Weighted RED (WRED) 441

being least likely to be dropped. Note, for instance, that the settings for assured forwarding (AF) DSCPs AF11, AF21, AF31, and AF41 are all identical. For the same reason, AF12, AF22, AF32, and AF42 have the same defaults, as do AF13, AF23, AF33, and AF43.

WRED and Queuing

WRED relies on the average queue depth concept, which calculates a rolling average of the queue depth of some queue. But which queue? Well, first consider a serial interface on a router, on which Weighted Fair Queuing (WFQ) is enabled by default. In this case, however, WFQ has been disabled, leaving a single first-in, first-out (FIFO) output queue on the interface. Figure 6-9 shows the basic idea.

Figure 6-9 FIFO Output Queue and WRED Interaction

Output Interface on a Router

Average Queue Depth Based on

Class Queue Actual Depth

Packet

Discarded

As was covered in depth in Chapter 4, “Congestion Management,” each interface has a TX Queue or TX Ring. If the TX Ring/TX Queue fills, IOS places new packets into the software queue(s) awaiting transmission. In this example, a single FIFO output queue is used, as shown. With WRED also enabled, WRED calculates the average queue depth of the single FIFO output queue. As new packets arrive, before being placed into the FIFO output queue, WRED logic decides whether the packet should be discarded, as described in detail earlier in this chapter.

With WRED enabled directly on a physical interface, IOS supports FIFO Queuing, and FIFO Queuing only! That fact certainly makes the explanation easier, because there is less to cover! So, WRED works just like Figure 6-9 when it is enabled directly on a physical interface, because WRED can only work with a single FIFO queue in that case.

You might recall that of all the queuing tools listed in Chapter 4, CBWFQ and Low Latency Queuing (LLQ, which is merely a variation of CBFWQ) are the only queuing tools that claim to be capable of using WRED. To use WRED with CBWFQ or LLQ, you need to configure

Weighted RED (WRED) 443

WRED Configuration

WRED requires very little configuration if you want to take the IOS defaults for the various tunable settings, such as per-precedence and per-DSCP thresholds. If you want to change the defaults, the configuration details can become quite large.

This section begins with a table of configuration commands (Table 6-6) and show commands (Table 6-7), followed by three separate examples.

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Example 6-1 WRED Default Configuration, R3, S0/0 (Continued)

class class-default set ip dscp default

!

call rsvp-sync

!

interface Ethernet0/0

description connected to SW2, where Server1 is connected ip address 192.168.3.253 255.255.255.0

ip nbar protocol-discovery half-duplex

service-policy input laundry-list

!

interface Serial0/0

description connected to FRS port S0. Single PVC to R1. no ip address

encapsulation frame-relay load-interval 30 random-detect

clockrate 128000

!

interface Serial0/0.1 point-to-point

description point-point subint global DLCI 103, connected via PVC to DLCI 101 ( R1)

ip address 192.168.2.253 255.255.255.0 frame-relay interface-dlci 101

!

! Lines omitted for brevity.

!

R3#show queueing interface serial 0/0

Interface Serial0/0 queueing strategy: random early detection (WRED)

Exp-weight-constant: 9 (1/512)

Mean queue depth: 37

R3#show queue s 0/0

Output queue for Serial0/0 is 57/0

Packet 1, linktype: ip, length: 64, flags: 0x88

source: 192.168.3.254, destination: 192.168.2.251, id: 0x053E, ttl: 253, TOS: 184 prot: 17, source port 18378, destination port 17260

Weighted RED (WRED) 447

Example 6-1 WRED Default Configuration, R3, S0/0 (Continued)

data: 0x47CA 0x436C 0x0028 0x0000 0x8012 0x3F73 0x4C7E 0x8D44 0x18D1 0x03FE 0xFC77 0xA2A7 0x35A2 0x54E7

Packet 2, linktype: ip, length: 64, flags: 0x88

source: 192.168.3.254, destination: 192.168.2.251, id: 0x0545, ttl: 253, TOS: 184 prot: 17, source port 16640, destination port 17178

data: 0x4100 0x431A 0x0028 0x0000 0x8012 0x6330 0x21B4 0x82AF 0x05C9 0x03FE 0x1448 0x8706 0xAFD9 0xD364

!

! Output omitted for brevity.

!

