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All original material copyright © 2012 by Aaron Balchunas (aaron@routeralley ),
- Routing Information Protocol -
RIP (Routing Information Protocol)
RIP is a standardized Distance Vector protocol, designed for use on smaller networks. RIP was one of the first true Distance Vector routing protocols, and is supported on a wide variety of systems.
RIP adheres to the following Distance Vector characteristics:
•
RIP sends out periodic routing updates (every 30 seconds ) •
RIP sends out the full routing table every periodic update •
RIP uses a form of distance as its metric (in this case, hopcount ) •
RIP uses the Bellman-Ford Distance Vector algorithm to determine the best “path” to a particular destination
Other characteristics of RIP include:
include和contain• RIP supports IP and IPX routing. • RIP utilizes UDP port 520 • RIP routes have an administrative distance of 120. • RIP has a maximum hopcount of 15 hops.
Any network that is 16 hops away or more is considered unreachable to RIP, thus the maximum diameter of the network is 15 hops. A metric of 16 hops in RIP is considered a poison route  or infinity metric.
If multiple paths exist to a particular destination, RIP will load balance between those paths (by default, up to 4) only if the metric (hopcount) is equal . RIP uses a round-robin system of load-balancing between equal metric routes, which can lead to pinhole congestion .
For example, two paths might exist to a particular destination, one going through a 9600 baud link, the other via a T1. If the metric (hopcount) is equal, RIP will load-balance, sending an equal amount of traffic down the 9600 baud link and the T1. This will (obviously) cause the slower link to become congested.
RIP Versions
RIP has two versions, Version 1 (RIPv1) and Version2 (RIPv2).
RIPv1 (RFC 1058) is classful, and thus does not include the subnet mask with its routing table updates. Because of this, RIPv1 does not support Variable Length Subnet Masks (VLSMs). When using RIPv1, networks must be contiguous, and subnets of a major network must be configured with identical subnet masks. Otherwise, route table inconsistencies (or worse) will occur.
RIPv1 sends updates as broadcasts to address 255.255.255.255.
RIPv2 (RFC 2543) is classless, and thus does include the subnet mask with its routing table updates. RIPv2 fully supports VLSMs, allowing discontiguous networks and varying subnet masks to exist.
Other enhancements offered by RIPv2 include:
•Routing updates are sent via multicast, using address 224.0.0.9
•Encrypted authentication can be configured between RIPv2 routers •Route tagging is supported (explained in a later section)
RIPv2 can interoperate with RIPv1. By default:
•RIPv1 routers will sent only Version 1 packets
•RIPv1 routers will receive both Version 1 and 2 updates
•RIPv2 routers will both send and receive only Version 2 updates
We can control the version of RIP a particular interface will “send” or “receive.”
Unless RIPv2 is manually specified, a Cisco will default to RIPv1 when configuring RIP.
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All original material copyright © 2012 by Aaron Balchunas (aaron@routeralley),
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All original material copyright © 2012 by Aaron Balchunas (aaron@routeralley ),
RIPv1 Basic Configuration
Routing protocol configuration occurs in Global Configuration mode. On Router A, to configure RIP, we would type:
Router(config)#  router rip
Router(config-router)#  network 172.16.0.0
Router(config-router)#  network 172.17.0.0
The first command, router rip , enables the RIP process.
The network  statements tell RIP which networks you wish to advertise to other RIP routers. We simply list the networks that are directly connected to our router. Notice that we specify the networks at their classful boundaries, and we do not specify a subnet mask.
To configure Router B:
Router(config)#  router rip
Router(config-router)#  network 172.17.0.0
Router(config-router)#  network 172.18.0.0
The routing table on Router A will look like:
RouterA#  show ip route
<eliminated irrelevant header>
Gateway of last resort is not set
C 172.16.0.0 is directly connected, Ethernet0
C 172.17.0.0 is directly connected, Serial0
R 172.18.0.0 [120/1] via 172.17.1.2, 00:00:00, Serial0
The routing table on Router B will look like:
RouterB#  show ip route
<eliminated irrelevant header>
Gateway of last resort is not set
C 172.17.0.0 is directly connected, Serial0
C 172.18.0.0 is directly connected, Ethernet0
R 172.16.0.0 [120/1] via 172.17.1.1, 00:00:00, Serial0
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All original material copyright © 2012 by Aaron Balchunas (aaron@routeralley ),
Limitations of RIPv1
The example on the previous page works fine with RIPv1, because the networks are contiguous and the subnet masks are consistent. Consider the
This particular scenario will still  work when using RIPv1, despite the fact that we’ve subnetted the m
ajor 10.0.0.0 network. Notice that the subnets are contiguous (that is, they belong to the same major network), and use the same subnet mask.
When Router A sends a RIPv1 update to Router B via Serial0, it will not include the subnet mask for the 10.1.0.0 network. However, because the 10.3.0.0 network is in the same major network as the 10.1.0.0 network, it will not  summarize  the address. The route entry in the update will simply state “10.1.0.0”.
Router B will accept this routing update, and realize that the interface receiving the update (Serial0) belongs to the same major network as the route entry of 10.1.0.0. It will then apply the subnet mask of its Serial0 interface to this route entry.
Router C will similarly send an entry for the 10.2.0.0 network to Router B. Router B’s routing table will thus look like:
RouterB#  show ip route
Gateway of last resort is not set
10.0.0.0/16 is subnetted, 4 subnets
C 10.3.0.0 is directly connected, Serial0
C 10.4.0.0 is directly connected, Serial1
R 10.1.0.0 [120/1] via 10.3.5.1, 00:00:00, Serial0
R 10.2.0.0 [120/1] via 10.4.5.1, 00:00:00, Serial1
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All original material copyright © 2012 by Aaron Balchunas (aaron@routeralley ),
Limitations of RIPv1 (continued)
Consider the following, slightly altered, example:
We’ll assume that RIPv1 is configured correctly on all routers. Notice that our networks are no longer contiguous. Both Router A and Router C contain subnets  of the 10.0.0.0 major network (10.1.0.0 and 10.2.0.0 respectively).
Separating these networks now are two Class C subnets (192.168.123.0 and 192.168.111.0).
Why is this a problem? Again, when Router A sends a RIPv1 update to Router B via Serial, it will not include the subnet mask for the 10.1.0.0 network. Instead, Router A will consider itself a border  router, as the 10.1.0.0 and 192.168.123.0 networks do not  belong to the same major network. Router A will summarize  the 10.1.0.0/16 network to its classful boundary of 10.0.0.0/8.
Router B will accept this routing update, and realize that it does not have a directly connected interface in scheme. Thus, it has no subnet mask to apply to this route. Because of this, Router B will install the summarized 10.0.0.0 route into its routing table.
Router C, similarly, will consider itself a border router between networks 10.2.0.0 and 192.168.111.0. Thus, Router C will also  send a summarized 10.0.0.0 route to Router B.

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