3Com 4800G Series, Configuration Manual

Page: 245 / 1246

Routing Protocol Overview 245
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The smaller the priority value, the higher the priority.
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The priority for a direct route is always 0, which you cannot change. Any other
type of routes can have their priorities manually configured.
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Each static route can be configured with a different priority.
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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.
In current implementations, routing protocols supporting load balancing are static
routing, RIP, OSPF, BGP and IS-IS.
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 priority to be the main route and all the rest backup routes.
Under normal circumstances, packets are forwarded through the main route.
When the main route goes down, the route with the highest priority among the
backup routes is selected to forward packets. When the main route recovers, the
route selection process is performed again and the main route is selected again to
forward packets.
Route Recursion The nexthops of some BGP routes (except EBGP routes) and static routes
configured with nexthops may not be directly connected. To forward the packets,
the outgoing interface to reach the nexthop must be available. Route recursion is
used to find the outgoing interface based on the nexthop information of the
route. Link-state routing protocols, such as OSPF and IS-IS, do not need route
recursion because they obtain nexthop information through route calculation.
Sharing of Routing
Information
As different routing protocols use different routing algorithms to calculate routes,
they may find different routes. In a large network with multiple routing protocols,
it is required for routing protocols to share their routing information. Each routing
protocol has its own route redistribution mechanism. For detailed information,
refer to the description about route redistribution in each routing protocol.
EBGP 255
UNKNOWN 256
Routing approach Priority