Cisco ASA 5505 - Manual (Page 482)

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Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 25 Configuring RIP
Information About RIP
The ASA supports both RIP Version 1 and RIP Version 2. RIP Version 1 does not send the subnet mask
with the routing update. RIP Version 2 sends the subnet mask with the routing update and supports
variable-length subnet masks. Additionally, RIP Version 2 supports neighbor authentication when
routing updates are exchanged. This authentication ensures that the ASA receives reliable routing
information from a trusted source.
RIP has advantages over static routes because the initial configuration is simple, and you do not need to
update the configuration when the topology changes. The disadvantage to RIP is that there is more
network and processing overhead than in static routing.
Routing Update Process
RIP sends routing-update messages at regular intervals and when the network topology changes. When
a router receives a routing update that includes changes to an entry, it updates its routing table to reflect
the new route. The metric value for the path is increased by 1, and the sender is indicated as the next hop.
RIP routers maintain only the best route (the route with the lowest metric value) to a destination. After
updating its routing table, the router immediately begins transmitting routing updates to inform other
network routers of the change. These updates are sent independently of the regularly scheduled updates
that RIP routers send.
RIP Routing Metric
RIP uses a single routing metric (hop count) to measure the distance between the source and a destination
network. Each hop in a path from source to destination is assigned a hop count value, which is typically
1. When a router receives a routing update that contains a new or changed destination network entry, the
router adds 1 to the metric value indicated in the update and enters the network in the routing table. The
IP address of the sender is used as the next hop.
RIP Stability Features
RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops
allowed in a path from the source to a destination. The maximum number of hops in a path is 15. If a
router receives a routing update that contains a new or changed entry, and if increasing the metric value
by 1 causes the metric to be infinity (that is, 16), the network destination is considered unreachable. The
downside of this stability feature is that it limits the maximum diameter of a RIP network to less than 16
hops.
RIP includes a number of other stability features that are common to many routing protocols. These
features are designed to provide stability despite potentially rapid changes in network topology. For
example, RIP implements the split horizon and hold-down mechanisms to prevent incorrect routing
information from being propagated.
RIP Timers
RIP uses numerous timers to regulate its performance. These include a routing-update timer, a
route-timeout timer, and a route-flush timer. The routing-update timer clocks the interval between
periodic routing updates. Generally, it is set to 30 seconds, with a small random amount of time added
whenever the timer is reset. This is done to help prevent congestion, which could result from all routers