latency queuing sounds like the best queuing apparatus possible, aloof based on the name. What

packet wouldn’t appetite to acquaintance low latency? As it turns out, for adjournment (latency) sensitive

traffic, LLQ is absolutely the queuing apparatus of choice. LLQ looks and acts aloof like CBWFQ in most

regards, except it adds the adequacy for some queues to be configured as low-latency queues. LLQ

schedules these specific queues as strict-priority queues. In added words, LLQ consistently services

packets in these antecedence queues first.

LLQ argot can sometimes be acclimated in a brace of altered ways. With a distinct action map that has

at atomic one low-latency queue, the action map ability be advised to be implementing LLQ,

while at the aforementioned time, that one low-latency chain is generally alleged “the LLQ.” Sometimes, a single

low-latency chain is alike alleged “the PQ” as a advertence to the bequest PQ-like behavior, or even

a “priority queue.”

While LLQ adds a low-latency chain to CBWFQ, it additionally prevents the chain starvation that occurs

with bequest PQ. LLQ absolutely polices the PQ based on the configured bandwidth. In effect, the

bandwidth accustomed to an LLQ antecedence chain is both the affirmed minimum and policed

maximum. (You may anamnesis from Chapter 12, that the DiffServ Expedited Forwarding PHB

formally defines the antecedence queuing and policing PHBs.) As a result, the packets that accomplish it out

of the chain acquaintance low latency, but some may be alone to anticipate craving the other

queues.

Figure 13-3 depicts the scheduler argumentation for LLQ. Agenda that the PQ argumentation is shown, but with the

policer analysis as well.

Queuing Tools: CBWFQ and LLQ 447

Figure 13-3 LLQ Scheduler Logic

LLQ agreement requires one added command in accession to the commands acclimated for CBWFQ

configuration. Instead of application the bandwidth command on a class, use the antecedence command:

priority {bandwidth-kbps | percent percentage} [burst]

This chic subcommand enables LLQ in the class, affluence bandwidth, and enables the policing

function. You can additionally configure the access admeasurement for the policer with this command, but the default

setting of 20 percent of the configured bandwidth is about a reasonable choice.

Example 13-4 shows a sample LLQ configuration, application the afterward criteria. Like Example 13-2, the

LLQ action is activated to R3’s s0/0 interface from Figure 13-1:

■ R3’s s0/0 bandwidth is 128 kbps.

■ Packets will already accept been apparent with acceptable DSCP values.

■ VoIP burden is already apparent DSCP EF and should be LLQed with 58 kbps of bandwidth.

■ AF41, AF21, and AF23 cartage should get 22, 20, and 8 kbps, respectively.

■ All added cartage should be placed into chic class-default, which should use WRED and WFQ.

Example 13-4 LLQ for EF, CBWFQ for AF41, AF21, AF23, and All Else

! The chic maps acclimated by the queue-on-dscp are not shown, but the names betoken what

! anniversary chic map has been configured to match. Agenda the antecedence 58 command makes

! chic dscp-ef an LLQ.

policy-map queue-on-dscp

class dscp-ef

priority 58

class dscp-af41

bandwidth 22

class dscp-af21

bandwidth 20

random-detect dscp-based

Any

Packets in

LLQ?

Put Packet in

TX Ring

Pick Next

Packet from

Other Non-LLQ

Queues

Yes

Wait Until TX

Ring Has More

Room

continues

LLQ for EF, CBWFQ for AF41, AF21, AF23, and All Else (Continued)

class dscp-af23

bandwidth 8

random-detect dscp-based

class class-default

fair-queue

random-detect dscp-based

! max-res has to be aloft or the action map would be rejected.

interface Serial0/0

bandwidth 128

encapsulation frame-relay

load-interval 30

max-reserved-bandwidth 85

service-policy achievement queue-on-dscp

! Below, for chic dscp-ef, agenda the byword “strict priority,” as able-bodied as the

! computed policing access of 1450 bytes (20% of 58 kbps and disconnected by 8 to convert

! the amount to a cardinal of bytes.)

R3# appearance policy-map queue-on-dscp

Policy Map queue-on-dscp

Class dscp-ef

Weighted Fair Queueing

Strict Priority

Bandwidth 58 (kbps) Access 1450 (Bytes)

! curve bare for brevity

! Agenda the statistics below. Any packets alone due to the policer would show

! up in the aftermost band below.

R3# appearance policy-map interface s 0/0 achievement chic dscp-ef

Serial0/0

Service-policy output: queue-on-dscp

Class-map: dscp-ef (match-all)

227428 packets, 14555392 bytes

30 additional offered amount 52000 bps, bead amount 0 bps

Match: ip dscp ef

Weighted Fair Queueing

Strict Priority

Output Queue: Conversation 40

Bandwidth 58 (kbps) Access 1450 (Bytes)

(pkts matched/bytes matched) 12194/780416

(total drops/bytes drops) 0/0