H3C Technologies H3C SR8800 - Manual (Page 19)

3
Routing preference
Different routing protocols may find different routes to the same destination. However, not all of those
routes are optimal. For route selection, routing protocols, direct routes, and static routes are assigned
different preferences. The route with the highest preference is preferred.
The preference of a direct route is always 0 and cannot be changed. All other types of routes can have
their preferences manually configured. Each static route can be configured with a different preference.
The following table lists the types of routes and the default preferences. The smaller the preference value,
the higher the preference.
Routing approach Preference
Direct route 0
OSPF 10
IS-IS 15
Static route 60
RIP 100
OSPF ASE 150
OSPF NSSA 150
IBGP 255
EBGP 255
Unknown (route from an untrusted source) 256
Load balancing
A routing protocol can be configured with multiple equal-cost routes to the same destination. These routes
have the same preference and will all be used to accomplish load balancing if there is no route with a
higher preference available.
Route backup
Route backup can help improve network reliability. With route backup, you can configure multiple routes
to the same destination, expecting the one with the highest preference to be the main route and all the
rest backup routes.
Under normal circumstances, packets are forwarded through the main route. When the link fails, the
route with the highest preference among the backup routes is selected to forward packets. When the link
recovers, the route selection process is performed again and the main route is selected again to forward
packets.
Route recursion
The next hops of some BGP routes (except EBGP routes) and static routes configured with next hops may
not be directly connected. The outgoing interface to reach the next hop must be available. Route
recursion is used to find the outgoing interface based on the next hop information of the route. Link-state
routing protocols, such as OSPF and IS-IS, do not need route recursion because they obtain next hop
information through route calculation.