Static Routing and Distance Vector Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 324

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IP Address —This is the destination route; it can be a network address, subnet, or host
route. You use special route 0.0.0.0 for the default route.

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Metric —The metric field is 32 bits in length. It contains a value between 1 and 15
inclusive, specifying the current metric for the destination. The metric is set to 16 to
indicate that a destination is not reachable.

Because RIP has a maximum hop count, it implements counting to infinity. For RIP, infinity is
16 hops. In the RIP message, no subnet masks accompany each route. Five 32-bit words are
repeated for each route entry that includes the following: AFI (16-bits), Unused must be zero
(16-bits), IP address, two more 32-bit unused fields, and the 32-bit metric. Five 32-bit words
equal 20 bytes for each route entry. Up to 25 routes are allowed in each RIP message. The
reason that only 25 routes are allowed is that the maximum datagram size is limited to 512 bytes
(not including the IP header). Calculating 25 routes

20 bytes each, plus the RIP header
(4 bytes), plus an 8-byte UDP header, equals 512 bytes.

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RIPv1 Timers
Cisco Systems uses four timers when implementing RIP. These timers are as follows:
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Update

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Invalid

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Holddown

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Flush

RIP sends its full routing table out all configured interfaces. The table is sent periodically as a
broadcast (255.255.255.255) to all hosts. The update timer specifies the frequency of the
periodic broadcasts. By default, the update timer is set to 30 seconds. Each route has a timeout
value associated with it. The timeout gets reset every time the router receives a routing update
containing the route. When the timeout value expires, the route is marked as unreachable by
marking it as invalid. The route is marked as invalid by setting the metric to 16. The route is
retained in the routing table. By default, the invalid timer is 180 seconds, or six update periods
(30 × 6 = 180).

A route entry marked as invalid is retained in the routing table until the flush timer expires. By
default, the flush timer is 240 seconds, which is 60 seconds longer than the expiration timer.

Cisco implements an additional timer for RIP—the holddown timer. You use the holddown
timer to stabilize routes by setting an allowed time for which routing information regarding
different paths is suppressed. After the metric for a route entry changes, no updates for the route
are accepted until the holddown timer expires. By default, the holddown timer is 180 seconds.