Information Technology Reference
In-Depth Information
prefixes over the sessions. The intraconfederation eBGP session follows iBGP rules for
prefix advertisement in some regards and eBGP rules in others. For example, NEXT_HOP,
MED, and LOCAL_PREF are preserved, yet AS_PATH is modified when sending the
updates.
To the external neighbors (peers outside the confederation), the sub-AS topology is invisible.
That is, the AS_PATH modified within the confederation is stripped in updates sent to
eBGP neighbors. To other autonomous systems, a confederation appears as a single AS.
Within each member AS, full iBGP mesh is required. Route reflection can also be deployed.
One distinct advantage of confederation is that there is no requirement that member auton-
omous systems use the same IGP. It is not necessary for each member AS to reveal its inter-
nal topology to other member autonomous systems. When different IGPs are used,
however, BGP next-hop reachability must be guaranteed within each member AS.
Figure 7-33 shows an example of confederation. A confederation has three types of
peerings:
•
External peerings, such as between R10 and R12
•
Confederation external peerings, such as between R4 and R8
•
Internal peerings, such as between R5 and R6 or R5 and R7
Figure 7-33
BGP Confederation
AS 65000
R1
R2
AS 200
Confederation
ID 100
R4
R3
R9
R11
R5
R8
R7
AS 65002
R6
AS 65001
R10
R12
