Specifications

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If nondeterministic routing table path selection behavior is not configured (that is,
if the path-selection cisco-nondeterministic statement is not included in the BGP
configuration), for paths with the same neighboring AS numbers at the front of the
AS path, prefer the path with the lowest multiple exit discriminator (MED) metric.
Confederation AS numbers are not considered when deciding what the neighbor
AS number is. When you display the routes in the routing table using the show
route command, they generally appear in order from most preferred to least
preferred.
Routes that share the same neighbor AS are grouped together in the command
output. Within a group, the best route is listed first and the other routes are
marked with the NotBest flag in the State field of the show route detail command.
If nondeterministic routing table path selection behavior is configured (that is, the
path-selection cisco-nondeterministic statement is included in the BGP
configuration), prefer the path with the lowest multiple exit discriminator (MED)
metric. When you display the routes in the routing table using the show route
command, they generally appear in order from most preferred to least preferred
and are ordered with the best route first, followed by all other routes in order from
newest to oldest.
In both cases, confederations are not considered when determining neighboring
ASs. Also, in both cases, a missing metric is treated as if a MED were present but
zero.
4. Prefer strictly internal paths, which include IGP routes and locally generated
routes (static, direct, local, and so forth).
5. Prefer strictly external (EBGP) paths over external paths learned through
interior sessions (IBGP).
6. For BGP, prefer the path whose next hop is resolved through the IGP route
with the lowest metric.
7. For BGP, prefer the route with the lowest IP address value for the BGP router
ID.
8. Prefer the path that was learned from the neighbor with the lowest peer IP
address.
Multiple Active Routes
The interior gateway protocols (IGPs) compute equal-cost multipath next hops,
and internal BGP (IBGP) picks up these next hops. When there are multiple,
equal-cost next hops associated with a route, the routing protocol process installs
only one of the next hops in the forwarding path with each route, randomly
selecting which next hop to install. For example, if there are three equal-cost
paths to an exit router and 900 routes leaving through that router, each of the
paths ends up with about 300 routes pointing at it. This mechanism provides load
distribution among the paths while maintaining packet ordering per destination.
Default Route Preference Values
The JUNOS software routing protocol process assigns a default preference value
to each route that the routing table receives. The default value depends on the
source of the route. The preference is a value from 0 through 255, with a lower
value indicating a more preferred route. In general, the narrower the scope of the
statement, the higher precedence its preference value is given, but the smaller
the set of routes it affects. To modify the default preference value for routes
learned by routing protocols, you generally apply routing policy when configuring
the individual routing protocols. You also can modify some preferences with other
configuration statements.