Limitations and features of the RIP Routing Protocol

In network protocols, routing protocols have always been the focus of our learning. In the routing protocol, it is also based on the RIP routing protocol. We need to master the RIP Protocol. Next, let's take a detailed look at the specific content of RIP.

I. Overview

The full name of the RIP Routing Protocol is the Routing Information Protocol (Routing Information Protocol). It is an internal gateway Protocol (IGP) used to transmit Routing Information in an autonomous system. the RIP Protocol is based on the Distance Vector algorithm Distance Vector Algorithms). It uses the "Number of hops", that is, metric, to measure the route Distance to the target address.

Ii. Restrictions of the Protocol

1. The Protocol stipulates that the metric of a valid route information cannot exceed 15, so that the Protocol cannot be applied to a large network, it should be said that this restriction is imposed by the designer because the Protocol is only applicable to small networks. for a target network with a metric of 16, it is considered inaccessible.

2. When the routing protocol is applied to reality, it is easy to see the phenomenon of "count to infinity", which makes the routing convergence very slow, it takes a long time for the route information to stabilize after the network topology changes.

3. The Protocol is based on the number of hops (that is, the number of routers passing through the packets) and the route is selected. This measure is not reasonable, the impact of network delay, reliability, line load, and other factors on transmission quality and speed is not considered.

Iii. RIP Route Protocol Version 1) Message format and Characteristics

1. RIP version 1) Message format

0 7 15 31

Command word 1 byte) Version 1 byte) must be 02 byte)

Address type identifier 2 bytes) must be 02 bytes)

IP address

Must be 0

Must be 0

Metric value 1-16)

You can have up to 24 other routes in the same format as the first 20 bytes)

When the "command word" field is 1, it indicates a RIP request, and when it is 2, it indicates a RIP response. the address type identifier is always 2 in actual application, that is, the address type is IP address. the "ip address" field indicates the destination network address, and the "Metric" field indicates the number of hops required to reach the destination network ".

2. Features of The RIP Routing Protocol

1) Route information update features:

When a vro is initially started, it only contains the route information of its direct connection network, and the metric value of its direct connection network is 1, then it sends a complete route table RIP request to other routers around the request. The "ip address" field is 0.0.0.0 ). the router updates its route table based on the received RIP response. The specific method is to add a new route table entry and add its metric value to 1. if you receive the same route information as the destination address of an existing table item, you can take the following three actions: first, if the source port of an existing table item is the same as the source port of the new table item, the route table is unconditionally updated based on the latest route information. In the second case, the existing table item and the new table item come from different ports, then compare their metric values to the most self-owned route table item with a smaller metric value. In the third case, the metric values of the old and new table items are equal, the common solution is to retain the old table items.

The router sends its route table once every 30 seconds to broadcast the route table in the form of a RIP response ). for a route entry, if no new route information is received after 180 seconds, mark it as invalid, that is, the metric value is marked as 16. if no update information is available after the other 120 seconds, the invalid information will be deleted.

2) processing of the "version" field in the RIP message in RIP version 1:

0: Ignore the message.

1: Version 1 message, check the field "must be 0" in the message. If it does not meet the requirements, ignore the message.

1: do not check fields "must be 0" in the message. Only valid fields specified in RFC 1058 are processed. therefore, machines running RIP version 1 can receive Packets Processing RIP version 2, but will lose the information of the new rules of RIP version 2.

3) Address handling in RIP Route Protocol Version 1

RIP version 1 cannot identify the subnet network address. Because the route update packet transmitted by RIP does not contain the subnet mask, the RIP route information is either the host address and used for point-to-point link routing; it is either A, B, or C-type network address, used for Ethernet Routing, or 0.0.0.0, that is, the default route information.

4) count to Infinity Counting to Infinity)

In the limitations section of RIP, we mentioned the possible phenomenon of infinite count. Next we will analyze the causes and processes of this phenomenon and investigate the following simple network:

C (destination network) ---- router A ------ router B

Under normal circumstances, for the target network, the metric value of router A is 1, and the metric value of router B is 2. When the link between the target network and router A fails, it is broken:

C (destination network) -- | -- router A ------ router B

Vroa A sets the metric value of the route table entry for the target network C to 16, indicating that the target network is inaccessible, prepare to update the route table every 30 seconds. If router A receives A route update packet from router B before this message is sent, B contains the routing information about metric 2 of C. According to the routing update method mentioned above, router A mistakenly considers that there is A route through router B that can reach the target network C, so as to update its route table. The metric value for the route table entry of the target network C is changed from 16 to 3, the port is changed to the port connected to router B. obviously, A will send this message to B, B will unconditionally update its route table, and change metric to 4. The message is sent from B to A, and A changes metric to 5 ...... Finally, the metric value of the route table about the target network C is changed to 16, and the correct route information is obtained. this phenomenon is called the "count to infinity" phenomenon. Although the convergence is finally completed, the convergence speed is very slow, and network resources are wasted to send these cyclic groups.

In addition, we can also see from here that there is a conflict in the choice of the maximum metric value. If you select too small, then the applicable network size is too small; if you choose too large, the convergence time will become long when the count to infinity occurs.