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Figure 2-7 Weighted fair queuing.

By: endeavor03

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Both C and E are FTP sessions, and they are high-volume traffic. A, B, and D are interactive sessions and they are low-volume traffic. Every session in this case is termed a conversation. If each conversation is serviced in a cyclic manner and gets a slot regardless of its arrival rate, the FTP sessions do not monopolize the bandwidth. Round trip delays for the interactive traffic, therefore, become predictable.

Weighted fair queuing provides an algorithmto identify data streams dynamically using an interface, and sorts them into separate logical queues. The algorithm uses various discriminators based on whatever network layer protocol information is available and sorts among them. For example, for IP traffic, the discriminators are source and destination address, protocol type, socket numbers, and TOS. This is how the two Telnet sessions (Sessions B and D) are assigned to different logical queues,as shown in Figure 2-7.

Ideally, the algorithm would classify every conversation that is sharing the wire so that each conversation receives its fair share of the bandwidth.Unfortunately,with such protocols as SNA, you 70-291

cannot distinguish one SNA session from another. For example, in DLSw+, SNA traffic is ultiplexed onto a single TCP session. Similarly in APPN, SNA sessions are multiplexed onto a ingle LLC2 session.

The weighted fair queuing algorithm treats these sessions as a single conversation. If you have many

TCP sessions, the TCP sessions get the majority of the bandwidth and the SNA traffic gets the minimum. For this reason, this algorithm is not recommended for SNA using DLSw+ TCP/IP encapsulation and APPN.

Weighted fair queuing, however, has many advantages over priority queuing and custom queuing.

Priority queuing and custom queuing require the installation of access lists; the bandwidth has to bepre-allocated and priorities have to be predefined. This is clearly a burden. Sometimes, network

administrators cannot identify and prioritize network traffic in real time.Weighted fair queuing sorts among individual traffic streams without the administrative burden associated with the other two types of queuing. 70-291 Exam

The easiest way to add bandwidth in a backbone network is to implement additional links. Routers provide built-in load balancing for multiple links and paths. You can use up to four paths to a destination network. In some cases, the paths need not be of equal cost.

Within IP, routers provide load balancing on both a per-packet and a per-destination basis. For per-destination load balancing, each router uses its route cache to determine the output interface. If IGRP or Enhanced IGRP routing is used, unequal-cost load balancing is possible. The router uses metrics to determine which paths the packets will take; the amount of load balancing can be adjusted

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This is how the two Telnet sessions (Sessions B and D) are assigned to different logical queues,as shown in Figure 2-7. MCSE Exam

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