BGP Peer Groups

BGP peer groups let several neighbors share one set of settings.

Take R1 in AS 65001.

Figure 1 – R1 and its three eBGP customers

It connects three eBGP customers across different Autonomous Systems.
All three share one security baseline: MD5 password (PMN-BGP) and timers (10 30).

Configure the Three Customers

Open R1 and paste the configuration for all three neighbors:

R1# configure terminal
R1(config)# router bgp 65001
R1(config-router)# neighbor 10.0.12.2 remote-as 65002
R1(config-router)# neighbor 10.0.12.2 password PMN-BGP
R1(config-router)# neighbor 10.0.12.2 timers 10 30
%BGP-5-ADJCHANGE: neighbor 10.0.12.2 Up

R1(config-router)# neighbor 10.0.13.2 remote-as 65003
R1(config-router)# neighbor 10.0.13.2 password PMN-BGP
R1(config-router)# neighbor 10.0.13.2 timers 10 30
%BGP-5-ADJCHANGE: neighbor 10.0.13.2 Up

R1(config-router)# neighbor 10.0.14.2 remote-as 65004
R1(config-router)# neighbor 10.0.14.2 password PMN-BGP
R1(config-router)# neighbor 10.0.14.2 timers 10 30
%BGP-5-ADJCHANGE: neighbor 10.0.14.2 Up
R1(config-router)# end

Spot the Problem

Your three sessions are up.

Now check your running config:

R1# show running-config | section router bgp
router bgp 65001
 bgp log-neighbor-changes
 neighbor 10.0.12.2 remote-as 65002
 neighbor 10.0.12.2 password PMN-BGP
 neighbor 10.0.12.2 timers 10 30
 neighbor 10.0.13.2 remote-as 65003
 neighbor 10.0.13.2 password PMN-BGP
 neighbor 10.0.13.2 timers 10 30
 neighbor 10.0.14.2 remote-as 65004
 neighbor 10.0.14.2 password PMN-BGP
 neighbor 10.0.14.2 timers 10 30

Count the lines: nine lines total.

Look at the duplicated lines. password and timers repeat word for word.
Six out of nine lines are pure copy-paste.

This design works, but it does not scale. Imagine doing this for 50 customers.
Changing a single password means 50 manual edits. Missing just one breaks that session.

Instead of repeating parameters neighbor by neighbor, you attach them to a single group name.
Neighbors join the group and inherit all baseline settings automatically.

What Happens Behind the Scenes

IOS sends BGP updates through an update group: a set of neighbors that share the same outbound policy.

Figure 2 – BGP update group: one update, three copies

The router builds the update once and copies it to every member.

IOS builds update groups on its own, with or without a peer group.
The real job of a peer group is to keep your configuration short and consistent.

Check the CLI Syntax

Ask IOS what parameter it expects after neighbor: