Path vector

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In routing protocols, the path vector paradigm is a method of ensuring loop-free paths toward a given destination. Its use is most prominent in the Border Gateway Protocol, the basis for routing in the Internet. A subset has been used for source routing in the the IEEE 802.5 Token Ring bridging in IBM System Network Architecture.

To take a BGP-oriented example, assume you are in Autonomous System 1 (AS1). You import route advertisements from other As, which, minimally, carry a path vector of AS numbers between you and the destination:

  • Example 1 to ASx, from AS2: ASx-AS8-AS3-AS2
  • Example 2 to ASx, from AS2: ASx-AS3-AS5-AS6-AS2-AS2
  • Example 3 to ASx, from AS7: ASx-AS3-AS5-AS9-AS7
  • Example 4 to ASx, from AS4: ASx-AS1-AS4

From the loop prevention standpoint, example 4 is immediately discarded, because it contains your AS number. Any route containing your AS is defined to form a loop.

In BGP practice, the most preferable route, in the absence of other tie-breaking factors, is the shortest loop-free AS path. Example 1, therefore, will be selected. As soon as other factors are considered, however, the path vector approach alone will not guarantee a route is loop-free.