IBM RackSwitch G8000, Application Manual

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© Copyright IBM Corp. 2011
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Chapter 18. Routing Information Protocol
In a routed environment, routers communicate with one another to keep track of
available routes. Routers can learn about available routes dynamically using the
Routing Information Protocol (RIP). IBM Networking OS software supports RIP
version 1 (RIPv1) and RIP version 2 (RIPv2) for exchanging TCP/IPv4 route
information with other routers.
Note: IBM N/OS 6.8 does not support IPv6 for RIP.
Distance Vector Protocol
RIP is known as a distance vector protocol. The vector is the network number
and next hop, and the distance is the metric associated with the network number.
RIP identifies network reachability based on metric, and metric is defined as hop
count. One hop is considered to be the distance from one switch to the next, which
typically is 1.
When a switch receives a routing update that contains a new or changed destination
network entry, the switch adds 1 to the metric value indicated in the update and
enters the network in the routing table. The IPv4 address of the sender is used as
the next hop.
Stability
RIP includes a number of other stability features that are common to many routing
protocols. For example, RIP implements the split horizon and hold-down
mechanisms to prevent incorrect routing information from being propagated.
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. The network destination network is
considered unreachable if increasing the metric value by 1 causes the metric to be
16 (that is infinity). This limits the maximum diameter of a RIP network to less than
16 hops.
RIP is often used in stub networks and in small autonomous systems that do not
have many redundant paths.
Routing Updates
RIP sends routing-update messages at regular intervals and when the network
topology changes. Each router “advertises” routing information by sending a routing
information update every 30 seconds. If a router doesn’t receive an update from
another router for 180 seconds, those routes provided by that router are declared
invalid. The routes are removed from the routing table, but they remain in the RIP
routes table. After another 120 seconds without receiving an update for those
routes, the routes are removed from respective regular updates.
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.
For more information, see the Configuration section, Routing Information Protocol
Configuration in the IBM Networking OS Command Reference .