D-Link DGS-3308FG Product Manual - Page 61

8-port Gigabit Ethernet Switch User's Guide 10 Update Response 11 Update Acknowledgement Table 5-12. RIP Command Codes A router or host can ask another router for routing information by sending a request command. Routers reply to requests using the response command. In most cases, however, routers broadcast unsolicited response messages periodically. The field VERSION contains the protocol version number (1 in this case), and is used by the receiver to verify it will interpret the message correctly. RIP 1 Address Conventions The generality of RIP is also evident in the way it transmits network addresses. The address format is not limited to use by TCP/IP. It can be used with multiple network protocol suites. Each network address reported by RIP can have an address of up to 14 octets. Of course, IP addresses need only 4. RIP specifies that the remaining octets must be zero. The field labeled FAMILY OF NET 1 identifies the protocol family under which the network address should be interpreted. RIP uses values assigned to address families under the 4BSD UNIX operating system (IP addresses are assigned a value of 2). In addition to normal IP addresses, RIP uses the convention that address 0.0.0.0 denotes a default route. RIP attaches a distance metric to every route it advertises, including default routes. Thus, it is possible to arrange for two routers to advertise a default route (for example, a route to the Internet) at different metrics, making one of them a primary path and the other a backup. The final field of each entry in a RIP message, DISTANCE TO NET 2, contains an integer count of the distance to the specified network. Distances are measured in router hops, but values are limited to the range 1 through 16, with the distance 16 used to signify infinity (unreachable). RIP 1 Route Interpretation and Aggregation Because RIP was originally designed to be used with classful addresses, version 1 did not include any provision for a subnet mask. When subnet addressing was added to IP, version 1 of RIP was extended to permit routers to exchange subnetted addresses. However, because RIP 1 update messages do not contain explicit mask information, an important restriction was added - a router can include host-specific or subnet-specific address in routing updates as long as all receivers can unambiguously interpret the addresses. In particular, subnet routes can be included in updates sent across a network that is part of the subnetted prefix, and only if the subnet mask used with the network is the same as the subnet mask used with the address. The restriction means the RIP 1 cannot be used to propagate variable-length subnet addresses or classless addresses. Note: RIP 1 can only be used with classful or fixed-length subnet addresses. If a router running RIP 1 connects to one or more networks that are subnets of a prefix N as well as to one or more networks that are not part of N, the router must prepare different update messages for the two types of interfaces. Updates sent over the interfaces that are subnets of N can include subnet routes, but updates sent over other interfaces cannot. Instead, when sending over other interfaces the router is required to aggregate the subnet information and advertise a single route to network N. RIP Version 2 Extensions The restriction on address interpretation means that version 1 of RIP cannot be used to propagate either variable length subnet addresses or the classless addresses used with CIDR. When version 2 of RIP (RIP2) was defined, the protocol was extended to include an explicit subnet mask along with each address. In addition, RIP2 updates include explicit next-hop information, which prevents routing loops and slow convergence. As a result, RIP2 offers significantly increased functionality as well as improved resistance to errors. 51