BGP Attributes comparaison

BGP attributes comparaison table

Characteristic	Attribute	Shared between	Value	BGP prefers	Significance	Config
well-known, mandatory	Next hop		by default not changed by IBGB
well-known, mandatory	Origin		i=IGP, INCOMPLETE=redistribution	IGP then EGP then INCOMPLETE	Route selection. lowest wins for IGP < EGP < INCOMPLETE
well-known, mandatory Discourages remote traffic from using a specific AS entry point	AS_PATH		sequence of AS	shorter	Detect routing loops Used by filter-list to stop propagation of routing updates from a specific AS, Affects route selection for external AS (to internal destination), disadvantage a local network route through one IBGP to get more traffic from external AS through another IBGP	neighbor x.x.x.x route-map yy out route-map : set as-path prepend xx route-map: match as-path xx
Encourages local traffic to use a specific AS exit point to reach a remote subnet	weight	local to router	0=learned routes 32768=self-originated	Largest	Local router significance. Change route selection in one IBGP router to define which path to reach external routes. Highest value chosen	neighbor x.x.x.x route-map yy in           route-map : set weight
well-known,discretionary Encourages local traffic to use a specific AS exit point to reach a remote AS	local preference	IBGP peers	default 100	Largest	Passed among IBGP routers, Changes route (to external destination) selection among IBGP routers. Highest value chosen	bgp default local-preference xx neighbor x.x.x.x route-map yy in route-map : set local-preference
Optional, non-transitive Discourages remote traffic from using a specific AS entry point to reach a local subnet	MED	EBGP peers set on router but sent to his EBGP peer	default 0	Smallest	Arrives in an AS and stay there, by default MED from different AS are not comparable. Exchanged between EBGP peers.	route-map : set metric
Optional,  transitive Mark traffic with a community so that neighbor can perform a specific action on it	Community				To mark specific traffic with a community (similar to tag) that then the remote router use route-map to apply specific action to this traffic.
well-known,discretionary	atomic_aggregate				set to indicate aggreagation happened
Optional,  transitive	aggregator		the router who aggregated the routes
	AS_Set				summarization of the AS_Path from all the routes that has been aggregated.

BGP route selection criteria

1. Prefer the path with the highest WEIGHT. 2. Prefer the path with the highest LOCAL_PREF. 3. Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP. 4. Prefer the path with the shortest AS_PATH. 5. Prefer the path with the lowest origin type. 6. Prefer the path with the lowest multi-exit discriminator (MED). 7. Prefer eBGP over iBGP paths. 8. Prefer the path with the lowest IGP metric to the BGP next hop. 9. Determine if multiple paths require installation in the routing table for BGP Multipath. 10. When both paths are external, prefer the path that was received first (the oldest one). 11. Prefer the route that comes from the BGP router with the lowest router ID. 12. If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length. 13. Prefer the path that comes from the lowest neighbor address.