BGP Best Path Selection Algorithm
BGP Best Path Selection Algorithm
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html
BGP Cost Community
https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/s_bgpcc.html
Why Routers Ignore Paths:
– path are marked “not synchronized” in sh ip bgp long-prefixes output
– next hop is inaccessible
– path from eBGP neighbor has local AS in AS_PATH
– bgp enforce-first-as is set and the update doesn’t list the neighbor AS as the first AS
– paths are marked “receive only” in sh ip bgp long-prefixes output
Best Path Algorithm:
Note: if extended community cost is configured for pre-bestpath, compare before anything else
-> lowest is best
-> set extcommunity cost pre-bestpath <community-id> <cost>
-> bgp bestpath cost-community ignore
-> this command was intended only to assist in troubleshooting path selection
– Important, next hop needs to be valid
– It’s not that difficult to remember whether higher or lower is best. With the first two (weight and local_preference), highest is best. Everything else, lowest, shortest, or oldest is best.
– The steps below include the configuration commands that can modify the default behavior of each step.
1. weight
-> highest is best
-> Cisco proprietary
-> locally significant
-> range 0 to 65,535
-> default is 0 for received routes
-> default is 32768 for locally generated routes
-> route-map WEIGHT permit 10
-> set weight 40000
-> neighbor 1.1.1.1 route-map WEIGHT in
-> neighbor 1.1.1.1 default-originate route-map WEIGHT in
-> network 1.1.1.0 mask 255.255.255.0 route-map WEIGHT
-> This works. As does applying the route-map to the redistribute and aggregate-address commands. But why would you do this to locally originated routes since it’s only locally significant.
2. local_preference
-> highest is best
-> significant only in the local AS
-> range 0 to 4294967295
-> default 100 for every route
-> bgp default local-preference 200
-> route-map LP permit 10
-> set local-preference 200
-> neighbor 1.1.1.1 route-map LP in
-> neighbor 1.1.1.1 default-originate route-map LP in
-> redistribute eigrp 1 route-map LP
-> redistribute ospf 1 route-map LP match internal external 1 external 2
-> network 1.1.1.0 mask 255.255.255.0 route-map LP
-> aggregate-address 1.1.2.0 255.255.255.252 attribute-map LP
3. locally originated path
-> locally generated routes are preferred over received routes
-> network command
-> aggregate-address command
-> redistribute command
4. AS_PATH
-> shortest is best
-> bgp bestpath as-path ignore
-> route-map PREPEND permit 10
-> set as-path prepend 100 100
-> neighbor 1.1.1.1 route-map PREPEND out
5. ORIGIN
-> lowest type is best
-> IGP < EGP < INCOMPLETE
6. multi-exit discriminator (MED)
-> metric
-> lowest is best
-> range 0 to 65,535
-> default is 0 for received routes
-> when MED is not assigned
-> set to the value of the IGP metric for routes injected into BGP
-> network or redistribute commands
-> used to influence a neighboring AS with multiple entry points
-> or a neighboring AS may be influencing the local AS with multiple entry points
-> compared only if the first AS in the AS_SEQUENCE is the same for multiple paths
-> confederation sub-ASs are ignored unless configured
-> bgp deterministic-med
-> compare routes advertised by different peers in the same AS
-> bgp always-compare-med
-> compare routes advertised by different ASs.
-> if used, must be configured in the entire AS to prevent routing loops
-> bgp bestpath med confed
-> compare all paths that consist only of AS_CONFED_SEQUENCE
-> bgp bestpath med confed missing-as-worst
-> bgp bestpath med missing-as-worst
-> route-map MED permit 10
-> set metric 100
-> neighbor 1.1.1.1 route-map MED out
-> neighbor 1.1.1.1 default-originate route-map MED out
-> redistribute eigrp 1 metric 100
-> redistribute ospf 1 metric 100 match internal external 1 external 2
-> redistribute eigrp 1 route-map MED
-> redistribute ospf 1 route-map MED match internal external 1 external 2
-> network 1.1.1.0 mask 255.255.255.0 route-map MED
-> aggregate-address 1.1.2.0 255.255.255.252 attribute-map MED
7. eBGP over iBGP
-> paths that contain as_confed_sequence and as_confed_set are considered internal
8. IGP metric
-> lowest is best
-> bgp bestpath igp-metric ignore
9. extended community cost
-> IGP point of insertion (POI)
-> lowest is best
-> route-map EXTCOMMUMITYCOST permit 10
-> set extcommunity cost 1 1
-> route-map EXTCOMMUMITYCOST permit 10
-> set extcommunity cost igp 1 1
-> neighbor 1.1.1.1 route-map EXTCOMMUMITYCOST out
-> neighbor 1.1.1.1 default-originate route-map EXTCOMMUMITYCOST out
-> redistribute eigrp 1 route-map EXTCOMMUMITYCOST
-> redistribute ospf 1 route-map EXTCOMMUMITYCOST match internal external 1 external 2
-> network 1.1.1.0 mask 255.255.255.0 route-map EXTCOMMUMITYCOST
-> aggregate-address 1.1.2.0 255.255.255.252 attribute-map EXTCOMMUMITYCOST
10. multipath
-> if multiple paths match the above steps,
-> determine if multiple paths require installation into the routing table
-> default, only one best path is installed into the routing table
-> maximum-paths 2
-> maximum-paths eibgp 2
-> maximum-paths ibgp 2
11. oldest route
-> when multiple paths are external, prefer the path that was received first
-> oldest is best
-> this step can be skipped by forcing BGP to compare router-ids
-> bgp bestpath compare-router-id
12. router-id
-> lowest is best
13. cluster length
-> lowest is best
-> router reflectors only
14. neighbor address
-> lowest is best
