3Com Router User Manual

50
CONGESTION AVOIDANCE
This chapter covers the following topics:
■Congestion Avoidance Overview
■WRED Configuration
■Displaying and Debugging Congestion Avoidance
■Congestion Avoidance Configuration Example 
Congestion Avoidance 
OverviewThe purpose of the congestion avoidance technology is to monitor the network 
traffic flow, predict the congestion and effectively prevent the congestion 
occurring at the bottleneck of the network. In a number of the congestion 
avoidance mechanisms, Random Early...

698CHAPTER 50: CONGESTION AVOIDANCE
and it causes the sudden increase and decrease of the network traffic, and the line 
traffic always fluctuates between the states of few or none and full.
RED and WREDRED and WRED can avoid global synchronization of TCP by dropping packets 
randomly. When the packets of a TCP connection are dropped, and transmission 
slows down, other TCP connections can still send packets at high rates, thus 
improving the utilization of the bandwidth.
RED and WRED avoids the TCP...

WRED Configuration699
■When the length of the queue is between the minimum threshold and 
maximum threshold, the WRED algorithm is used to calculate and determine 
whether the packet is dropped. The specific method is that each incoming 
packet is allocated with a random number, which is compared with the drop 
probability of the current queue, if it is larger than the drop probability, the 
packet is dropped. The longer the queue is, the higher the drop probability 
is--but there is a maximum drop...

700CHAPTER 50: CONGESTION AVOIDANCE
exponent is the filtering coefficient for calculating the average queue length, and 
the range of the value is 1 to 16, and the default value is 9.
When exponent=0 and the queue length exceeds the threshold, WRED will act 
accordingly. When exponent is higher, WRED will act slowly to the change of 
queue status.
This configuration should be performed after enabling WRED in the interface view.
Set the Priority 
Parameters for WREDYou can set WRED drop lower threshold...

Displaying and Debugging Congestion Avoidance701
Displaying and 
Debugging 
Congestion AvoidanceTa b l e 741   Display and Debug Congestion Avoidance
Congestion Avoidance 
Configuration 
Example 
1Configure a WFQ queue.
[Router] interface ethernet 0
[Router-Ethernet0] qos wfq
2Enable WRED.
[Router-Ethernet0] qos wred
3Configure the exponent to calculate the average WRED queue length.
[Router-Ethernet0] qos wred weighting-constant 1
4Configure the lower threshold, upper threshold, and drop probability...

702CHAPTER 50: CONGESTION AVOIDANCE 

XII
DIAL-UP
Chapter 51Configuring DCC
Chapter 52Configuring Modem 

704 

51
CONFIGURING DCC 
This chapter covers the following topics:
■DCC Overview
■Configuring DCC 
■Displaying and Debugging DCC
■DCC Configuration Examples
■Troubleshooting DCC
DCC OverviewDial Control Center (DCC) is the routing technique adopted when the routers 
interconnect via a PSTN (Public Switched Telephone Network) or ISDN (Integrated 
Services Digital Network). In DCC, the routers are interconnected through PSTN.  
The connections are established through dialing when data transmissions are...

706CHAPTER 51: CONFIGURING DCC
DCC Configuration 
Methods3Com routers provide two DCC configuration methods: circular DCC, and 
resource-shared DCC. With distinguishing features, these two methods are 
applicable to different applications. In applications, the participating parties of a 
call can flexibly select either method as needed. In other words, one party can 
adopt circular DCC while the other party adopt resource-shared DCC to originate 
a call. 
Circular DCC
Circular DCC has the following...