Path vector

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.