Split Horizon For Rip - Cisco NCS 6000 Series Configuration Manual

Implementing RIP

Split Horizon for RIP

Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing
protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks
information about routes from being advertised by a router out of any interface from which that information
originated. This behavior usually optimizes communications among multiple routers, particularly when links
are broken.
If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be
sourced by every secondary address. One routing update is sourced per network number unless split horizon
is disabled.
Note
The split horizon feature is enabled by default. In general, we recommend that you do not change the default
state of split horizon unless you are certain that your operation requires the change in order to properly advertise
routes.
Route Timers for RIP
RIP uses several timers that determine such variables as the frequency of routing updates, the length of time
before a route becomes invalid, and other parameters. You can adjust these timers to tune routing protocol
performance to better suit your internetwork needs, by making the following timer adjustments to:
• The rate (time in seconds between updates) at which routing updates are sent
• The interval of time (in seconds) after which a route is declared invalid
• The interval (in seconds) during which routing information regarding better paths is suppressed
• The amount of time (in seconds) that must pass before a route is removed from the RIP topology table
• The amount of time delay between RIP update packets
The first four timer adjustments are configurable by the timers basic command. The output-delay command
changes the amount of time delay between RIP update packets. See
configuration details.
It also is possible to tune the IP routing support in the software to enable faster convergence of the various IP
routing algorithms and quickly drop back to redundant routers, if necessary. The total result is to minimize
disruptions to end users of the network in situations in which quick recovery is essential.
Route Redistribution for RIP
Redistribution is a feature that allows different routing domains, to exchange routing information. Networking
devices that route between different routing domains are called boundary routers, and it is these devices that
inject the routes from one routing protocol into another. Routers within a routing domain only have knowledge
of routes internal to the domain unless route redistribution is implemented on the boundary routers.
When running RIP in your routing domain, you might find it necessary to use multiple routing protocols
within your internetwork and redistribute routes between them. Some common reasons are:
• To advertise routes from other protocols into RIP, such as static, connected, OSPF, and BGP.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Split Horizon for RIP
Customizing RIP, on page 372 for
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