384 CHAPTER 16: IP R OUTINGImportantConsiderationsConsider the following issues when you implement RIP on your module:n Use RIP-2 rather than RIP-1 if possible, because RIP-2 uses subnetmasking and the next hop field. Subnet mask advertising allows youto use VLSM. (See “Variable Length Subnet Masks (VLSMs)” earlier inthis chapter for more information.)n Set RIP as follows:n RIP-1 — learnn RIP-2 — enabledIn this way, the module keeps track of the RIP-1 and RIP-2 addressroutes in its routing table and forwards the routes as well.n 3Com recommends that you not advertise RIP-1 and RIP-2 together. Ifyou do, two different sets of IP addresses may go into to the routingtable for every one RIP advertisement, which quickly reduces theefficiency of the routing table.Routing Policies IP routing policies allow you to control how routes are sent from andreceived by the routing table in your module. Both RIP and OSPF haverouting 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:n Import policies — Import policies control what routes are added tothe routing table. (That is, the import policies control which routesyour module can accept from other routers.) When RIP or OSPFforwards a route to the routing table, the router searches its importpolicies before adding the route to the routing table.n Export policies — Export policies control what routes from therouting table are advertised by the RIP and OSPF protocols to otherrouters. (That is, export policies control which routes your module canforward to other routers.) When RIP or OSPF are preparing a routeadvertisement, the router searches its export policies beforeadvertising the route to the network.You can create up to 128 routing policies total. The total is sharedbetween OSPF and RIP policies.