R3#show interfaces s 0/0

Serial0/0 is up, line protocol is up Hardware is PowerQUICC Serial

Description: connected to FRS port S0. Single PVC to R1. MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 20/255, rxload 4/255 Encapsulation FRAME-RELAY, loopback not set

Broadcast queue 0/64, broadcasts sent/dropped 2726/0, interface broadcasts 252

2

Last input 00:00:02, output 00:00:00, output hang never Last clearing of "show interface" counters 01:38:28

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 4391 Queueing strategy: random early detection(RED)

30 second input rate 29000 bits/sec, 58 packets/sec

30 second output rate 122000 bits/sec, 91 packets/sec

23863 packets input, 1535433 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

2 input errors, 0 CRC, 2 frame, 0 overrun, 0 ignored, 0 abort 36688 packets output, 5638653 bytes, 0 underruns

0 output errors, 0 collisions, 4 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=up DSR=up DTR=up RTS=up CTS=up

The WRED part of the configuration is quite short. The configuration shows the random-detect interface subcommand under serial 0/0. As you will see later, the command actually disables WFQ if configured. The rest of the highlighted configuration commands show the CB marking configuration, which implements the functions listed before the example. (For more information about CB marking, see Chapter 3, “Classification and Marking.”)

After the configuration, the show queueing interface serial 0/0 command output lists the WRED settings, and the statistics for each precedence value. The defaults for the exponential

448 Chapter 6: Congestion Avoidance Through Drop Policies

weighting constant, and the per-precedence defaults of minimum threshold, maximum threshold, and MPD are all listed. In addition, the command lists statistics for bytes/packets dropped by WRED, per precedence value. For those of you who did not memorize the DSCP values, you may not be able to correlate the DSCP values set by CB marking, and the precedence values interpreted by WRED. WRED just looks at the first 3 bits of the IP ToS byte when performing precedence-based WRED. So, DSCP best effort (BE) equates to precedence 0, DSCP AF21 and AF23 both equate to precedence 2, and DSCP expedited forwarding (EF) equates to precedence 5.

The show queueing command also lists a column of statistics for tail drop as well as random drops. In this example, WRED has dropped several packets, and the queue has filled, causing tail drops, as shown with the nonzero counters for random drops and tail drops in the show queueing command output.

The show queue serial 0/0 command lists the same type of information seen in earlier chapters. However, this command lists one particularly interesting item relating to WRED in the first line of the command output. The actual queue depth for the single FIFO queue used with WRED is listed at 57 entries in this particular example. The earlier show queueing command lists an average queue depth of 37 just instants before. These two numbers just give us a small reminder that WRED decides to drop based on average queue depth, as opposed to actual queue depth.

Finally, the show interfaces command at the end of the example reminds us that WRED does not work with any other queuing method directly on the interface. The command uses the statement “Queueing strategy: random early detection (RED)” to remind us of that fact. WRED uses a single FIFO queue, and measures its average queue depth based on the queue depth of the FIFO queue.

The second WRED configuration example uses WRED on R3’s S0/0 interface again, but this time with DSCP WRED, and a few changes to the defaults. In fact, Example 6-2 just shows the changed configuration, with most of the configuration staying the same. For instance, the same CB marking configuration is used to mark the traffic, so the details are not repeated in the example. The example uses the familiar network diagram that was also used in the preceding example.

Example 6-2 DSCP-Based WRED on R3 S0/0

R3#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Weighted RED (WRED) 449

Example 6-2 DSCP-Based WRED on R3 S0/0 (Continued)

source: 192.168.3.254, destination: 192.168.2.251, id: 0x0545, ttl: 253, TOS: 184 prot: 17, source port 16640, destination port 17178

data: 0x4100 0x431A 0x0028 0x0000 0x8012 0xAB15 0x21E1 0x71CF 0x05C9 0x03FE 0x7AA3 0x770B 0x2408 0x8264

Packet 2, linktype: ip, length: 64, flags: 0x88

source: 192.168.3.254, destination: 192.168.2.251, id: 0x053E, ttl: 253, TOS: 184 prot: 17, source port 18378, destination port 17260

R3#show queueing interface s 0/0

Interface Serial0/0 queueing strategy: random early detection (WRED)

Exp-weight-constant: 5 (1/32)

Mean queue depth: 38

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The configuration begins with a change from precedence-based WRED to DSCP-based WRED using the random-detect dscp-based interface subcommand. The random-detect dscp af21 50 60 changes the default minimum and maximum thresholds for AF21 to 50 and 60, respectively, with the random-detect dscp af23 20 30 changing these same values for AF23. In addition, although Cisco does not recommend changing the exponential weighting constant, the configuration does offer an example of the syntax with the random-detect exponential- weighting-constant 5 command. By setting it to a smaller number than the default (9), WRED will more quickly change the average queue depth calculation, more quickly reacting to changes in the queue depth.

