027-BGP - Border Gateway Protocol

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Introduction to BGP: 

• Path Vector protocol
• Administrative Distance: eBGP = 20 , iBGP = 200
• Protocol: IP , TCP port 179
• Authentication : MD5

BGP Attributes Categories

• Well-known mandatory (WM) - attributes must be supported and included in routing updates
• Well-known discretionary (WD) - attributes must be supported but may not be included routing updates
• Optional transitive (OT) - attributes don't have to be supported, but Marked as partial & passed onto peers
• Optional nontransitive (ON) - attributes don't have to be supported, and can be ignored.

BGP Attributes

1. Weight (O)- Cisco proprietary, a 16-bit value used only by local router.
2. Origin (WM) - The source of the route (IGP > EGP > unknown)
3. AS Path (WM) - An ordered list of the ASs the route has traversed
4. Next Hop (WM) - Specifies the next-hop address for the route
5. Local Preference (WD) - Communicated between iBGP peers to favor a route out of the AS
6. Multi Exit Discriminator (ON) - Advertised to eBGP peers to indicate a preferred entrance into the local AS
7. Atomic Aggregate (WD) - Notes that route summarization has been performed
8. Aggregator (OT) - Identifies the router and AS where summarization was performed
9. Community (OT) - Provides route tagging capability
10. Originator ID (ON) - Identifies a route reflector
11. Cluster List (ON) - Records the route reflector clusters the route has traversed

 NOTE:

An AS Path can be one of two types as distinguished by its type code:

• AS Sequence - An ordered list
• AS Set - An unordered list

An AS Set is used to record AS numbers lost when aggregation is performed. The Atomic Aggregate

attribute does not have to be included to indicate aggregation has been performed if an AS Set is

included.

BGP Path Selection Process:

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