Describe the different metrics that BGP can use in building a routing.docx
- 1. Describe the different metrics that BGP can use in building a routing table Solution The metrics used by BGP in building a routing table are 1) Weight — weight is the first criterion used by the router and it is set locally on the user’s router. The Weight is not passed to the following router updates. In case there are multiple paths to a certain IP address, BGP always selects the path with the highest weight. The weight parameter can be set either through neighbor command, route maps or via the AS-path access list. 2) Local Preference — this criterion indicates which route has local preference and BGP selects the one with the highest preference. Local Preference default is 100. 3) Network or Aggregate — this criterion chooses the path that was originated locally via an aggregate or a network, as the aggregation of certain routes in one is quite effective and helps to save a lot of space on the network. 4) Shortest AS_PATH — this criterion is used by BGP only in case it detects two similar paths with nearly the same local preference, weight and locally originated or aggregate addresses. 5) Lowest origin type — this criterion assigns higher preference to Exterior Gateway Protocol (EGP) and lower preference to Interior Gateway Protocol (IGP). 6) Lowest multi-exit discriminator (MED) — this criterion, representing the external metric of a route, gives preference to the lower MED value. 7) eBGP over iBGP — just like the “Lowest origin type― criterion, this criterion prefers eBGP rather than iBGP. 8) Lowest IGP metric — this criterion selects the path with the lowest IGP metric to the BGP next hop. 9) Multiple paths — this criterion serves as indication whether multiple routes need to be installed in the routing table.
- 2. 10) External paths — out of several external paths, this criterion selects the first received path. 11) Lowest router ID — this criterion selects the path which connects to the BGP router that has the lowest router ID. 12) Minimum cluster list — in case multiple paths have the same router ID or originator, this criterion selects the path with the minimum length of the cluster list. 13) Lowest neighbor address — this criterion selects the path, which originates from the lowest neighbor address.