What does the preference field mean in the routing table?

The Preference field represents the routing priority in the routing table. Equipment manufacturers will specify different priorities for different routing protocols in their respective products.

Routers often support multiple routing protocols. The support of multiple routing protocols involves the selection and coordination of multiple routes. In order to solve this problem, the concept of preference is introduced in routing parameters.

Each routing protocol generally sets a fixed preference value. The smaller the preference value, the higher the priority of the route corresponding to the protocol. Under normal circumstances, the routing priority is specified as follows:

直接路由 0 OSPF 路由 10 IS-IS的level 1的路由 15 IS-IS的level 2的路由 18 NSFnet主干的SPF路由 19 缺声网关和EGP缺省 20 重定向路由 30 由route socket得到的路由 40 由网管加入的路由 50 路由器发现的路由 55 静态路由 60 Cisco IGRP的路由 80 DCN的hello的路由 90 Berkeley的RIP路由 100 点对点接口聚集的路由 110 down状态的接口路由 120 聚集的缺省路由优先级 130 OSPF的扩展路由 140 BGP的路由 170 EGP的路由 200

The concept of routing priority is that a new route with a higher priority can replace a route with a lower priority and the same destination, and vice versa. However, in specific implementation, it is possible that certain changes will be made. For example, RIP-2 under quidway2501 provides a command to change the routing priority of RIP. This command can be used to change the priority of RIP routes. When configuring static routing in quidway2501, you can also specify the priority of the route. This is done for specific applications. However, we do not encourage changing priorities.

What needs to be distinguished is the two concepts of routing cost (metric) and routing priority (preference). Metric is for the same routing protocol. For different protocols, since they represent different meanings, it is meaningless to compare the metrics of different protocols. Therefore, to make a choice between two routes with the same information of different protocols, you can only compare the routes. priority. On the contrary, preference is for different protocols. The priority of routes with the same protocol is generally the same. At this time, metric is the criterion for choosing between two routes with the same information sink.

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