BGP Path Selection

• Best Path Selection:
  • Default chooses single best path.
  • Best path installed in RIB/FIB
  • Advertised to other BGP peers.
  • Decision process
    • RFC 4271
• Prerequisites:
  • Next hop must be in routing table
    • prevents route recursion failure.
  • Synchronization rule must be met or disabled.
  • AS-Path must not contain local-AS
    • Standard EBGP Loop prevention
    • Can be disabled with ‘allow-as in’
  • First ASN in path must be neighbor’s ASN
    • ‘bgp enforce-first-as’ command.

• Path Selection Order
  • Weight
    • Cisco proprietary
    • Locally significant
    • Higher value is preferred
  • Local Preference
    • Higher value is preferred
    • Not advertised to EBGP peers.
    • Carried through confederation EBGP.
  • Locally Originated
    • Locally originated gets weight of 32768
  • AS-Path
    • Smaller length is preferred
  • Origin
    • IGP over EBGP over Incomplete
      • IGP origination is from network statement.
      • Incomplete is from redistribution.
        • Network statement preferred over redistribution.
  • MED
    • Smaller value preferred.
    • Only compared for peerings to same provider by default.
      • Typically only used for comparing the same route from the same provider over multiple links.

Tie Breakers:

• EBGP over iBGP
  • If learned from EBGP, it’s not your prefix.
  • EBGP always preferred.
• IGP Metric to Next-Hop
  • Can use multi-path if all equal after this step.
  • Hidden command to allow multipath if AS_Path is not the same (has to be same length)
    • ‘bgp bestpath as-path multipath-relax’

• Additional Tie Breakers:
  • Oldest
  • Lowest RID
  • Shortest cluster list
  • Lowest Neighbor Address
• Exceptions:
  • ‘bgp bestpath as-path ignore’
  • ‘bgp always-compare-med’
    • Compares MED for routes locally originated in Confederation.
  • ‘bgp bestpath med missing-as-worst’
    • Assign 4,294,967,294 to NULL MED
  • ‘bgp deterministic med’
    • Compare MED against all possible paths.
  • IGP Metric
    • ‘bgp bestpath igp-metric ignore’
      • IOS-XE 3.4S
  • Router-ID
    • ‘no bgp bestpath compare-router-id’

Manipulating Best Path Selection

• Outbound routing policy affects inbound traffic
• Inbound routing policy affects outbound traffic
• Longest match routing is above all
  • Affects both directions.

• Attributes for influencing outbound path selection:
  • Weight and Local Pref
    • Set inbound
    • Affects outbound traffic

• Attributes for influencing inbound path selection:
  • AS-Path and MED
    • Set outbound
    • Affects inbound traffic

• Multipath Load Balancing
  • MP load balancing for external links with unequal bandwidth
    • Enabled for IPv4, IPv6, VPNv4, VRF AF
    • For iBGP, eBGP, eiBGP
  • Still only one best path advertised to peers.