3Com Switch 7750 Configuration Guide Guide

264 CHAPTER 30: IP ROUTING PROTOCOL OVERVIEW
Routing Protocols and
Preferences
Different routing protocols may discover different routes to the same destination,
but only one route among these routes and the static routes is optimal. In fact, at
any given moment, only one routing protocol can determine the current route to a
specific destination. Routing protocols (including static routing) are endowed with
different preferences. When there are multiple routing information sources, the
route discovered by the routing protocol with the highest preference will become
the current route. Routing protocols and their default route preferences (the
smaller the value, the higher the preference is) are shown in
Table 191.
In the table, "0" is used for directly connected routes, and "255" is used for
routes from untrusted source.
Except for direct routing, you can manually configure the preferences of various
dynamic routing protocols as required. In addition, you can configure different
preferences for different static routes.
Traffic Sharing and
Route Backup
Traffic sharing
The Switch 7750 Family supports multi-route mode, allowing the configuration of
multiple routes that reach the same destination and have the same preference.
The same destination can be reached via multiple different routes, whose
preferences are equal. When there is no route with a higher preference to the
same destination, the multiple routes will be adopted. Then, the packets destined
for the same destination will be forwarded through these routes in turn to
implement traffic sharing.
Route backup
The Switch 7750 Family supports route backup. When the main route fails, the
system automatically switches to a backup route to improve network reliability.
To achieve route backup, you can configure multiple routes to the same
destination according to actual situation. One of the routes has the highest
preference and is called primary route. The other routes have descending
preferences and are called backup routes. Normally, the router sends data through
the main route. When line failure occurs on the main route, the main route will
hide itself and the router will choose the one whose preference is the highest
among the remaining backup routes as the path to send data. In this way, the
Tabl e 191 Routing protocols and corresponding route preferences
Routing protocol or type Preference of the corresponding route
DIRECT 0
OSPF 10
IS-IS 15
STATIC 60
RIP 100
OSPF ASE 150
OSPF NSSA 150
UNKNOWN 255
IBGP 256
EBGP 256