WRED Configuration
Because WRED manages drops based on chain depth, WRED charge be configured alongside a
particular queue. However, best queuing mechanisms do not abutment WRED; as a result, WRED
can be configured alone in the afterward locations:
■ On a concrete interface (with FIFO queuing)
■ For a non-LLQ chic central a CBWFQ activity map
■ For an ATM VC
To use WRED anon on a concrete interface, IOS absolutely disables all added queuing mechanisms
and creates a distinct FIFO queue. WRED again manages the chain with attention to drops. For
CBWFQ, WRED is configured in a chic central a activity map, in the aforementioned area as the
bandwidth and antecedence commands discussed beforehand in this chapter.
The random-detect command enables WRED, either beneath a concrete interface or beneath a class
in a activity map. This command enables WRED to use IPP, and not DSCP. The random-detect
dscp-based command both enables WRED and tells it to use DSCP for free the traffic
profile for a packet.
To change WRED agreement from the absence WRED contour for a accurate IPP or DSCP, use
the afterward commands, in the aforementioned area as the added random-detect command:
random-detect antecedence antecedence min-threshold max-threshold [mark-probdenominator]
random-detect dscp dscpvalue min-threshold max-threshold [mark-probabilitydenominator]
Finally, abacus of the rolling boilerplate chain abyss can be afflicted through configuring a
parameter alleged the exponential weighting constant. A low exponential weighting connected means
that the old boilerplate is a baby allotment of the calculation, consistent in a added bound alteration average.
The ambience can be afflicted with the afterward command, although alteration it is not
recommended:
random-detect exponential-weighting-constant exponent
Note that earlier, Archetype 13-4 showed basal WRED agreement central some classes of a
CBWFQ configuration.
Modified Arrears Round-Robin
MDRR is a queuing affection implemented alone in the Cisco 12000 alternation router family. Because
the 12000 alternation does not abutment CBWFQ and LLQ, MDRR serves in abode of these features. Its
main claims to acclaim are bigger candor than bequest queuing methods such as antecedence queuing and
custom queuing, and that it supports a antecedence chain (like LLQ). For the CCIE Routing and
Switching condoning exam, you charge to accept how MDRR works at the conceptual level, but
you don’t charge to apperceive how to configure it.
MDRR allows classifying cartage into seven round-robin queues (0–6), with one added priority
queue. When no packets are placed into the antecedence queue, MDRR commonly casework its queues
in a round-robin approach, cycling through anniversary chain already per cycle. With packets in the priority
queue, MDRR has two options for how to accommodate the antecedence chain in the chain service
algorithm:
■ Austere antecedence mode
■ Alternating mode
Strict antecedence approach serves the antecedence chain whenever cartage is present in that queue. The benefit
is, of course, that this cartage gets the aboriginal account behindhand of what is activity on in the added queues.
The downside is that it may advance to chain starvation in added queues if there is consistently cartage in the
priority queue. In this mode, the antecedence chain additionally can get added than the configured bandwidth
percentage, because this chain is served added than already per cycle.
By contrast, alternating approach serves the antecedence chain in amid confined anniversary of the added queues.
Let’s say that bristles queues are configured: 0, 1, 2, 3, and the antecedence chain (P). Assuming that there
is consistently cartage in anniversary queue, actuality is how it would be processed: 0, P, 1, P, 2, P, 3, P, and so on.
The aftereffect is that chain starvation in non-priority queues does not occur, because anniversary chain is
being served. The check of this approach is that it can account jitter and added cessation for the
traffic in the antecedence queue, compared to austere antecedence mode.
456 Affiliate 13: Congestion Management and Avoidance
Two agreement in MDRR, different to this queuing method, advice to differentiate MDRR from other
queuing tools:
■ Breakthrough amount (QV)
■ Deficit
MDRR supports two types of scheduling, one of which uses the aforementioned accepted algorithm as the
legacy CQ affection in Cisco IOS routers (other than the 12000 series). MDRR removes packets
from a chain until the breakthrough amount (QV) for that chain has been removed. The QV quantifies
a cardinal of bytes and is acclimated abundant like the byte calculation is acclimated by the CQ scheduler. MDRR
repeats the activity for every queue, in adjustment from 0 through 7, and again repeats this round-robin
process. The end aftereffect is that anniversary chain gets some allotment bandwidth of the link.
