Cisco ASR 9000 Series Configuration Manual page 38

BGP Functional Overview
BGP Functional Overview
BGP uses TCP as its transport protocol. Two BGP routers form a TCP connection between one another (peer
routers) and exchange messages to open and confirm the connection parameters.
BGP routers exchange network reachability information. This information is mainly an indication of the full
paths (BGP autonomous system numbers) that a route should take to reach the destination network. This
information helps construct a graph that shows which autonomous systems are loop free and where routing
policies can be applied to enforce restrictions on routing behavior.
Any two routers forming a TCP connection to exchange BGP routing information are called peers or neighbors.
BGP peers initially exchange their full BGP routing tables. After this exchange, incremental updates are sent
as the routing table changes. BGP keeps a version number of the BGP table, which is the same for all of its
BGP peers. The version number changes whenever BGP updates the table due to routing information changes.
Keepalive packets are sent to ensure that the connection is alive between the BGP peers and notification
packets are sent in response to error or special conditions.
Note For information on configuring BGP to distribute Multiprotocol Label Switching (MPLS) Layer 3 virtual
private network (VPN) information, see the Cisco ASR 9000 Series Aggregation Services Router MPLS
Configuration Guide
For information on BGP support for Bidirectional Forwarding Detection (BFD), see the
Cisco ASR 9000 Series Aggregation Services Router Interface and Hardware Configuration Guide and
the Cisco ASR 9000 Series Aggregation Services Router Interface and Hardware Command Reference.
BGP Router Identifier
For BGP sessions between neighbors to be established, BGP must be assigned a router ID. The router ID is
sent to BGP peers in the OPEN message when a BGP session is established.
BGP attempts to obtain a router ID in the following ways (in order of preference):
• By means of the address configured using the bgp router-id command in router configuration mode.
• By using the highest IPv4 address on a loopback interface in the system if the router is booted with saved
• By using the primary IPv4 address of the first loopback address that gets configured if there are not any
If none of these methods for obtaining a router ID succeeds, BGP does not have a router ID and cannot establish
any peering sessions with BGP neighbors. In such an instance, an error message is entered in the system log,
and the show bgp summary command displays a router ID of 0.0.0.0.
After BGP has obtained a router ID, it continues to use it even if a better router ID becomes available. This
usage avoids unnecessary flapping for all BGP sessions. However, if the router ID currently in use becomes
invalid (because the interface goes down or its configuration is changed), BGP selects a new router ID (using
the rules described) and all established peering sessions are reset.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
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loopback address configuration.
in the saved configuration.
Implementing BGP
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