Multicast and Routing Guide K/KA/KB.15.15

13 Border Gateway Protocol (BGP)
Introduction
BGPv4 (RFC 4271) is the defacto internet exterior gateway protocol used between ISPs.
The characteristics of BGP are:
• Controls route propagation and the selection of optimal routes, rather than route discovery
and calculation, which makes BGP different from interior gateway protocols such as OSPF
and RIP.
• Uses TCP to enhance reliability.
• Supports CIDR.
• Reduces bandwidth consumption by advertising only incremental updates, which allows
advertising large amounts of routing information on the Internet.
• Eliminates routing loops completely by adding AS path information to BGP routes.
• Provides policies to implement flexible route filtering and selection.
• Provides scalability.
A router that advertises BGP messages is called a BGP speaker. The BGP speaker establishes peer
relationships with other BGP speakers to exchange routing information. When a BGP speaker
receives a new route or a route better than the current one from another AS, it advertises the route
to all the other BGP peers in the local AS.
BGP can be configured to run on a router in the following two modes:
• iBGP (internal BGP)
• eBGP (external BGP)
When a BGP speaker peers with another BGP speaker that resides in the same autonomous system,
the session is referred to as an iBGP session. When a BGP speaker peers with a BGP speaker that
resides in a different autonomous system, the session is referred to as an eBGP session.
Configuring BGP globally
Table 31 Global BGP configuration commands
CLI referenceDefaultDescriptionCommand syntax
(page 290)Not enabled.Configures a BGP routing
process.
router bgp as-#
no router bgp
(page 290)Configures a fixed router ID
for the local Border
bgp router-id
router-id
Gateway Protocol (BGP)
routing process.
no bgp router id
(page 290)To specify the networks to
be advertised by the Border
[no] network
ipv4/mask [route-map
route-map-name]
Gateway Protocol (BGP)
routing processes, use the
network command.
(page 290)To adjust BGP network
timers, use the bgp timers
[no] bgp timers
keep-alive hold-time
command in router
configuration mode.
(page 291)DisabledRe-enables the state
contained within this node
[no] enable
disable
and all child nodes of the
Introduction 289