ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1826 –
CONFEDERATIONS
Confederations simply divides an autonomous system into smaller groups.
It splits up an AS into multiple sub-ASes, where full mesh connections are
maintained only within each sub-As, and sub-ASes are connected by eBGP.
The overall AS is known as a confederation, while the sub-ASes may also
be referred to as member ASes. The entire confederation has a unique AS
number, while the member ASes may have AS numbers obtained from
public AS number space, or use AS number from private AS number space.
Figure 558: Connections for BGP Confederation
To prevent looping within the confederation, the AS-Confed-Set and AS-
Confed-Sequence path attributes are added. These attributes function in
the same manner as AS-Set and AS-Sequence. The following additional
requirements are applied for route advertisements passed between
member ASes:
◆ The Local-Pref for a route may be passed from one member AS to
another member AS. This exception to normal practice is allowed within
the confederation since this attribute is meant for use by the entire AS.
◆ The Next-Hop for a route set by the first BGP speaker in the
confederation may be passed from one member AS to another member.
◆ When a route advertisement is passed from one member AS to another,
the AS-Confed-Sequence must be inserted into the AS-Path along with
the AS number of the member AS to help prevent looping.
Border routers that also peer with outside ASes have to modify routing
information that leaves the confederation so that the internal structure of
the confederation remains hidden to exterior peers, primarily because this
information is of no use to another external AS. The information stripped
from route advertisements and update messages sent outside of the
confederation include AS-Confed-Sequence and AS-Confed-Set. Neither
are AS numbers of member ASes advertised to exterior peers.
Autonomous
System
AS16478
Public Domain
Router
Router
Router
Router
AS100
Member AS
iBGP
eBGP
Router
Router
Router
Router
AS200
Member AS
iBGP
Router
Router
Router
Router
AS300
Member AS
iBGP
eBGP