Couple of amendments. Asterisks mark several factors that are
Cisco-implementation specific, although there are knobs to turn them
to the IETF specified behavior.
"Frank Wells" <[EMAIL PROTECTED]> wrote,
>This is the BGP attribute decision process:
>
>1 BGP Path Selection starts; if the next hop is inaccessible, do not
>consider it.
>
>2 Consider larger BGP administrative weights first.
* weight, with a scope of a single router, is Cisco-specific
>
>3 If the routers have the same weight, consider the route with
>higher local preference.
>
>4 If the routes have the same local preference, prefer the route
>that the specified router originated.
>
>5 If no route was originated, prefer the shorter AS path.
* This is Cisco, not IETF, behavior, and can be disabled. The IETF
idea seems to have been that local-preference would be set as a
consequence of AS path filters.
>
>6 If the AS paths are of the same length, prefer external paths over
>internal paths.
>
>7 If all paths are external, prefer the lowest origin code (IGP
>[Interior Gateway Protocol] <EGP <INCOMPLETE).
>
>8 If origin codes are the same, prefer the path with the lowest
>MULTI_EXIT_DISC (MED). A missing metric is treated as zero.
-- clarification:
HIGHER weight and local preference are better
LOWER MED is better
* treating the missing metric as 0 is Cisco-specific; the IETF
says to treat a missing MED as the highest possible value.
** By default, MEDs are comparable only between the same AS,
unless always-consider-med is configured
>
>9 If IGP synchronization is disabled and only an internal path
>remains, prefer the path through the closest IGP neighbor.
>
>10 Prefer the route with the lowest IP address value for the BGP router ID.
* unless BGP load balancing is enabled. If this feature is on, routes
that are otherwise identical (i.e., to the same AS) will be added up
to the BGP maximum-paths value.
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