Configuring WRED for Frame Relay
This section demonstrates the configuration tasks required to enable WRED for Frame Relay. WRED can be configured at the interface level or within the policy-map to initiate the WRED drop mechanism on a per-class basis for CBWFQ or LLQ.
Enabling WRED on an Interface
To enable WRED on an interface, use the following commands, beginning in the global configuration mode:
Step 1. Enter the interface configuration mode of the Frame Relay interface on which you want to enable WRED.
Step 2. Enable WRED with the random-detect interface configuration command.
Step 3. (optional) Change the weight factor used in calculating the average queue length with random-detect exponential-weighting-constant exponent. An integer value between 1 and 16 is accepted.
Step 4. (optional) Use the random-detect precedence precedence min-threshold max-threshold mark-prob-denominator interface configuration command to configure the parameters for packets with a specific IP precedence value. By default, the minimum threshold for IP precedence 0 corresponds to half the maximum threshold for the interface. The default output queue size is 40. Use this command to configure the parameter for each IP precedence value. Note that RED can be enabled by configuring the same values for each IP precedence class.
Step 5. (optional) To enable flow-based WRED, first enable WRED and then flow-based WRED with random-detect flow interface configuration command. To adjust the flow threshold multiplier, use the random-detect flow average-depth-factor scaling-factor interface configuration command. To adjust the maximum flow count, use the random-detect flow count number interface configuration command.
Enabling WRED in a Traffic Policy
To enable WRED in a traffic policy within a CBWFQ structure, use the following commands, beginning in global configuration mode:
Step 1. Enter the policy-map configuration mode by specifying the name of the traffic policy with the policy-map policy-map global configuration command.
Step 2. In the policy-map, specify the name of traffic class with the class class-name command.
Step 3. In the policy-map-class configuration mode, enable WRED with the random-detect command.
Step 4. (optional) You can configure the parameters for IP precedence in WRED or turn on flow-based WRED using the same commands shown in the previous section, which described configuring WRED for the interface.
Scenario: Comparison of Tail Drop and WRED for Frame Relay
Figure 21-2 shows the network topology used in this scenario to observe the difference between Tail Drop and WRED for Frame Relay.
Figure 21-2. Network to Verify Tail Drop and WRED
[View full size image]
In Figure 21-2, routers R1 and R2 are connected to a low-speed Frame Relay network. The access speed of the Frame Relay connection is deliberately set to a slow 9600 bps so that congestion can be speedily built up and observed at router R1's egress interface. Two PCs are connected to the LAN segments attached to the routers, as shown in the diagram. The PCs are running a TCP Session Emulator application. One of them simulates a TCP client, and the other acts as a TCP server. When the Frame Relay is up and running, TCP connections are established between the PCs. This creates the TCP traffic required to achieve congestion, allowing the observation of WRED and Tail Drop in action.
The configurations of the routers in Figure 21-2 are shown in Example 21-3.
Example 21-3. Configurations of the Routers in Figure 21-2
! Router R1
<output omitted>
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.0
!
interface Serial4/3
no ip address
encapsulation frame-relay
no fair-queue
!
interface Serial4/3.102 point-to-point
ip address 172.16.1.1 255.255.255.252
frame-relay interface-dlci 102
!
router eigrp 1
network 10.0.0.0 0.0.0.255
network 172.16.1.0 0.0.0.3
no auto-summary
! Router R2
<output omitted>
interface FastEthernet0/1
ip address 10.0.1.1 255.255.255.0
!
interface Serial2/2
no ip address
encapsulation frame-relay
no fair-queue
!
interface Serial2/2.201 point-to-point
ip address 172.16.1.2 255.255.255.252
frame-relay interface-dlci 201
!
router eigrp 1
network 10.0.1.0 0.0.0.255
network 172.16.1.0 0.0.0.3
no auto-summary
The TCP Session Emulator running at the PCs is used to generate TCP traffic onto the network to emulate TCP session flows between a TCP client and a TCP server. The dozens of TCP connections initiated are sufficient to flood the slow 9.6 kbps Frame Relay link.
