The RIP protocol adopts the distance vector algorithm, which is less applicable in actual use. By default, RIP uses a very simple metric system: the distance is the number of links required to reach the destination site, ranging from 1 to 15, with a value of 16 indicating infinity. The RIP process uses UDP port 520 to send and receive RIP packets. RIP packets are sent in the form of broadcast every 30s. In order to prevent the occurrence of "broadcast storm", the subsequent packets will be sent after a random delay. In RIP, if a route is not brushed within 180s, the corresponding distance is set to infinity, and the entry is deleted from the routing table. There are two types of RIP packets: request packets and response packets.
Routing Information Protocol (RIP) is the most widely used Interior Gateway Protocol (IGP). (IGP) is a routing protocol used on internal networks (and in rare cases, networks connected to the Internet), which allows routers to dynamically adapt to changes in network connections by exchanging information continuously, including every Which networks can a router reach, how far are those networks, etc. IGP is an application layer protocol and uses UDP as the transport protocol.
While RIP is still frequently used, most believe it will and is being replaced by routing protocols such as OSPF and IS-IS. Of course, we also see EIGRP, a more adaptive routing protocol belonging to the same basic protocol class as RIP (Distance Vector Routing Protocol), also getting some use.
Features of RIP
(1) Only exchange information with neighboring routers. Two routers are adjacent if the communication between the two routers does not pass through the other router. The RIP protocol stipulates that no information is exchanged between non-adjacent routers.
(2) The information exchanged by the router is all the information currently known by the router. That is, its own routing table.
(3) The routing information is exchanged at a fixed time, for example, every 30 seconds, and then the router updates the routing table according to the received routing information. (can also be configured to trigger an update)
RIP anti-loop mechanism
1-Maximum hop count: Define the maximum hop count (maximum 15 hops). When the hop count is 16 hops, the target is unreachable.
2-Split horizon: Routes learned from an interface will not be broadcast back to the interface. Cisco can turn off the split horizon function for each interface.
3-Route Poisoning: When the topology changes, the router will mark the failed route as possibly down and assign an unreachable metric.
4-Poison reverse: The route learned from an interface will be sent back to the interface, but it has been poisoned, and the hop count is set to 16, making it unreachable.
5-Trigger update (trigger update): Once a route crash is detected, the route refresh message is broadcast immediately without waiting for the next refresh cycle.
6-holddown timer: prevents frequent flipping of the routing table and increases the stability of the network.
RIP (Routing Information Protocol) is an internal dynamic routing protocol based on the DV algorithm. It is the first standard IP routing protocol supported for all major vendors, networks. For more complex environments, RIP should generally not be used.
How RIP works
1. Initialization - When RIP is initialized, it will send request packets from each interface participating in the work. The request packet will request a complete routing table from all RIP routers. The request is done by sending a broadcast over the LAN or to the next hop address on a point-to-point link. This is a special request that requests a complete routing update from a neighboring device.
2. Receive requests - RIP has two types of messages, response and receive messages. Each route entry in the request packet is processed to establish metrics and paths for the route. RIP uses the hop count measurement. A value of 1 means a directly connected network, and 16 means that the network is unreachable. The router will return the entire routing table as a reply to the received message.
3. Response received - The router receives and processes the response, and it updates routing table entries by adding, deleting, or modifying them.
4. Regular routing update and timing - routers send the entire routing table to neighbor routers in the form of reply messages every 30 seconds. When the router receives a new route or an update of an existing route, it will set a timeout of 180 seconds. If there is no update information for 180 seconds, the hop count of the route is set to 16. The router advertises the route with a metric of 16 until the refresh timer removes the route from the routing table. The refresh timer is set to 240 seconds, or 60 seconds longer than the expired timer. Cisco also uses a third timer, called the hold timer. The 180 seconds after receiving a route with a higher metric is the time to suppress the timer. During this period, the router does not update the routing table with the new information it receives, which provides an extra period of time for the network to converge. time.
5. Trigger routing update - when a certain routing metric changes, the router only sends the route related to the change, and does not send the complete routing table.