Important Considerations; Routing Policies - 3Com corebuilder 3500 Implementation Manual

300 C 11: I
HAPTER NTERNET
Important
Considerations

Routing Policies

P (IP)
ROTOCOL
Consider the following issues when you implement RIP on your system:
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 —
learn
RIP-2 —
enabled
In this way, the system 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 system. Both RIP and OSPF have
routing policy capabilities. This section describes the RIP routing policies;
OSPF routing policies are discussed in Chapter 14.
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 system 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 system 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. The total is shared between
OSPF and RIP policies.