3Com Switch 8800 Advanced Software V5 Configuration Guide

190 CHAPTER 19: IP ROUTING OVERVIEW
This chapter focuses on unicast routing protocols. For information on multicast
routing protocols, refer to
“IPv6 Multicast Routing and Forwarding Configuration”
on page 515.
Version of IP protocol
IPv4 routing protocols: RIP, OSPF, BGP and IS-IS.
IPv6 routing protocols: RIPng, OSPFv3, BGP4+, IPv6 IS-IS.
Routing Protocols and
Routing Priority
Different routing protocols may find different routes to the same destination.
However, not all of those routes are optimal. In fact, at a particular moment, only
one protocol can uniquely determine the current optimal routing to the
destination. For the purpose of route selection, each routing protocol (including
static routes) is assigned a priority. The route found by the routing protocol with
the highest priority is preferred.
The following table lists some routing protocols and the default priorities for
routes found by them:
n
■ 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 configured with a different priority.
■ IPv4 and IPv6 routes have their own respective routing tables.
Load Balancing and
Route Backup
Load Balancing
In multi-route mode, a routing protocol can be configured with multiple
equal-cost routes to the same destination. These routes have the same priority and
will all be used to accomplish load balancing if there is no route with a higher
priority available.
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 routes for load balancing.
Routing approach Priority
DIRECT 0
OSPF 10
IS-IS 15
STATIC 60
RIP 100
OSPF ASE 150
OSPF NSSA 150
IBGP 255
EBGP 255
UNKNOWN 255