Border Gateway Protocol BGP

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Border Gateway Protocol (BGP)
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BGP

• What
• Why
• Who
• When
• Where

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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

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Border Gateway Protocol (BGP)

• Used for carrying routing information between
  ASs
• Path vector protocol
• Incremental
• Runs over TCP (port 179)
• Conveys information about AS path topology

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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

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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

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BGP is not needed

• if singularly homed
• if not providing downstream routing
• Use a default route!

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Load Balancing

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

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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.

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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

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Summary

• Use BGP only when needed!
• IGP and conventional BGP network problems
• BGP gives flexibility and control