Wednesday, May 4, 2011

BGP Path Attributes and Route Selection

BGP Path Attributes

Mandatory Well-Known
 Origin (i, e, ?)
 AS-Path (sequence of AS-Numbers to access network/IP)
 Next-Hop (ip address)
Discretionary Well-Known
 Local Preference (for routing policy)
 Atomic Aggregate (flags route if aggregated)
Optional Non-Transitive
 Multi-Exit Discriminator (MED) - multiple entry pts to one AS
 Originator-ID - for route reflector environment
 Cluster-List - for route reflector environment
Optional Transitive
 Aggregator - IP address & AS of routers that aggregated routes
 Community - for route tagging

BGP Route Selection Order:
  1. Prefer highest weight (local to router)
  2. Prefer highest local preference (global within AS)
  3. Prefer routes that the router originated
  4. Prefer shorter AS paths
  5. Prefer lowest origin code (IGP < EGP < Incomplete)
  6. Prefer lowest MED
  7. Prefer external (EBGP) paths over internal (IBGP)
    1. For IBGP paths, prefer nonreflected routes (no originator-ID) over reflected routes
    2. For reflected routes, prefer shorter cluster-list
  8. For IBGP paths, prefer path through closest IGP neighbor
  9. For EBGP paths, prefer oldest (most stable) path
  10. Prefer paths from router with the lower BGP router-ID

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