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7.8.1.0 - Force10 Networks

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Figure 77 illustrates the choices parameters BGP goes through to select the best path. The list following the<br />

illustration details the path selection criteria.<br />

Figure 77 BGP Best Path Selection<br />

Largest<br />

Weight<br />

No, or Not Resulting in a Single Route<br />

Highest<br />

Local Pref<br />

Locally<br />

Originated<br />

Path<br />

Shortest<br />

AS Path<br />

Lowest<br />

Origin<br />

Code<br />

Lowest<br />

MED<br />

Learned<br />

via EBGP<br />

A Single Route is Selected and Installed in the Forwarding Table<br />

Best Path selection details<br />

Lowest<br />

NEXT-HOP<br />

Cost<br />

Tie Breakers<br />

Short<br />

Cluster<br />

List<br />

from<br />

Lowest<br />

BGP ID<br />

Lowest<br />

Peering<br />

Addr<br />

1. Prefer the path with the largest WEIGHT attribute.<br />

2. Prefer the path with the largest LOCAL_PREF attribute.<br />

• Prefer the path that was locally originated via a network command, redistribute command<br />

or aggregate-address command.<br />

• Routes originated via the network or redistribute commands are preferred over routes<br />

originated via the aggregate-address command.<br />

3. Prefer the path with the shortest AS_PATH (unless the bgp bestpath as-path ignore command is<br />

configured, then AS_PATH is not considered). The following criteria apply:<br />

• An AS_SET has a path length of 1, no matter how many ASs are in the set.<br />

132 Border Gateway Protocol

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