MDRR deals with the CQ scheduler’s botheration by alleviative any “extra” bytes beatific during a cycle
as a “deficit.” If too abounding bytes were taken from a queue, abutting time about through the queues,
the cardinal of added bytes beatific by MDRR is subtracted from the QV. In effect, if added than the QV
is beatific from a chain in one pass, that abounding beneath bytes are taken in the abutting pass. As a result,
averaged over abounding passes through the cycle, the MDRR scheduler provides an exact bandwidth
reservation.
Figure 13-7 shows an archetype of how MDRR works. In this case, MDRR is application alone two
queues, with QVs of 1500 and 3000, respectively, and with all packets at 1000 bytes in length.
Figure 13-7 MDRR: Making Up Deficits
Queue1
P6 P5 P4 P3 P2 P1
Queue2
P12 P11 P10 P9 P8 P7
Beginning Q1 Arrears Calculation = 1500
Ending Q1 Arrears Calculation = -500
Beginning Q1 Arrears Calculation = 3000
Ending Q1 Arrears Calculation = 0
Note: All Packets are 1000 bytes long!
1st MDRR Canyon Through the Queues
Queue1
Queue2
P12 P11 P10
Beginning Q1 Arrears Calculation = -500 + 1500 = 1000
Ending Q1 Arrears Calculation = 0
Beginning Q1 Arrears Calculation = 0 + 3000 = 3000
Ending Q1 Arrears Calculation = 0
P6 P5 P4 P3
2nd MDRR Canyon Through the Queues
LAN Switch Congestion Management and Avoidance 457
Some altercation of how to adapt Amount 13-7 may advice you abstract what is activity on. The figure
shows the activity during the aboriginal round-robin canyon in the top bisected of the figure, and the activity during
the added canyon in the lower bisected of the figure. The archetype begins with six packets (labeled P1
through P6) in chain 1, and six packets (labeled P7 through P12) in chain 2. Anniversary arrowed line
to the appropriate abandon of the queues, pointing to the right, represents the best by MDRR to accelerate a
single packet.
When a chain aboriginal fills, the queue’s arrears adverse (DC) is set to the QV for that queue, which is
1500 for chain 1 and 3000 for chain 2. In Amount 13-7, MDRR begins by demography one packet from
queue 1, decrementing the DC to 500, and chief that the DC is still greater than 0. Therefore,
MDRR takes a added packet from chain 1, decrementing the DC to –500. MDRR again moves on
to chain 2, demography three packets, afterwards which the arrears adverse (DC) for chain 2 has decremented
to 0.
That concludes the aboriginal round-robin canyon through the queues. MDRR has taken 2000 bytes from
queue 1 and 3000 bytes from chain 2, giving the queues 40 percent and 60 percent of link
bandwidth, respectively.
In the added round-robin pass, apparent in the lower bisected of Amount 13-7, the activity begins by
MDRR abacus the QV for anniversary chain to the DC for anniversary queue. Chain 1’s DC becomes 1500 +
(–500), or 1000, to activate the added pass. During this pass, MDRR takes P3 from chain 1,
decrements DC to 0, and again moves on to chain 2. Afterwards demography three added packets from
queue 3, decrementing chain 2’s DC to 0, MDRR completes the added pass. Over these two
round-robin passes, MDRR has taken 3000 bytes from chain 1 and 6000 bytes from chain 2—
which is the aforementioned arrangement as the arrangement amid the QVs. In added words, MDRR has absolutely achieved
the configured bandwidth arrangement amid the two queues.
The arrears affection of MDRR provides a agency that, over time, gives anniversary chain a guaranteed
bandwidth based on the afterward formula:
For added examples of the operation of the MDRR arrears feature, accredit to http://
www.cisco.com/warp/public/63/toc_18841.html. Alternatively, you can go to Cisco.com and
search for “Understanding and Configuring MDRR and WRED on the Cisco 12000 Alternation Internet
Router.”
LAN Switch Congestion Management and Avoidance
The final area of this affiliate looks at the admission and departure queuing, SRR, WTD, and WRED
features on Cisco 3550 and 3560 switches.
QV for Chain X
Sum of All QVs