NetworkSec - Layer 3 - Border Gateway Protocol (BGP)

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Border Gateway Protocol (BGP)

The goal of Inter-domain routing

• to find loop free path to the intended destination.
• concerned with reachability than optimality

Finding path anywhere close to optimal is considered to be a great achievement

• Scalability
  • An Internet backbone router must be able to forward any packet destined anywhere in the Internet
  • Having a routing table that will provide a match for any valid IP address
• Autonomous nature of the domains
  • It is impossible to calculate meaningful path costs for a path that crosses multiple ASs
  • A cost of 1000 across one provider might imply a great path
  • but it might mean an unacceptable bad one from another provider
• Issues of trust
  • Provider A might be unwilling to believe certain advertisements from provider B

Each AS has:

• One BGP speaker that advertises:
  • local networks
  • other reachable networks (transit AS only)
  • gives path information
• In addition to the BGP speakers, the AS has one or more border “gateways” which need not be the same as the speakers
• The border gateways:
  • routers through which packets enter and leave the AS

BGP does not belong to either of the two main classes of routing protocols (distance vectors and link-state protocols)

• BGP advertises complete paths as an enumerated lists of ASs to reach a particular network

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BGP Example

• Speaker for AS 2 advertises reachability to P and Q
• Network 128.96, 192.4.153, 192.4.32, 192.4.3, can be reached directly from AS 2.
• Speaker for backbone network then advertises
• Networks 128.96, 192.4.153, 192.4.32, 192.4.3 can be reached along the path <AS 1, AS 2>.
• Speaker can also cancel previously advertised paths

BGP Issues

• the AS numbers carried in BGP need to be unique
  • Example,
  • AS 2 can only recognize itself in the AS path if no other AS identifies itself in the same way
  • AS numbers are 16-bit numbers assigned by a central authority
  • AS is the B segment level

Integrating Interdomain and Intradomain Routing



• All routers run iBGP and an intradomain routing protocol.
• Border routers (A, D, E) also run eBGP to other ASs 

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to be continued…

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