Information Technology Reference
In-Depth Information
3.
Prefer the path with the largest weight. (This step is Cisco specific, and weight is lo-
calized to the router.)
4.
Prefer the path with the largest local preference. iBGP uses this path only to reach the
preferred external BGP router.
5.
Prefer the path that was locally originated via a
network
or
aggregate
BGP subcom-
mand or through redistribution from an IGP. Local paths sourced by
network
or
redistribute
commands are preferred over local aggregates sourced by the
aggregate-
address
command. (This step is Cisco specific.)
6.
If no route was originated, prefer the route that has the shortest autonomous system
path. (This step is Cisco specific.)
7.
If all paths have the same autonomous system path length, prefer the path with the
lowest origin type. Paths with an origin type of IGP (lower) are preferred over paths
originated from an EGP such as BGP, and EGP origin is preferred over a route with an
incomplete origin. (This step is Cisco specific.)
8.
If the origin codes are the same, prefer the path with the lowest MED attribute. An
eBGP peer uses this attribute to select a best path to the autonomous system. (This
step is a tiebreaker, as described in the RFC that defines the BGP.)
9.
If the paths have the same MED, prefer the external (eBGP) path over the internal
(iBGP) path. (This step is Cisco specific.)
10.
If the paths are still the same, prefer the path through the closest IGP neighbor (best
IGP metric). (This step is a tiebreaker, as described in the RFC that defines the BGP.)
11.
Prefer the path with the BGP neighbor with the lowest router ID. (The RFC that de-
fines the BGP describes the router ID.)
After BGP decides on a best path, it marks it with a > sign in the
show ip bgp
table and
adds it to the IP routing table.
BGP Summary
The characteristics of BGP follow:
BGP is an Exterior Gateway Protocol (EGP) used in routing in the Internet. It is an in-
terdomain routing protocol.
■
BGP is a path-vector routing protocol suited for strategic routing policies.
■
It uses TCP port 179 to establish connections with neighbors.
■
BGPv4 implements CIDR.
■
eBGP is used for external neighbors. It is used between different autonomous systems.
■
iBGP is used for internal neighbors. It is used within an autonomous system.
■
BGP uses several attributes in the routing-decision algorithm.
■
It uses confederations and route reflectors to reduce BGP peering overhead.
■
■
bound traffic.
