1. Scaling Service Provider Networks
Introducing Confederations

2. Outline Overview IBGP Transit AS Problems
Splitting a Transit AS with BGP Confederations
AS-Path Propagation Within the BGP Confederation
AS-Path Processing in BGP Confederations
Intra-Confederation EBGP Session Properties
Summary

3. IBGP Transit AS Problems
IBGP requires a full mesh between all BGP-speaking routers.
Large number of TCP sessions
Unnecessary duplication of routing traffic
Solutions
Route reflectors modify IBGP split-horizon rules.
BGP confederations modify IBGP AS-path processing.

4. Splitting a Transit AS with BGP Confederations
Splitting the AS into smaller autonomous systems would reduce the number of BGP sessions, but extra AS numbers are not available.
Confederations enable internal AS numbers to be hidden and announce only one (external) AS number to EBGP neighbors.

5. AS-Path Propagation Within the BGP Confederation
IBGP session
The AS path is not changed.
Intra-confederation EBGP session
The intra-confederation AS number is prepended to the AS path.
EBGP session with external peer
Intra-confederation AS numbers are removed from the AS path.
The external AS number is prepended to the AS path.

6. AS-Path Propagation Within the BGP Confederation (Cont.)

7. AS-Path Processing in BGP Confederations
Intra-confederation AS path is encoded as a separate segment of the AS path.
The intra-confederation AS path is displayed in parentheses when you are using Cisco IOS show commands.
All routers within the BGP confederation have to support BGP confederations.
A router not supporting BGP confederations will reject an AS path with unknown segment type.

8. Intra-Confederation EBGP Session Properties
Behaves like EBGP session during session establishment
The EBGP neighbor has to be directly connected, or you have to configure ebgp-multihop on the neighbor.
Behaves like IBGP session when propagating routing updates
The local preference, MED, and next-hop attributes are retained.
The whole confederation can run one IGP, providing optimal routing based on the next-hop attribute in the BGP routing table.

9. Summary
IBGP requires a full mesh between all BGP-speaking routers; route reflectors modify IBGP split-horizon rules, and BGP confederations modify IBGP AS-path processing.
The full-mesh requirement is relaxed through introduction of member autonomous systems into which the original autonomous system is split.
The additional autonomous system numbers are hidden from the outside world by modified AS-path update procedures.
The intra-confederation segment is removed from the AS path by the egress confederation router prior to prepending the official AS number when sending a BGP update to an external AS.
Intra-confederation EBGP sessions act like EBGP sessions from a session-establishment perspective, and they act like IBGP sessions from the BGP attribute-propagation perspective.