At Step 4 of the BGP path selection process, the router defaults to a simple rule: a shorter BGP AS-Path length is always preferred. Because of this, you can look at incoming AS-Paths to control how traffic leaves your fabric, or you can intentionally “lengthen” your own AS-Path via AS-Path prepending to influence how external networks route traffic back into your data center.
In this video, I’ll show you exactly how to manipulate the inbound and outbound traffic paths and verify the changes in the NX-OS BGP table.
Summary
| Component | Description | Example |
|---|---|---|
| AS_SEQ | An ordered list of ASNs through which the route has been advertised. Used to prevent BGP loops and in path selection, influenced by AS_PATH prepending inbound or outbound. | 100 200 300 |
| AS_SET | An unordered list of ASNs through which the route has been advertised. Used when doing summarization (aggregation) to prevent routing loops. | {100, 200, 300} |
| AS_CONFED_SEQ | Like AS_SEQ but holds only confederation ASNs. Used to prevent BGP loops inside a real AS when using confederation, but not inside a sub-AS. | (65001 65002 65003) |
| AS_CONFED_SET | Like AS_SET, but holds only confederation ASNs. Used when doing summarization (aggregation) inside confederations. | {65001 65002 65003} |
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Additional Resources:
BGP Path Attributes: Your BGP Path Selection Master Guide
