RIP
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route to be sent to the neighbor gateways the routes learnt from the neighbor gateways;
poison reverse split horizon not only deletes the abovementioned routes, but set the costs
of those routes to infinite. ―Triggering update‖ mechanism defines whenever route metric
changed by the gateway, the gateway advertise the update packets immediately,
regardless of the 30 second update timer status.
There two versions of RIP, version 1 and version 2. RFC1058 introduces RIP-I
protocol, RFC2453 introduces RIP-II, which is compatible with RFC1723 and RFC1388.
RIP-I updates packets by packets broadcast, subnet mask and authentication is not
supported. Some fields in the RIP-
I packets are not used and are required to be all 0‘s; for
this reason, such all 0's fields should be checked when using RIP-I, the RIP-I packets
should be discarded if such fields are non-zero. RIP-II is a more improved version than
RIP-I. RIP-II sends route update packets by multicast packets (multicast address is
224.0.0.9). Subnet mask field and RIP authentication filed (simple plaintext password and
MD5 password authentication are supported), and support variable length subnet mask.
RIP-II used some of the zero field of RIP-I and require no zero field verification. switch
send RIP-II packets in multicast by default, both RIP-I and RIP-II packets will be accepted.
Each layer3 switch running RIP has a route database, which contains all route entries
for reachable destination, and route table is built based on this database. When a RIP
layer3 switch sent route update packets to its neighbor devices, the complete route table
is included in the packets. Therefore, in a large network, routing data to be transferred and
processed for each layer3 switch is quite large, causing degraded network performance.
Besides the above mentioned, RIP protocol allows route information discovered by
the other routing protocols to be introduced to the route table. It can also be as the
protocol exchanging route messages with CE on PE routers, and supports the VPN
route/transmitting examples.
The operation of RIP protocol is shown below:
1
．
Enable RIP. The switch sends request packets to the neighbor layer3
switches by broadcasting; on receiving the request, the neighbor devices
reply with the packets containing their local routing information.
2
．
The Layer3 switch modifies its local route table on receiving the reply packets
and sends triggered update packets to the neighbor devices to advertise
route update information. On receiving the triggered update packet, the
neighbor lay3 switches send triggered update packets to their neighbor lay3
switches. After a sequence of triggered update packet broadcast, all layer3
switches get and maintain the latest route information.
In addition, RIP layer3 switches will advertise its local route table to their neighbor
devices every 30 seconds. On receiving the packets, neighbor devices maintain their local
route table, select the best route and advertise the updated information to their own