BGP Best Path Selection – Cisco

   January 15, 2008  Leave a comment

1. Largest Weight

– Cisco proprietary. Only valid on the router it is configured on. It is not passed to other routers.


2. Highest Local Preference

– Highest Local Pref is preferred. Local Pref does not get passed between different AS’s
– Due to the above reason, customers are sending/using BGP community string with their
announcements
– Local Pref does get passed between confederations (sub-AS’s).

3. Locally Originated

– Prefer the path that was locally originated via a network or aggregate BGP subcommand, or through redistribution from an IGP. Local paths sourced by network or redistribute commands are preferred over local aggregates sourced by the aggregate-address command.

4. Shortest AS Path

– An AS_SET counts as 1, no matter how many ASs are in the set
– The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the AS_PATH length.
– (**Confederation hops ARE NOT counted as hops)

5. Lowest Origin Type

– Prefer the path with the lowest origin type: IGP is lower than EGP, and EGP is lower than
INCOMPLETE. (IGP < EGP < Incomplete)
– IGP origin is created when there is a ‘network statement’
– EGP origin is created via ‘redistribute ’
– Incomplete origin is created via ‘redistribute ’

6. Lowest MED(multi-exit discriminator)

– Only compared when the AS path entering our network is the same. EX. 777 2 33 and 777 2 44
– metrics compared….777 2 44 and 777 3 44 – metrics NOT compared. NOTE – MED is different than IGP metric. MED is assigned by the end-user, the IGP metric is determined via the ISIS settings on our backbone.
**Unlike local preference, metric is exchanged between ASs. A metric is carried into an AS but does not leave the AS

7. eBGP over iBGP

– routes are preferred over iBGP routes. eBGP routes are those learned via eBGP sessions (ie different AS’s). iBGP routes are learned via iBGP sessions (same AS).

8. Lowest IGP metric

– IGP metrics are the internal routing protocols that BGP uses to reach the next-hop ip address (MCI backbone uses ISIS and MPLS).
– Prefer the path with the lowest IGP metric to the BGP next hop. Continue, even if bestpath is already selected.

9. Maximum paths

– Check if multiple paths need to be installed in the routing table for BGP Multipath. continue, if bestpath is not selected yet

10. Oldest one(External)

– When both paths are external, prefer the path that was received first (the oldest one). This step minimizes route-flap, since a newer path will not displace an older one, even if it would be the preferred route based on the next decision criteria.

11. Lowest Router ID

– Prefer the route coming from the BGP router with the lowest router ID
– If a path contains route-reflector (RR) attributes, the originator ID is substituted for the router ID in the path selection process.

– The router ID is the highest IP address on the router, with preference given to loopback addresses. It can also be set manually using the bgp router-id command.

12. Minimum Cluster ID/list length

– This will only be present in BGP route-reflector environments

13. Lowest IP

– Prefer the path coming from the lowest neighbor address. This is the IP address used in the BGP neighbor configuration, and corresponds to the remote peer used in the TCP connection with the local router.


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