Important Considerations; Routing Policies - 3Com 4007 Implementation Manual

384 C 16: IP R
HAPTER
Important
Considerations

Routing Policies

OUTING
Consider the following issues when you implement RIP on your module:
Use RIP-2 rather than RIP-1 if possible, because RIP-2 uses subnet
masking and the next hop field. Subnet mask advertising allows you
to use VLSM. (See "Variable Length Subnet Masks (VLSMs)" earlier in
this chapter for more information.)
Set RIP as follows:
RIP-1 —
RIP-2 —
In this way, the module keeps track of the RIP-1 and RIP-2 address
routes in its routing table and forwards the routes as well.
3Com recommends that you not advertise RIP-1 and RIP-2 together. If
you do, two different sets of IP addresses may go into to the routing
table for every one RIP advertisement, which quickly reduces the
efficiency of the routing table.
IP routing policies allow you to control how routes are sent from and
received by the routing table in your module. Both RIP and OSPF have
routing policy capabilities. This section describes the RIP routing policies;
OSPF routing policies are discussed in Chapter 19.
There are two basic types of routing policies:
Import policies — Import policies control what routes are added to
the routing table. (That is, the import policies control which routes
your module can accept from other routers.) When RIP or OSPF
forwards a route to the routing table, the router searches its import
policies before adding the route to the routing table.
Export policies — Export policies control what routes from the
routing table are advertised by the RIP and OSPF protocols to other
routers. (That is, export policies control which routes your module can
forward to other routers.) When RIP or OSPF are preparing a route
advertisement, the router searches its export policies before
advertising the route to the network.
You can create up to 128 routing policies total. The total is shared
between OSPF and RIP policies.
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