© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Transit Autonomous Systems Working with a Transit AS

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Outline Overview Transit AS Tasks External Route Propagation Internal Route Propagation Packet Forwarding in an AS Core Router IBGP Requirements in a Transit AS Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Transit AS Tasks Propagate routes between remote autonomous systems Route packets between remote networks

© 2005 Cisco Systems, Inc. All rights reserved. BGP v External Route Propagation

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Internal Route Propagation

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Packet Forwarding in an AS

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Core Router IBGP Requirements in a Transit AS All core routers must have all external routes. Core routers must receive BGP routes. –Redistribution of BGP routes into IGP is not scalable. –Default routing is not applicable in transit AS core.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Summary Routers in a transit AS receive routing information updates from neighboring autonomous systems, propagate the information through their own AS, and send it to other neighboring autonomous systems. Two autonomous systems usually exchange routing information over an EBGP session. A BGP session between two routers in the same AS is called an IBGP session. For packets to be properly forwarded in a transit AS, all routers must have external routing information. The only feasible method of distributing external routing information to all routers in the transit AS is through IBGP.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.22-9