Pre—Preference of the route. Among routes to the same destination, the route with the highest
preference is optimal.
Cost—If multiple routes to a destination have the same preference, the one with the smallest cost is
the optimal route.
Dynamic routing protocols
Static routes work well in small, stable networks. They are easy to configure and require fewer system
resources. However, in networks where topology changes occur frequently, a typical practice is to
configure a dynamic routing protocol. Compared with static routing, a dynamic routing protocol is
complicated to configure, requires more routers resources, and consumes more network resources.
Dynamic routing protocols dynamically collect and report reachability information to adapt to topology
changes. They are suitable for large networks.
Dynamic routing protocols can be classified by different criteria, as shown in
Table 2 Categories of dynamic routing protocols
An AS refers to a group of routers that use the same routing policy and work under the same
Routing protocols, including static and direct routing, each by default have a preference. If they find
multiple routes to the same destination, the router selects the route with the highest preference as the
The preference of a direct route is always 0 and cannot be changed. You can configure a preference for
each static route and each dynamic routing protocol. The following table lists the route types and default
preferences. The smaller the value, the higher the preference.
Table 3 Route types and default route preferences
IGPs—Work within an AS. Examples include RIP, OSPF, and IS-IS.
EGPs—Work between ASs. The most popular EGP is BGP.
Distance-vector protocols—Examples include RIP and BGP. BGP is also considered
a path-vector protocol.
Link-state protocols—Examples include OSPF and IS-IS.
Unicast routing protocols—Examples include RIP, OSPF, BGP, and IS-IS.
Multicast routing protocols—Examples include PIM-SM and PIM-DM.
IPv4 routing protocols—Examples include RIP, OSPF, BGP, and IS-IS.
IPv6 routing protocols—Examples include RIPng, OSPFv3, IPv6 BGP, and IPv6 IS-IS.