Extreme Networks ExtremeWare XOS 10.1, Manual

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Overview of RIP
ExtremeWare XOS 10.1 Concepts Guide 187
Overview of RIP
RIP is an Interior Gateway Protocol (IGP) first used in computer routing in the Advanced Research
Projects Agency Network (ARPAnet) as early as 1969. It is primarily intended for use in homogeneous
networks of moderate size.
To determine the best path to a distant network, a router using RIP always selects the path that has the
least number of hops. Each router that data must traverse is considered to be one hop.
Routing Table
The routing table in a router using RIP contains an entry for every known destination network. Each
routing table entry contains the following information:
• IP address of the destination network
• Metric (hop count) to the destination network
• IP address of the next router
•
Timer that tracks the amount of time since the entry was last updated
The router exchanges an update message with each neighbor every 30 seconds (default value), or if
there is a change to the overall routed topology (also called triggered updates). If a router does not receive
an update message from its neighbor within the route timeout period (180 seconds by default), the
router assumes the connection between it and its neighbor is no longer available.
Split Horizon
Split horizon is a scheme for avoiding problems caused by including routes in updates sent to the
router from which the route was learned. Split horizon omits routes learned from a neighbor in updates
sent to that neighbor.
Poison Reverse
Like split horizon, poison reverse is a scheme for eliminating the possibility of loops in the routed
topology. In this case, a router advertises a route over the same interface that supplied the route, but the
route uses a hop count of 16, defining it as unreachable.
Triggered Updates
Triggered updates occur whenever a router changes the metric for a route, and it is required to send an
update message immediately, even if it is not yet time for a regular update message to be sent. This will
generally result in faster convergence, but may also result in more RIP-related traffic.
Route Advertisement of VLANs
VLANs that are configured with an IP address, but are configured to not route IP or are not configured
to run RIP, do not have their subnets advertised by RIP. Only those VLANs that are configured with an
IP address and are configured to route IP and run RIP have their subnets advertised.