Administrator Guide

In BGP, routers with an established TCP connection are called neighbors or peers. After a connection is established, the
neighbors exchange full BGP routing tables with incremental updates afterward. In addition, neighbors exchange KEEPALIVE
messages to maintain the connection.
In BGP, neighbor routers or peers can be classified as internal or external. External BGP peers must be connected physically
to one another (unless you enable the EBGP multihop feature), while internal BGP peers do not need to be directly connected.
The IP address of an EBGP neighbor is usually the IP address of the interface directly connected to the router. First, the BGP
process determines if all internal BGP peers are reachable, then it determines which peers outside the AS are reachable.
NOTE: Sample Configurations for enabling BGP routers are found at the end of this chapter.
1. Assign an AS number and enter ROUTER BGP mode.
CONFIGURATION mode
router bgp as-number
● as-number: from 0 to 65535 (2 Byte) or from 1 to 4294967295 (4 Byte) or 0.1 to 65535.65535 (Dotted format).
Only one AS is supported per system.
NOTE: If you enter a 4-Byte AS number, 4-Byte AS support is enabled automatically.
a. Enable 4-Byte support for the BGP process.
NOTE: This command is OPTIONAL. Enable if you want to use 4-Byte AS numbers or if you support AS4 number
representation.
CONFIG-ROUTER-BGP mode
bgp four-octet-as-support
NOTE:
Use it only if you support 4-Byte AS numbers or if you support AS4 number representation. If you are
supporting 4-Byte ASNs, enable this command.
Disable 4-Byte support and return to the default 2-Byte format by using the no bgp four-octet-as-support
command. You cannot disable 4-Byte support if you currently have a 4-Byte ASN configured.
Disabling 4-Byte AS numbers also disables ASDOT and ASDOT+ number representation. All AS numbers are displayed in
ASPLAIN format.
b. Enable IPv4 multicast or IPv6 mode.
CONFIG-ROUTER-BGP mode
address-family [ipv4 | ipv6}
Use this command to enter BGP for IPv6 mode (CONF-ROUTER_BGPv6_AF).
2. Add a neighbor as a remote AS.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | peer-group name} remote-as as-number
● peer-group name: 16 characters
● as-number: from 0 to 65535 (2 Byte) or from 1 to 4294967295 (4 Byte) or 0.1 to 65535.65535 (Dotted format)
Formats: IP Address A.B.C.D
You must Configure Peer Groups before assigning it a remote AS.
3. Enable the BGP neighbor.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | peer-group-name} no shutdown
NOTE:
When you change the configuration of a BGP neighbor, always reset it by entering the clear ip bgp command in
EXEC Privilege mode.
To view the BGP configuration, enter show config in CONFIGURATION ROUTER BGP mode. To view the BGP status, use
the show ip bgp summary command in EXEC Privilege mode. The first example shows the summary with a 2-byte AS
number displayed (in bold); the second example shows that the summary with a 4-byte AS number using the show ip bgp
summary command (displays a 4–byte AS number in bold).
R2#show ip bgp summary
BGP router identifier 192.168.10.2, local AS number 65123
BGP table version is 1, main routing table version 1
Border Gateway Protocol IPv4 (BGPv4)
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