Overview of route information protocol (RIP)

I. background
The route information protocol (RIP) uses the number of hops as the distance vector protocol of metric. RIP is widely used in global Internet routing. It is an internal gateway protocol that implements routing functions within the autonomous system. External gateway protocol (such as edge gateway protocol BGP) is used for routing between different Autonomous Systems. The predecessor of RIP is the Xerox protocol GWINFO. Later versions of routed were pronounced as/rutdi/) encapsulated in the Berkeley standard release Unix (BSD) in 1982 ). RIP itself is developed into the protocol of the special network, and some protocol families use the RIP variant, such as: AppleTalk route table maintenance protocol RTMP) and Banyan VINES route table protocol RIP) it is based on the IP version of RIP. The latest enhanced version of RIP is the RIP2 specification, which allows more information to be contained in the RIP group and provides a simple authentication mechanism.
Ip rip is formally defined in two documents: RFC 1058 and 1723. RFC 1058 (1988) describes the implementation of the first version of RIP. RFC 1723 (1994) is an update of RIP, allowing the RIP group to carry more information and security features.
This article briefly introduces the basic capabilities and features of RIP, including the route update process, RIP route metric, route stability and timer.
Ii. route update
RIP sends route update information at the regular interval and when the network topology changes. When a vro receives an Update route entry that contains a table entry, it updates its route table. The value of metric for this path is added with 1, and the sender is recorded as the next hop. The RIP Router only maintains the path with the minimum metric value in the destination Optimal Path ). After updating the route table, the router immediately sends a route update notification to other routers, which is irrelevant to the periodically sent update information.
Iii. RIP route metric
RIP uses a single route metric hop count) to measure the distance from the source network to the target network. Each hop in the path from the source to the destination is assigned a hop value, which is usually 1. When the router receives the route update information that contains the new or changed destination network table items, it adds the metric value to 1 and saves it to the route table. the IP address of the sender acts as the next hop address.
RIP restricts the maximum number of hops from the source to the target to prevent routing loops. The maximum value is 15. If the vro receives the route update information containing the new or changed table items and adds the metric value to 1 to become an infinite number (16), the target network cannot be reached.
Iv. RIP Stability
To adapt to fast network topology changes, RIP defines the same stability characteristics as other routing protocols. For example, RIP implements the split-horizon and hold-down mechanisms to prevent route information from being transmitted incorrectly. In addition, the limit on the number of hops of RIP also prevents the infinite growth of routing loops.
5. RIP Timer
RIP uses some timers to control its performance, including the route update timer, route timeout, and route clearing timer. The interval at which the route update timer records periodic updates, usually 30 seconds. A small random number of seconds is added when the timer is reset to prevent conflicts. Each route table entry has a related route time-out timer. When the route time-out timer expires, the path is marked as invalid, but it is still saved in the routing table, it is not cleared until the route clearing timer expires.
Vi. RIP group format
The following describes the grouping formats of ip rip and IP RIP2.
1. RIP group format

· Command -- indicates whether the group is a request or a response. The request group requires the router to send all or part of its route table. The Response Group can be an active periodic route update or response to the request. Large route tables can use multiple RIP groups to transmit information.
· Version number -- indicates the RIP version used. This field can notify different versions of incompatibility.
· Zero -- unused.
· Address family flag (AFI)-specifies the address family in use. RIP is designed to carry routing information of different protocols. Each item has an address family flag to indicate the address type used. The AFI of the IP is 2.
· Address -- specify the IP address of the item.
· Metric -- indicates the number of hops that have taken place during the target process ). The valid path is between 1 and 15, and 16 indicates the path is inaccessible.
Note: up to 25 AFI, address, and metric fields can be contained in an ip rip group. That is, up to 25 IP address entries can be contained in an RIP group.
2. RIP2 group format
The RIP2 specification (RFC1723) allows the RIP group to contain more information and provides a simple authentication mechanism, such.
· Command -- indicates whether the group is a request or a response. The request group requires the router to send all or part of its route table. The Response Group can be an active periodic route update or response to the request. Large route tables can use multiple RIP groups to transmit information.
· Version -- indicates the RIP version used. The value is 2 in the RIP group that implements RIP2 or performs authentication.
· Not used -- the value is 0.
· Address family flag (AFI)-specifies the address family in use. RIP is designed to carry routing information of different protocols. Each item has an address family flag to indicate the address type used. The AFI of the IP is 2. If the AFI of the first item is 0 xFFFF, the remaining part of the item is the authentication information. Currently, the only authentication type is a simple password.
· Route tag-provides a method to distinguish between internal routes learned by RIP) and external Routes learned by other protocols.
· IP Address -- specify the IP address of the item.
· Subnet mask-the subnet mask that contains this item. If this field is 0, the subnet mask is not specified.
· Next hop -- specify the IP address of the next hop.
· Metric -- indicates the number of hops that have taken place during the target process ). The valid path is between 1 and 15, and 16 indicates the path is inaccessible.
Note: up to 25 AFI, address, and metric fields can be contained in an ip rip group. That is, up to 25 IP address entries can be contained in an RIP group. If AFI indicates authentication information, there are only 24 route table entries.

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