Routing Protocol and Route Priority

Route Priority

Dynamic routes of different routing protocols and static routes may have the same destination, but not all these routes are optimal. At a certain moment, only one routing protocol determines the current route to a certain destination. To select the optimal route, each routing protocol and static route are configured with priorities, and the route with the highest priority becomes the optimal route. Table 1 lists routing protocols and their default priorities.

In Table 1, 0 indicates a direct route, and 255 indicates any route learned from unreliable sources. The smaller the value, the higher the priority.

The priorities of routing protocols can be configured, except for direct routes. In addition, the priority of each static route can be different.

The NetEngine 8000 F defines external and internal priorities. The external priority refers to the priority set by users for each routing protocol. Table 1 lists the default external priorities.

When different routing protocols are configured with the same priority, the system selects the optimal route based on the internal priority. For the internal priority of each routing protocol, see Table 2.

For example, two routes, an OSPF route and a static route, can reach 10.1.1.0/24, and the priorities of the two routes are set to 5. In this case, the NetEngine 8000 F selects the optimal route based on the internal priorities listed in Table 2. The internal priority of OSPF (10) is higher than that of the static route (60). Therefore, the system selects the route discovered by OSPF as the optimal route.

• If multiple OSPFv2 processes learn routes to the same destination and the external and internal priorities of the routes are the same, the system selects the route with the smallest link cost; if the link costs of the routes are the same, the routes participate in load balancing. If multiple OSPFv3 processes learn routes to the same destination and the external and internal priorities of the routes are the same, the system selects the route with the smallest process ID.
• If multiple IS-IS processes learn routes to the same destination and the external and internal priorities of the routes are the same, the system selects the route with the smallest link cost; if the link costs of the routes are the same, the routes participate in load balancing.
• If multiple RIP or RIPng processes learn routes to the same destination and the external and internal priorities of the routes are the same, the system selects the route with the smallest link cost; if the link costs of the routes are the same, the routes participate in load balancing.