Setting Local Preference with Route Maps

This topic describes how to use route maps to set the BGP local preference attribute.

This topic describes how to use route maps to set the BGP local preference attribute.

60% Bandwidth Utilization

60% Bandwidth Utilization

AS 65001

10% Bandwidth'

Utilization 75% Bandwidth Utilization

AS 65004

10% Bandwidth'

Utilization 75% Bandwidth Utilization

20% Bandwidth Utilization

20% Bandwidth Utilization

BGP is designed for manipulating routing paths.

Unlike local routing protocols, BGP was never designed to choose the quickest path. BGP was designed to manipulate traffic flow to maximize or minimize bandwidth use. This figure demonstrates a common situation that can result when you are using BGP without any policy manipulation.

Using default settings for path selection in BGP can cause uneven use of bandwidth. In the figure, router A in AS 65001 is using 60 percent of its outbound bandwidth to router X in 65004, but router B is using only 20 percent of its outbound bandwidth. If this utilization is acceptable to the administrator, then no manipulation is needed.

But if the load averages 60 percent and has temporary bursts above 100 percent of the bandwidth, this situation will cause lost packets, higher latency, and higher CPU usage because of the number of packets being routed. When another link to the same locations is available and is not heavily used, it makes sense to divert some of the traffic to the other path. To change outbound path selection from AS 65001, the network administrator must manipulate the local preference attribute.

To determine which path to manipulate, the administrator performs a traffic analysis on Internet-bound traffic by examining the most heavily visited addresses, web pages, or domain names. This information can usually be found by examining network management records or firewall accounting information.

6-104 Building Scalable Cisco Internetworks (BSCI) v3.0

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