Tail drop advanced

WRED

Drops packets with a certain CoS when a certain threshold is reached

When 50% of queue is filled, drop CoS 0 and 1

When 60% is filled, drop CoS 2 and 3

Defines a min and max threshold

CoS 0 and 1 has a min threshold of 50 and max of 70. When queue reaches 50, WRED starts dropping packets in random and when it reaches 70 will drop all packets

WRR – Weighted Round Robin

Strict Priority

Queues emptied in round robin fashion based on the weights

By default, has 2 queues.

Queue 1 is served 70% of the time and Queue 2 is served 30% of the time

Always the priority queue will be emptied first.

After each packet from the other queues are send, the priority queue will be checked and emptied

Map CoS to queue thresholds

Cos 0 maps to Queue 1 Threshold 1

Cos 1 maps to Queue 1 threshold 2

set qos map 2q2t tx 1 1 cos 0

set qos map 2q2t tx 1 2 cos 1

Define the threshold levels for tail drop or WRED

Threshold 1 is 80

Threshold 2 is 100

set qos drop-threshold 2q2t tx queue 1 80 100 – tail drop

set qos wred 2q2t tx queue 1 80 100 — WRED

Queue serving time

Queue 1 served 5/260

Queue 2 served 255/260

set qos wrr 2q2t 5 255

Assigning buffer length

Queue 1 has 80 (low priority queue is assigned more buffer)

Queue 2 has 20

set qos txq-ratio 2q2t 80 20

RED

WRED

FWRED

Packet drop

Random

Based on IP Precedence

Packets with higher IP Precedence are less likely to be dropped than packets with lower precedence

Based on the flow.

Statistical drop

none

Drops more packets from larger users than smaller

Flows that respond to packet drops are protected against those that do not

Non-IP Traffic

no special consideration

considered to belong to IP Precedence 0

Command

random-detect

random-detect exponential-weighing-constant

random-detect precedence

random-detect

random-detect flow

random-detect flow average-depth-factor

random-detect flow count

RSVP Aware

no

yes

FRF.12

Frame relay does not distinguish between Voice and data packets

It fragments packets that are larger than the configured fragment size

frame-relay fragment

Set the size to be larger than the voice packet size

FRF.11 Annex C

Frame relay distinguishes between voice and data packets

Only data packets are fragmented

PPP multilink

ppp multilink

ppp multilink interleave

ppp multilink fragment-delay

CIR

Average rate

Higher than guaranteed rate

Lower than access rate

Allows for bursting

When using VoIP/VoFR, must be same as the guaranteed rate. Same as MinCIR

Default: 56000 bps

MinCIR

Guaranteed rate

Default: half of CIR

Bc

Amount of data send per Tc

For Data = CIR/8 so Tc = 125 ms

For Voice = CIR/100 so Tc = 10ms

Be

Amount of excess data to be send during the first Tc interval once credit is build up

Used only if CIR is less than Access rate

For voice, set Be to 0

Default = 0

BECN

When it receives a BECN, the traffic rate is dropped by 25%

For each BECN (one per time interval), the rate is dropped by 25%, until it reaches the MinCIR value

It must allow 16 intervals of no BECN, before it traffic rate is increased

The rate is increased by the byte limit/16. Hence it takes much longer to get back to CIR level

Use

Assumption

Input

Output

Erlang B

To find the number of trunks

Blocked calls are cleared/re-routed and never to return on the same trunks

Traffic volume in Erlang

Percentage of Blocked calls

Number of lines

Erlang Extended B

To find the number of trunks

Blocked calls are retried and will come back to the same trunks

Traffic volume in Erlang

Percentage of Blocked calls

Recall factor

Number of lines

Erlang C

To find the number of agents required in call center

Calls per hour

Duration of call

Average delay of call

Number of agents