Accton Technology ES4626 - Manual (Page 390)

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count” will be resulted. In other words, the route of unreachable layer3 switch will be
selected with the metrics increasing progressively. This greatly affects the route selection
and route aggregation time.
To prevent “infinite count”, RIP provides mechanism such as “split horizon” and
“triggered update” to solve route loop. “Split horizon” is done by avoiding sending to a
gateway routes leaned from that gateway. There are two split horizon methods: “simple
split horizon” and “poison reverse split horizon”. Simple split horizon deletes from the
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.
ES4624-SFP/ES4626-SFP 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:
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.