Bgp uses which algorithm

The list of the selection criteria is presented below in the same order in which BGP uses them to select the optimal routes to be injected into the IP Routing table:. 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. Local Preference default is BGP best path selection algorithm also provides a mechanism to discard paths that are not considered as candidates for the best path. With the full Internet BGP routing table being upward of K routes and with a BGP router having the potential to be receiving multiple copies of that routing table from multiple providers, it has to have some way to compare those multiple BGP routing tables and select only the best route to go into the IP routing table on the router.

You should note that Cisco BGP routers have weight as the first criteria in the table where other brands of routers do not. Cisco's official BGP Best Path Selection Algorithm documentation lists these criteria, but below I will list each of them with my own commentary and examples. Unless there are no options in place to influence the result, the BGP Best Path selects the best path based on the shortest Autonomous System AS , one or more networks under a single administrative control.

However, many times options like weight, local preference, and MED are put in place by network administrators. Let's look at the selection criteria, in order, that BGP uses to select the best routes to install into the IP Routing table:.

There are some commands that you will use over and over with the BGP metrics. They are show ip bgp and show ip bgp summary. In fact, I usually create aliases for these commands to make using them quicker see my article " Enter Commands More Efficiently with Cisco Command Aliases ". Real User Monitoring. Resolve performance issues faster, optimize conversions, and make better business decisions. Network Insights. Get complete visibility of every network layer to detect and resolve issues quickly.

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Who We Are. Log In. Log In Log In. Chapter 4. Introduction The Border Gateway Protocol BGP is responsible for finding the best route for data transmission between two endpoints connected across the internet. It is a routing protocol that exchanges routing and reachability information between AS on the internet. Connection to peer AS: An enterprise or any given organization that owns an AS can be connected to one or more upstreams.

To each upstream, there can be one or more connections and these connections can be all to the same device or spread among multiple devices. Optional transitive attributes are marked as partial when not recognized.

Optional transitive attributes include: - Aggregator - Community Optional non-transitive : Possibly recognized by BGP routers but not passed to peers. States if the information from the Update is partial or complete.

A path with the shortest AS Path is preferred skippable via router configuration. A path with the lowest origin type is preferred. By default, the MED is compared only if the neighbour AS is the same for the paths that are compared this is configurable. There are multiple commands related to how and when to treat the MED value of the paths, which, due to its complexity, is outside the scope of this article. A path that is an External BGP path is preferred versus internal.

At this point, if multipath is configured, the Router installs the routes. If not, the algorithm continues to the next step.