notes: iBGP Synchronization.

- If an AS is a transit AS, BGP will not advertise a route until all routers in that AS have learned the external route via IGP.
- Legacy rule which is disabled from IOS 12.2(8)T+.
- Designed to prevent black holes when non-bgp routers are in the transit path and don’t carry routes about the external next-hop destinations.

Enable automatic summarization when:
  - Summarization of IGP-to-BGP redistributed routes to major network boundaries are required.
  - Using classful network command to summarize subnets to a major network boundaries.

Disable automatic summarization when:
- Summarization on IGP-to-BGP redistribution are not desired.
- Using classless variant of the network command.

Solutions:
- Run BGP on every router in the transit path.
- Redistribute BGP into IGP.
- Tunnel BGP over GRE, IPIP, MPLS etc.