The command output from the various show commands do not differ much compared to when DSCP-based WRED is enabled. The format now includes DSCP values rather than precedence values, as you may notice with the counters that point out drops for both AF21 and AF23, which were previously both treated as precedence 2.

WRED suffers from the lack of concurrent queuing tool support on an interface. However, WRED can be enabled inside a CBWFQ class, operating on the queue for the class, effectively enabling WRED concurrently with CBWFQ. The final WRED example shows a configuration for WRED using LLQ.

The last WRED configuration example repeats base configuration similar to one of the CBWFQ examples from Chapter 4. Voice, HTTP, and FTP traffic compete for the same bandwidth, with WRED applied per-class for the two HTTP classes and one FTP class. Note that because voice traffic is drop sensitive, WRED is not enabled for the low-latency queue. Because WRED can be enabled per class in conjunction with CBWFQ, WRED calculates average queue depth based on the per-class queue.

The criteria for each type of traffic is as follows:

•R3’s S0/0 is clocked at 128 kbps.

•VoIP payload is marked with DSCP EF using CB marking on ingress to R3 E0/0. On egress of R3’s S0/0 interface, DSCP EF traffic is placed in its own queue, without WRED. This class gets 58 kbps.

•Any HTTP traffic whose URL contains the string “important” anywhere in the URL is marked with AF21 using CB marking on ingress to R3 E0/0. On egress of R3’s S0/0 interface, DSCP AF21 traffic is placed in its own queue, with WRED. This class gets 20 kbps.

•Any HTTP traffic whose URL contains the string “not-so” anywhere in the URL is marked with AF23 using CB marking on ingress to R3 E0/0. On egress of R3’s S0/0 interface, DSCP AF23 traffic is placed in its own queue, with WRED. This class gets 8 kbps.

•All other traffic is marked with DSCP BE using CB marking on ingress to R3 E0/0. On egress of R3’s S0/0 interface, DSCP BE traffic is placed in its own queue, with WRED. This class gets 20 kbps.

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Example 6-3 WRED Used in LLQ Classes dscp-af21, dscp-af23, and class-default (Continued)

policy-map queue-on-dscp class dscp-ef

priority 58 class dscp-af21 bandwidth 20

random-detect dscp-based class dscp-af23

bandwidth 8 random-detect dscp-based

class class-default fair-queue

random-detect dscp-based

!

interface Ethernet0/0

description connected to SW2, where Server1 is connected ip address 192.168.3.253 255.255.255.0

ip nbar protocol-discovery half-duplex

service-policy input laundry-list

!

interface Serial0/0

description connected to FRS port S0. Single PVC to R1. bandwidth 128

no ip address encapsulation frame-relay load-interval 30 max-reserved-bandwidth 85

service-policy output queue-on-dscp clockrate 128000

!

interface Serial0/0.1 point-to-point

description point-point subint global DLCI 103, connected via PVC to DLCI 101 ( R1)

ip address 192.168.2.253 255.255.255.0 frame-relay interface-dlci 101

!

R3#show policy-map interface serial 0/0

Serial0/0

Service-policy output: queue-on-dscp

Class-map: dscp-ef (match-all) 46437 packets, 2971968 bytes

30 second offered rate 0 bps, drop rate 0 bps Match: ip dscp ef

Weighted Fair Queueing Strict Priority

Output Queue: Conversation 264 Bandwidth 58 (kbps) Burst 1450 (Bytes)

454 Chapter 6: Congestion Avoidance Through Drop Policies

Example 6-3 WRED Used in LLQ Classes dscp-af21, dscp-af23, and class-default (Continued)

(depth/total drops/no-buffer drops) 11/46/0 exponential weight: 9

mean queue depth: 5