RIP is the originator of dynamic routing. Although it is not used much now, you can learn more about it when you have time.

1. Introduction to RIP

RIP (Routing Information Protocol) is a relatively simple interior gateway protocol (IGP), which is mainly used in smaller networks, such as campus networks and regional networks with simpler structures.

RIP is generally not used in more complex environments and large networks. Since RIP is simpler to implement, it is much easier to configure and maintain than OSPF and IS-IS.

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2. Working Mechanism of RIP

1. Basic concepts of RIP

RIP is a protocol based on the distance vector algorithm. It exchanges routing information through UDP packets and uses port number 520.

RIP uses the number of hops to measure the distance to the destination address. The number of hops is called the metric. In RIP, the number of hops from a router to a network directly connected to it is 0, the number of hops to a network reachable through a router is 1, and so on. To limit the convergence time, RIP stipulates that the metric value is an integer between 0 and 15. A hop number greater than or equal to 16 is defined as infinite, that is, the destination network or host is unreachable.

Due to this limitation, RIP is not suitable for large networks. To improve performance and prevent routing loops, RIP supports Split Horizon and Poison Reverse functions.

2. RIP routing database

Each router running RIP manages a routing database that contains routing entries to all reachable destinations. These routing entries contain the following information:

• Destination address: The address of the host or network.
• Next hop address: The IP address of the interface of the adjacent router that needs to be passed through to reach the destination.
• Outgoing interface: The outgoing interface through which this router forwards packets.
• Metric: The cost of this router to reach the destination.
• Routing time: The time from the last time the routing entry was updated to now. Each time the routing entry is updated, the routing time is reset to 0.
• Route Tag: Used to identify external routes. Routing information can be flexibly controlled based on the route tag in the routing policy. For more information about routing policies, see "Routing Policies" in "Layer 3 Technology - IP Routing Configuration Guide."

3. RIP prevents routing loops

The RIP protocol announces its own routing table to neighbors, which may cause routing loops. This can be avoided through the following mechanisms:

• Counting to infinity: Defines routes with a metric value of 16 as unreachable (infinity). When a routing loop occurs, the metric value of a route will increase to 16, and the route is considered unreachable.
• Triggered Updates: RIP avoids the possibility of routing loops between multiple routers by triggering updates, and can accelerate the convergence of the network. Once the metric of a route changes, it immediately sends an update message to the neighboring routers instead of waiting until the update cycle arrives.
• Split Horizon: The routes learned by RIP from a certain interface will not be sent back to the neighboring router from that interface. This not only reduces bandwidth consumption, but also prevents routing loops.
• Poison Reverse: After RIP learns a route from an interface, it sets the metric of the route to 16 (unreachable) and sends it back to the neighbor router from the original interface. In this way, useless information in the other party's routing table can be cleared.

3. RIP Version

There are two versions of RIP: RIP-1 and RIP-2.

1. RIP-1 is a classful routing protocol that only supports publishing protocol messages in broadcast mode.

The protocol message cannot carry mask information, it can only identify the routes of natural network segments such as A, B, and C, so RIP-1 does not support discontiguous subnets.

2. RIP-2 is a classless routing protocol. Compared with RIP-1, it has the following advantages:

• Supports routing tags, and routing can be flexibly controlled based on routing tags in routing policies.
• The message carries mask information and supports route aggregation and CIDR (Classless Inter-Domain Routing).
• Supports specifying the next hop, and can select the optimal next hop address on the broadcast network.
• Supports multicast routing to send update messages. Only RIP-2 routers can receive update messages, reducing resource consumption.
• Supports authentication of protocol messages, and provides two methods: plain text authentication and MD5 authentication, to enhance security. RIP-2 has two message transmission methods: broadcast and multicast. By default, multicast is used to send messages, and the multicast address used is 224.0.0.9. When the interface runs RIP-2 broadcast mode, it can also receive RIP-1 messages.