Chapter 50
| IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1079 –
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 4: 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:
â—† 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.
â—† 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.
â—† 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
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
To Next Page
Loading...
Need help?
General