H3C S3610-28P Operation Manual page 348

Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
II. RIP routing table
A RIP router has a routing table containing routing entries of all reachable destinations,
and each routing entry contains:
Destination address: IP address of a host or a network.
Next hop: IP address of the adjacent router's interface to reach the destination.
Egress interface: Packet outgoing interface.
Metric: Cost from the local router to the destination.
Route time: Time elapsed since the routing entry was last updated. The time is
reset to 0 every time the routing entry is updated.
Route tag: Identifies a route, used in a routing policy to flexibly control routes. For
information about routing policy, refer to Routing Policy Configuration.
III. RIP timers
RIP employs four timers, update, timeout, suppress, and garbage-collect.
The update timer defines the interval between routing updates.
The 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.
The suppress timer defines how long a RIP route stays in the suppressed state.
When the metric of a route is 16, the route enters the suppressed state. In the
suppressed state, only routes which come from the same neighbor and whose
metric is less than 16 will be received by the router to replace unreachable routes.
The 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.
IV. Routing loops prevention
RIP is a distance vector (D-V) routing protocol. Since a 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 the 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 useless 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.
2-2
Chapter 2 RIP Configuration