Prefer the path with the highest weight. The default value is 0 for all routes that are not originated by the local router. You can learn how it works in the BGP weight attribute lesson. The local preference is used within an autonomous system and exchanged between iBGP routers. We prefer the path with the highest local preference. The default value is To learn more, take a look at the BGP local preference attribute lesson.
Prefer the path that the local router originated. In the BGP table, you will see next hop 0. Prefer the path with the shortest AS path length. For example, AS path 1 2 3 is preferred over AS path 1 2 3 4 5.
T, and E introduced this command. The router ID is the same for multiple paths because the routes were received from the same router. The current best path can be lost when, for example, the neighbor that offers the path goes down.
The router ID is the highest IP address on the router, with preference given to loopback addresses. Also, you can use the bgp router-id command to manually set the router ID. Note : If a path contains route reflector RR attributes, the originator ID is substituted for the router ID in the path selection process.
If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length. It allows clients to peer with RRs or clients in other clusters. The address corresponds to the remote peer that is used in the TCP connection with the local router. In this example, 9 paths are available for the network The show ip bgp network command displays the entries in the BGP routing table for the given network.
BGP selects the best path out of these 9 paths by considering various attributes that are explained in this document. In the output shown here, BGP compares the available paths and selects Path 6 as the best path based on its lower router-ID. The extended community attribute, which is called BGP Cost Community , provides a way to customize the best path selection process. An additional step, in which cost communities are compared, is added to the algorithm that the How the Best Path Algorithm Works section describes.
This step comes after the required step point of insertion in the algorithm. The path with the lowest cost value is preferred. Note : Be aware of these items: - This step is skipped if you have issued the bgp bestpath cost-community ignore command.
The cost number value determines the preference for the path. The path with the lowest cost number value is preferred. Paths that are not specifically configured with the cost number value are assigned a default cost number value of 2,,, An Autonomous System is a single network or a set of networks and routers, which are under the control of one administrative entity. Nevertheless, network administrators frequently manipulate such options as local preference, lowest multi-exit discriminator and weight.
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. For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement. Emulate the Cisco IOS default behavior cisco-non-deterministic. This mode evaluates routes in the order that they are received and does not group them according to their neighboring AS.
With cisco-non-deterministic mode, the active path is always first. All inactive, but eligible, paths follow the active path and are maintained in the order in which they were received, with the most recent path first.
Ineligible paths remain at the end of the list. These advertisements are received in quick succession, within a second, in the order listed. Path 3 is received most recently, so the routing device compares it against path 2, the next most recent advertisement. The cost to the IBGP peer is better for path 2, so the routing device eliminates path 3 from contention. When comparing paths 1 and 2, the routing device prefers path 1 because it is received from an EBGP peer.
This allows the routing device to install path 1 as the active path for the route. We do not recommend using this configuration option in your network. It is provided solely for interoperability to allow all routing devices in the network to make consistent route selections. Always comparing MEDs whether or not the peer ASs of the compared routes are the same always-compare-med.
0コメント