ROUTING INFORMATION PROTOCOL
RIP (Routing Information Protocol) is a way for routers, which connect networks using the Internet Protocol (IP), to share information about how to route traffic among networks. RIP is classified by the Internet Engineering Task Force (IETF) as an Interior Gateway Protocol (IGP), one of several protocols for routers moving traffic around within a larger autonomous system network — e.g., a single enterprise’s network that may be compromised of many separate local area networks (LANs) linked through routers. Although once the most widely used IGP, Open Shortest Path First (OSPF) routing has largely replaced RIP in corporate networks. Each RIP router maintains a routing table, which is a list of all the destinations (networks) it knows how to reach, along with the distance to that destination.
RIP uses a distance vector algorithm. to decide which path to put a packet on to get to its destination. It stores in its routing table the distance for each network it knows how to reach along with the addresses of the “next hop” router — another router that is on one of the same networks — through which a packet has to travel to get to that destination. If it receives an update on a route, and the new path is shorter, it will update its table entry with the length and next-hop addresses of the shorter path; if the new path is longer, it will wait through a “hold-down” period to see if later updates reflect the higher value as well, and only update the table entry if the new, longer path is stable.
HOW DOES IT WORK?
Using RIP, each router sends its entire routing table to its closest neighbours every 30 seconds. The neighbours in turn will pass the information on to their nearest neighbours, and so on, until all RIP hosts within the network have the same knowledge of routing paths, a state known as convergence.
If a router crashes or a network connection is severed, the network discovers this because that router stops sending updates to its neighbours, or stops sending and receiving updates along the severed connection. If a given route in the routing table isn’t updated across six successive update cycles
( that is, for 180 seconds ) a RIP router will drop that route, letting the rest of the network know via its own updates about the problem and begin the process of reconverging on a new network topology.
RIP uses a modified hop count as a way to determine network distance. By default, if a router’s neighbour owns a destination network, that route has one hop, described as a cost of 1. RIP allows only 15 hops in a path. If a packet can’t reach a destination in 15 hops, the destination is considered unreachable. Paths can be assigned a higher cost ( as if they involved extra hops) if the enterprise wants to limit or discourage their use. RIP has been supplanted mainly due to its simplicity and its inability to scale to very large and complex networks. Other routing protocols push less information of their own onto the network, while RIP pushes its whole routing table every 30 seconds. As a result, other protocols can converge more quickly, use more sophisticated routing algorithms, include latency, packet loss, actual monetary cost and other link characteristics, as well as hop count with arbitrary weighting.
There are only two message types used by RIP, Request message and Response message. The names are as self descriptive as they can be. When a RIP enabled router interface comes up, it sends out a Request message. The other RIP enabled routers in the network are responding with Response messages. When the first router receives Response messages, it installs the new received routes in its routing table. If the router already has a route in its table, but it gets one with a better hop count, the old route is replaced. After that, the router sends its own routing table to its neighbours.
The default Administrative Distance for RIP is 120. In routing, the AD is used as a reference of trustworthiness. The lower the value, the higher the priority of that route. For example, if you have a route received through RIP, which has an AD of 120, and you have the same route received from a higher priority protocol like OSPF, which has an AD of 110. OSPF will be in charge to route the packets through its route, even if the routing protocol metric is greater. Due to its lack of scaling capabilities, RIP is the least-preferred protocol from all Interior Gateway Protocol (IGPs).
You can only enable one RIP routing process on the ASA. After you enable the RIP routing process, you must define the interfaces that will participate in that routing process using the network command. By default, the ASA sends RIP Version 1 updates and accepts RIP Version 1 and Version 2 updates. To enable the RIP routing process, perform the following step :
The no router rip command can be used to remove entire RIP configuration that has been enabled. Once this is cleared, you must reconfigure RIP again using the router rip command. GENERATING A DEFAULT ROUTE To generate a default route in RIP, use the following steps :
CONFIGURING INTERFACES FOR RIP
If you have an interface that you do not want to participate in RIP routing, but that is attached to a network that you want advertised, you can configure a network command that covers the network to which the interface is attached, and use the passive-interface command to prevent that interface from sending RIP advertisements. Additionally, you can specify the version of RIP that is used by the ASA for updates.
DISABLING ROUTE SUMMARIZATION
RIP Version 1 always uses automatic route summarisation. This feature cannot be disabled for RIP Version 1. RIP Version 2 uses automatic route summarisation by default. The RIP routing process summarises on network number boundaries. This can cause routing problems if you have non- contiguous networks.
To disable automatic router summarisation, enter the following command in router configuration mode for the RIP routing process :
FILTERING NETWROKS IN RIP
To filter the networks received in updates, perform the following steps :
REDISTRIBUTING ROUTES INTO THE RIP ROUTING PROCESS
Routes can be redistributed from the OSPF, EIGRP, static, and connected routing processes into the RIP routing process. To redistribute a route into the RIP routing process, perform the following steps :
CONFIGURING RIP SEND/RECEIVE VERSION ON AN INTERFACE
One can override the globally-set version of RIP the ASA uses to send and receive RIP updates on a per-interface basis. To configure the RIP send and receive version, perform the following steps :
ENABLING RIP AUTHENTICATION
To enable RIP authentication on an interface, perform the following steps :
The following commands can be used to monitor or debug the RIP routing process. Debugging output is assigned high priority in the CPU process and can render the system unusable. It is best to use debug commands during periods of lower network traffic and fewer users. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system performance.
To monitor or debug various RIP routing statistics, perform the following tasks :
CONFIGURATION EXAMPLE FOR RIP
The following example shows how to enable and configure RIP with various optional processes :
FEATURE HISTORY FOR RIP
ADVANTAGES OF RIP
- Very easy to understand and configure.
- Almost guaranteed to be supported by all routers.
- Supports load balancing.
- Generally loop free.
LIMITATIONS OF RIP
- Classful Routing
RIP uses glassful routing and it does not support classless routing. For example, if a network that is connected to a
router running RIP is using VLSM, all the nodes/hosts on that network should sue the same network mask.
- Router Authentication
There was no support for authentication on RIP version 1 which made the routers vulnerable to attacks. With RIP version 2, MD5 authentication came into the place and instead of plain text password, encrypted passwords were used for the authentication. Router authentication is used when a router receives a route update from its neighbour router. However, other mechanisms were implemented to prevent wrong information from being populated to other routers in RIP v2.
- Split Horizon Route Advertisement
To prevent the routing loops, routers do not broadcast the update on the interfaces it receives an update from.
- Route Poisoning with Hold Down Timer
The concepts of route poisoning and hold down timer are utilised to prevent packet looping and making sure that no routers send a packet to an invalid router.
For instance, when a Router A detects that one of the networks connected directly to its interface is down, it sets its distance as 16 (hop count 16 is counted as infinity in RIP) and broadcasts the update to other connected routers.
It might take some time in the network for route poisoned message to reach every other router. In the meantime, the router that advertised the message, Router A, would not accept any update from any other router until the hold-down timer expires.
Once the hold-down timer expires, the router starts accepting updates from other routers. In this case of RIP, the hold-down timer is 180 seconds, six times the update timer.
RIP, like all routing protocols, is designed to disseminate the network information pertinent to routers. At the most basic level, routers need to know what networks are reachable and how far away they are. RIP does this, and it is still widely used today. RIP has its share of detractors, but as one of the oldest and simplest routing protocols, understanding it is key to understanding routing.
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