Route Selection - Juniper J2300 User Manual

Peer ASs establish links through an external peer BGP session. As a result,
all route advertisement between the external peers takes place by means of
the EBGP mode of information exchange. To propagate the routes through
the AS and advertise them to internal peers, BGP uses IBGP. To advertise the
routes to a different peer AS, BGP again uses EBGP.
To avoid routing loops, IBGP does not advertise routes learned from an internal
BGP peer to other internal BGP peers. For this reason, BGP cannot propagate
routes throughout an AS by passing them from one router to another. Instead,
BGP requires that all internal peers be fully meshed so that any route advertised
by one router is advertised to all peers within the AS.
As a network grows, the full mesh requirement becomes increasingly difficult to
manage. In a network with 1000 routers, the addition of a single router requires
that all the routers in the network be modified to account for the new addition. To
combat these scaling problems, BGP uses route reflection and BGP confederations.
For information about route reflection, see "Scaling BGP for Large Networks"
on page 280. For information about routing confederations, see "Scaling
BGP for Large Networks" on page 280.

Route Selection

A local BGP router uses the following primary criteria to select a route
from the routing table for the forwarding table:
1.
2.
3.
4.
5.
If more than one route remains after all these criteria are evaluated, the local BGP
router evaluates a set of secondary criteria to select the single route to a destination
Next-hop accessible—If the next hop is inaccessible, the local router does not
consider the route. The router must verify that it has a route to the BGP
next-hop address. If a local route to the next hop does not exist, the local route
does not include the router in its forwarding table. If such a route exists,
route selection continues.
Highest local preference—The local router selects the route with the highest
local preference value. If multiple routes have the same preference, route
selection continues. (For more information, see "Local Preference" on page
278.)
Shortest AS path—The local router selects the route with the fewest entries in
the AS path. If multiple routes have the same AS path length, route selection
continues. (For more information, see "AS Path" on page 279.)
Lowest origin—The local router selects the route with the lowest origin value. If
multiple routes have the same origin value, route selection continues. (For
more information, see "Origin" on page 279.)
Lowest MED value—The local router selects the route with the lowest multiple
exit discriminator (MED) value. If multiple routes have the same MED
value, route selection continues. (For more information, see "Multiple Exit
Discriminator" on page 280.)
Routing Overview
277
BGP Overview