This document describes the requirements, limitations, and benefits
when you use peer groups with Border Gateway Protocol (BGP).
The major benefit you achieve when you specify a BGP peer group is that
a BGP peer group reduces the amount of system resources (CPU and memory)
necessary in an update generation. In addition, a BGP peer group also
simplifies the BGP configuration. A BGP peer group reduces the load on system
resources by allowing the routing table to be checked only once, and updates to
be replicated to all peer group members instead of being done individually for
each peer in the peer group. Based on the number of peer group members, the
number of prefixes in the table, and the number of prefixes advertised, this
can significantly reduce the load. It is recommended that you group together
peers with identical outbound announcement policies.
Cisco recommends that you have an in-depth understanding of BGP.
The information in this document is based on the fact that BGP peer
groups have been supported in Cisco IOS® software since Cisco IOS Software
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
Technical Tips Conventions for more information on document
You can group BGP neighbors who share the same outbound policies
together in what is called a BGP peer group. Instead of configuring each
neighbor with the same policy individually, a peer group allows you to group
the policies which can be applied to individual peers thus making efficient
update calculation along with simplified configuration.
Peer groups have these requirements:
All members of a peer group must share identical outbound
announcement policies (such as distribute-list, filter-list, and route-map),
except for default-originate, which is handled on a per-peer basis even for
peer group members.
You can customize the inbound update policy for any member of a peer
A peer group must be either internal (with internal BGP (iBGP)
members) or external (with external BGP (eBGP) members). Members of an external
peer group have different autonomous system (AS)
Cisco IOS Software Releases earlier than 11.1(18)CC have the
limitations described in this section. Failure to adhere to these rules can
result in inconsistent routing.
If you use peer groups for clients of a route reflector, all the
clients must be fully meshed.
If you use an eBGP peer group, transit cannot be provided among the
peer group members.
All eBGP peer group members must be from the same subnet to avoid
non-connected next hop announcements.
However, these limitations were removed starting with Cisco IOS
Software Releases 11.1(18)CC, 11.3(4), and 12.0. Only the router on which the
peer groups are defined needs to be upgraded to the new code.
Note: Before you declare any device as a route-reflector-client using
peer-group, you must first configure a neighborship with that device. Failure
to do so results in the removal of the route-reflector-client from the
configuration when a reboot is performed.
This behavior was first found in Cisco IOS Software Releases
12.0(25)S01 and 12.2(15)T02 and is corrected in Cisco IOS Software Release 12.2
Note: The total number of BGP peers and the configurable limit and the
maximum number of established BGP peers that are supported on a router depends
on many variables, such as:
Total number of routes in the BGP table
Level of stability of the routes
Number of routes sent to each peer
Similarity between routes sent to different
Devices available memory and processor
Typically BGP peers on a router can be grouped into peer groups based
on their outbound update policies. A list of peer-groups used commonly by ISPs
are listed here:
Normal iBGP peer group for normal iBGP peers
iBGP client peer group for reflection peers on a route
eBGP full-routes for peers to receive full Internet
eBGP customer-routes for peers to receive only routes from direct
customers of the ISP. (You can configure some members with
default-originate to receive the default route in addition to
the customer routes.
eBGP default-routes for peers to receive the default route, and
possibly a few other routes.
BGP for detailed examples of how to configure BGP peer groups.