Border Gateway Protocol (BGP)

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BGP

• What• Why• Who• When• Where

BGP Version 4

• RFC 1771• Intelligent route selection

based on most specific prefix and shortest Autonomous System (AS) path

• Classless Inter Domain Routing (CIDR)

• Current Internet standard for inter-domain (AS) exterior routing

Border Gateway Protocol (BGP)

• Used for carrying routing information between AS’s

• Path vector protocol• Incremental• Runs over TCP (port 179)• Conveys information about AS

path topology

General Operation

• Learns multiple paths via internal and external BGP speakers

• Picks the best path and installs in the IP forwarding table

• Policies applied by influencing the best path selection

When should I use BGP?

• Dual- or multi-homed• Providing partial or full internet

routing to a downstream customer

• Anytime the AS path information is required

BGP is not needed…

• … if singularly homed• … if not providing downstream

routing• Use a default route!

Load Balancing

• BGP itself does not load-balance traffic; it chooses & installs a “best” route.

BGP Load Balancing Problem

• “Since BGP picks a ‘best’ route based upon most specific prefix and shortest AS_PATH, it becomes non-trivial to figure out how to manually direct specific portions of internal traffic (prefixes) in a distributed fashion across multiple external gateways.”

Asymmetric Routing Paradigm

• In the Internet, it is impossible to control end-to-end routing and paths traversed

• Packets may not always traverse the same downstream path as they do when forwarded upstream

Summary

• Use BGP only when needed!• IGP and conventional BGP

network problems• BGP gives flexibility and

control