TP-Link T3700G-28TQ T3700G-28TQ V1 UG - Page 169

RIP timers, Routing loops prevention, Operation of RIP, RIP Version, After receiving such information

Page 169 highlights

 Route time: Time elapsed since the routing entry was last updated. The time is reset to 0 every time the routing entry is updated.  RIP timers RIP employs three timers: update, timeout and garbage-collect.  Update timer: defines the interval between routing updates.  Timeout timer: defines the route aging time. If no update for a route is received within the aging time, the metric of the route is set to 16 in the routing table.  Garbage-collect: timer defines the interval from when the metric of a route becomes 16 to when it is deleted from the routing table. During the garbage-collect timer length, RIP advertises the route with the routing metric set to 16. If no update is announced for that route after the garbage-collect timer expires, the route will be deleted from the routing table.  Routing loops prevention RIP is a distance vector (D-V) routing protocol. Since an RIP router advertises its own routing table to neighbors, routing loops may occur. RIP uses the following mechanisms to prevent routing loops.  Counting to infinity: The metric value of 16 is defined as unreachable. When a routing loop occurs, the metric value of the route will increment to 16.  Split horizon: A router does not send the routing information learned from a neighbor to this neighbor to prevent routing loops and save bandwidth.  Poison reverse: A router sets the metric of routes received from a neighbor to 16 and sends back these routes to the neighbor to help delete such information from the neighbor's routing table.  Triggered updates: A router advertises updates once the metric of a route is changed rather than after the update period expires to speed up network convergence.  Operation of RIP The following procedure describes how RIP works. 1) After RIP is enabled, the router sends request messages to neighboring routers. Neighboring routers return Response messages including information about their routing tables. 2) After receiving such information, the router updates its local routing table, and sends triggered update messages to its neighbors. All routers on the network do the same to keep the latest routing information. 3) By default, an RIP router sends its routing table to neighbors every 30 seconds. 4) RIP ages out routes by adopting an aging mechanism to keep only valid routes.  RIP Version RIP has two versions, RIPv1 and RIPv2. RIPv1, a classful routing protocol, supports message advertisement via broadcast only. RIPv1 protocol messages do not carry mask information, which means it can only recognize routing information of natural networks such as Class A, B, and C. That is why RIPv1 does not support discontinuous subnets. RIPv2 is a classless routing protocol. Compared with RIPv1, RIPv2 has the following advantages.  Supporting route tags. Route tags are used in routing policies to flexibly control routes. 158

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Route time: Time elapsed since the routing entry was last updated. The time is reset to 0
every time the routing entry is updated.
RIP timers
RIP employs three timers: update, timeout and garbage-collect.
Update timer: defines the interval between routing updates.
Timeout timer: defines the route aging time. If no update for a route is received within the
aging time, the metric of the route is set to 16 in the routing table.
Garbage-collect: timer defines the interval from when the metric of a route becomes 16 to
when it is deleted from the routing table. During the garbage-collect timer length, RIP
advertises the route with the routing metric set to 16. If no update is announced for that
route after the garbage-collect timer expires, the route will be deleted from the routing
table.
Routing loops prevention
RIP is a distance vector (D-V) routing protocol. Since an RIP router advertises its own routing table
to neighbors, routing loops may occur.
RIP uses the following mechanisms to prevent routing loops.
Counting to infinity: The metric value of 16 is defined as unreachable. When a routing
loop occurs, the metric value of the route will increment to 16.
Split horizon: A router does not send the routing information learned from a neighbor to
this neighbor to prevent routing loops and save bandwidth.
Poison reverse: A router sets the metric of routes received from a neighbor to 16 and
sends back these routes to the neighbor to help delete such information from the
neighbor’s routing table.
Triggered updates: A router advertises updates once the metric of a route is changed
rather than after the update period expires to speed up network convergence.
Operation of RIP
The following procedure describes how RIP works.
1)
After RIP is enabled, the router sends request messages to neighboring routers. Neighboring
routers return Response messages including information about their routing tables.
2)
After receiving such information, the router updates its local routing table, and sends triggered
update messages to its neighbors. All routers on the network do the same to keep the latest
routing information.
3)
By default, an RIP router sends its routing table to neighbors every 30 seconds.
4)
RIP ages out routes by adopting an aging mechanism to keep only valid routes.
RIP Version
RIP has two versions, RIPv1 and RIPv2.
RIPv1, a classful routing protocol, supports message advertisement via broadcast only. RIPv1
protocol messages do not carry mask information, which means it can only recognize routing
information of natural networks such as Class A, B, and C. That is why RIPv1 does not support
discontinuous subnets.
RIPv2 is a classless routing protocol. Compared with RIPv1, RIPv2 has the following advantages.
Supporting route tags. Route tags are used in routing policies to flexibly control routes.
158