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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)                      9  TCP/IP Border Gateway Protocol (BGP/BGP-4)                           9  BGP Fundamentals and General Operation

BGP Path Attributes and Algorithm Overview 1 2 3 BGP General Operation and Messaging

Criteria for Assigning Preferences to Routes

Obviously, if a BGP speaker only knows of a single route to a network, it will install and use that route (assuming there are no problems with it). The assigning of preferences amongst routes only becomes important when more than one route has been received by a BGP speaker for a particular network. Preferences can be determined based on a number of different criteria. Just a few typical ones include:

• The number of autonomous systems between the router and the network (fewer generally being better).
  
  
• The existence of certain policies that may make certain routes unusable; for example, a route may pass through an AS that this AS is not willing to trust with its data.
  
  
• The origin of the path—that is, where it came from.

In the case where a set of routes to the same network are all calculated to have the same preference, a “tie-breaking” scheme is used to select from among them. Additional logic is used to handle special circumstances, such as the case of overlapping networks (see the description of the Atomic_Aggregate path attribute for an example of this).

The selection of routes for dissemination to other routers in Phase 3 is based on a rather complex algorithm that I cannot do justice here. Route advertisement is guided by the routing policies we discussed earlier in this section. Different rules are used to select routes for advertising to internal peers compared to external peers.

BGP Path Attributes and Algorithm Overview 1 2 3 BGP General Operation and Messaging