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EES4710BD 10 Slots L2/L3/L4 Chassis Switch
switch will be selected with the metrics increasing progressively. This greatly affects the route
selection and route aggregation time.
To avoid “infinite count”, RIP provides a 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
above-mentioned routes, but sets the costs of those routes to infinite. “Triggering update”
mechanism defines whenever route metric are changed by the gateway, the gateway will advertise
the update packets immediately, regardless of the 30 second update timer status.
There are 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 advertisement, subnet mask and authentication are 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. 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 use of multicast
(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 uses some of the zero field of RIP-I and requires no zero field verification. ES4710BD
layer 3 switches by default send RIP-II packets by multicast. Both RIP-I and RIP-II packets are
accepted.
Each layer 3 switch running RIP has a route database, which contains all route entries for reachable
destinations, the route table is built based on this database. When a RIP layer 3 switch sends route
update packets to neighboring devices, the complete route table is included in the packets. Therefore,
in a large network, routing data to be transferred and processed for each layer 3 switch is quite large,
causing degradation of network performance.
Besides the above-mentioned, RIP protocol allows route information discovered by the other routing
protocols to be introduced into the route table.
The operation of RIP protocol is shown below:
1. Enable RIP. The switch sends request packets to the neighboring layer 3 switches by
broadcasting; on receiving the request, the neighbor devices reply with the packets containing
their local routing information.
2. The Layer 3 switch modifies its local route table upon receiving the reply packets and sends
triggered update packets to the neighboring devices to advertise the route update information.
On receiving the triggered update packet, the neighboring layer 3 switches sends triggered
update packets to their neighboring layer 3 switches. After a sequence of triggered updates by
packet broadcast, all layer 3 switches get and maintain the latest route information.
In addition, RIP layer 3 switches will advertise its local route table to their neighbor devices
every 30 seconds. On receiving the packets, neighboring devices maintain their local route table,
select the best route and advertise the updated information to their own neighbor devices, so that the
updated routes are globally valid. Moreover, RIP uses a timeout mechanism for outdated route, that
is, if a switch does not receive regular update packets from a neighbor within a certain interval
(invalid timer interval), it considers the route from that neighbor invalid, after holding the route for a
certain interval (holddown timer interval), it will delete that route.