In the next section, when the TCP sessions are initiated between the PCs, observe the behavior of Tail Drop and WRED in response to a spike in traffic on router R2's serial interface.
Enabling WRED on an Interface
To enable WRED on an interface, use the following commands, beginning in the global configuration mode:
Step 1. Enter the interface configuration mode of the Frame Relay interface on which you want to enable WRED.
Step 2. Enable WRED with the random-detect interface configuration command.
Step 3. (optional) Change the weight factor used in calculating the average queue length with random-detect exponential-weighting-constant exponent. An integer value between 1 and 16 is accepted.
Step 4. (optional) Use the random-detect precedence precedence min-threshold max-threshold mark-prob-denominator interface configuration command to configure the parameters for packets with a specific IP precedence value. By default, the minimum threshold for IP precedence 0 corresponds to half the maximum threshold for the interface. The default output queue size is 40. Use this command to configure the parameter for each IP precedence value. Note that RED can be enabled by configuring the same values for each IP precedence class.
Step 5. (optional) To enable flow-based WRED, first enable WRED and then flow-based WRED with random-detect flow interface configuration command. To adjust the flow threshold multiplier, use the random-detect flow average-depth-factor scaling-factor interface configuration command. To adjust the maximum flow count, use the random-detect flow count number interface configuration command.
Enabling WRED in a Traffic Policy
To enable WRED in a traffic policy within a CBWFQ structure, use the following commands, beginning in global configuration mode:
Step 1. Enter the policy-map configuration mode by specifying the name of the traffic policy with the policy-map policy-map global configuration command.
Step 2. In the policy-map, specify the name of traffic class with the class class-name command.
Step 3. In the policy-map-class configuration mode, enable WRED with the random-detect command.
Step 4. (optional) You can configure the parameters for IP precedence in WRED or turn on flow-based WRED using the same commands shown in the previous section, which described configuring WRED for the interface.
Scenario: Comparison of Tail Drop and WRED for Frame Relay
Figure 21-2 shows the network topology used in this scenario to observe the difference between Tail Drop and WRED for Frame Relay.
Figure 21-2. Network to Verify Tail Drop and WRED
[View full size image]
In Figure 21-2, routers R1 and R2 are connected to a low-speed Frame Relay network. The access speed of the Frame Relay connection is deliberately set to a slow 9600 bps so that congestion can be speedily built up and observed at router R1's egress interface. Two PCs are connected to the LAN segments attached to the routers, as shown in the diagram. The PCs are running a TCP Session Emulator application. One of them simulates a TCP client, and the other acts as a TCP server. When the Frame Relay is up and running, TCP connections are established between the PCs. This creates the TCP traffic required to achieve congestion, allowing the observation of WRED and Tail Drop in action.
The configurations of the routers in Figure 21-2 are shown in Example 21-3.
Example 21-3. Configurations of the Routers in Figure 21-2
! Router R1
<output omitted>
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.0
!
interface Serial4/3
no ip address
encapsulation frame-relay
no fair-queue
!
interface Serial4/3.102 point-to-point
ip address 172.16.1.1 255.255.255.252
frame-relay interface-dlci 102
!
router eigrp 1
network 10.0.0.0 0.0.0.255
network 172.16.1.0 0.0.0.3
no auto-summary
! Router R2
<output omitted>
interface FastEthernet0/1
ip address 10.0.1.1 255.255.255.0
!
interface Serial2/2
no ip address
encapsulation frame-relay
no fair-queue
!
interface Serial2/2.201 point-to-point
ip address 172.16.1.2 255.255.255.252
frame-relay interface-dlci 201
!
router eigrp 1
network 10.0.1.0 0.0.0.255
network 172.16.1.0 0.0.0.3
no auto-summary
The TCP Session Emulator running at the PCs is used to generate TCP traffic onto the network to emulate TCP session flows between a TCP client and a TCP server. The dozens of TCP connections initiated are sufficient to flood the slow 9.6 kbps Frame Relay link.
In the next section, when the TCP sessions are initiated between the PCs, observe the behavior of Tail Drop and WRED in response to a spike in traffic on router R2's serial interface.
