Path vector: Difference between revisions
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In [[routing protocol]]s, 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: <code>ASx-AS8-AS3-AS2</code> | |||
*Example 2 to ASx, from AS2: <code>ASx-AS3-AS5-AS6-AS2-AS2</code> | |||
*Example 3 to ASx, from AS7: <code>ASx-AS3-AS5-AS9-AS7</code> | |||
*Example 4 to ASx, from AS4: <code>ASx-AS1-AS4</code> | |||
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. | |||
Latest revision as of 15:00, 20 March 2024
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.