RIP Characteristics and RIP Timers

Posted: 12th June 2022 by ccna7guru in Packet Tracer, SRWE
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RIP Characteristics: RIP has the following key characteristics:

  • RIP is a distance vector routing protocol.
  • RIP uses hop count as its only metric for path selection.
  • Advertised routes with hop counts greater than 15 are unreachable.
  • Messages are broadcast every 30 seconds.

RIP Timers
In addition to the update timer, the IOS implements three additional timers for RIP:

  1. Invalid
  2. Flush
  3. Holddown

Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the default), the route is marked as invalid by setting the metric to 16. The route is retained in the routing table until the flush timer expires.

Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table.

Holddown Timer. This timer stabilizes routing information and helps prevent routing loops during periods when the topology is converging on new information. Once a route is marked as unreachable, it must stay in holddown long enough for all routers in the topology to learn about the unreachable network. By default, the holddown timer is set for 180 seconds.

The timer values can be verified with two commands: show ip route and show ip protocols. Notice in the output from show ip route that each route learned through RIP shows the elapsed time since the last update, expressed in seconds.

show ip protocols

Bounded Updates EIGRP: Unlike other distance vector routing protocols, EIGRP does not send periodic updates. Instead, EIGRP sends bounded updates about a route when a path changes or the metric for that route changes. When a new route becomes available or when a route needs to be removed, EIGRP sends an update only about that network instead of the entire table. This information is sent only to those routers that need it.

EIGRP uses updates that are:

  • Non-periodic because they are not sent out on a regular basis.
  • Partial updates sent only when there is a change in topology that influences routing information.
  • Bounded, meaning the propagation of partial updates are automatically bounded so that only those routers that need the information are updated.               

Triggered Updates: To speed up the convergence when there is a topology change, RIP uses triggered updates. A triggered update is a routing table update that is sent immediately in response to a routing change. Triggered updates do not wait for update timers to expire. The detecting router immediately sends an update message to adjacent routers. The receiving routers, in turn, generate triggered updates that notify their neighbors of the change.

Triggered updates are sent when one of the following occurs:

  • An interface changes state (up or down)
  • A route has entered (or exited) the “unreachable” state
  • A route is installed in the routing table

Using only triggered updates would be sufficient if there were a guarantee that the wave of updates would reach every appropriate router immediately. However, there are two problems with triggered updates:

  • Packets containing the update message can be dropped or corrupted by some link in the network.
  • The triggered updates do not happen instantaneously. It is possible that a router that has not yet received the triggered update will issue a regular update at just the wrong time, causing the bad route to be reinserted in a neighbor that had already received the triggered update.

What is a Routing Loop?

A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network. A routing loop can occur when two or more routers have routing information that incorrectly indicates that a valid path to an unreachable destination exists.   The loop may be a result of:

  • Incorrectly configured static routes
  • Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol)
  • Inconsistent routing tables not being updated due to slow convergence in a changing network
  • Incorrectly configured or installed discard routes

Distance vector routing protocols are simple in their operations. Their simplicity results in protocol drawbacks like routing loops. Routing loops are less of a problem with link-state routing protocols but can occur under certain circumstances.

Note: The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.

What are the Implications of Routing Loops?

A routing loop can have a devastating effect on a network, resulting in degraded network performance or even a network downtime.

A routing loop can create the following conditions:

  • Link bandwidth will be used for traffic looping back and forth between the routers in a loop.
  • A router’s CPU will be strained due to looping packets.
  • A router’s CPU will be burdened with useless packet forwarding that will negatively impact the convergence of the network.
  • Routing updates may get lost or not be processed in a timely manner. These conditions would introduce additional routing loops, making the situation even worse.
  • Packets may get lost in “black holes.”

There are a number of mechanisms available to eliminate routing loops, primarily with distance vector routing protocols. These mechanisms include:

  • Defining a maximum metric to prevent count to infinity
  • Holddown timers
  • Split horizon
  • Route poisoning or poison reverse
  • Triggered updates
RIP v1 Message Format