3Com 4500 PWR 26-Port Configuration Manual page 283

Rout ing Protocols and Routing Priority
Different routing protocols may find different r
However, not all of those routes are optimal. In fact, at a particular moment, only one protocol
uniquely determine the current optimal routing to the destination. For the purpose of route selection,
each routing protocol (including static routes) is assigned a pri
protocol with th e highest priority is pr
T he followi ng table lists some routing protocols and the default priorities for routes found by them:
Table 21-1 Routing protocols and priorities of their d
Routing approach
DIRECT
OSPF
STATIC
RIP
OSPF ASE
OSPF NSSA
UNKNOWN
The smaller the priority value, the higher the priority.
The priority for a direct route is always 0, which you cannot change. Any other type of routes can
have their priorities manually configured.
Each static route can be configu
Load Shar ing and Route Backup
L oad sharing
A given routing protocol may find several routes with the same metric to the same destination, and if this
protocol has the highest priority among all the active protocols, these routes will be considered valid and
are used to forward packets, thus achieving load sha
R oute backup
You can configure multiple routes to the
be the primary route and all the rest backup routes.
Route backup can help improve network reliability. Automatic switching can happen between the
primary route and a backup route.
Under normal circumstances, packets are forwarded through the primary route. When the primary route
goes down, the route with the highest priority among the backup routes is selected to forward packets.
When the primary route recovers, the route selection process is performed again and the primary route
is selected again to forward packets.
outes (including static routes) to the same destination.
eferred.
efau lt route
0
10
60
100
150
150
255
red with a different priority.
ring.
same destination, expecting the one with the highest priority to
21-4
ority. The route found by the routing
Priority
can