RIP Protocol Fundamentals: Configuration, Timers, and Optimization

Routing Information Protocol (RIP) Essentials

RIPv1 vs. RIPv2: Classful vs. Classless Routing

What is the difference between RIPv1 and RIPv2? RIPv1 is a classful routing protocol, while RIPv2 is classless.

Non-IP Routing Protocols Similar to RIP

List similar non-IP routing protocols to RIP (e.g., from Xerox, Novell, etc.): Similar protocols to IP RIP include:

• Xerox Network Systems protocol suite (XNS)
• Novell’s IPX RIP
• AppleTalk’s Routing Table Maintenance Protocol (RTMP)

UNIX Implementation of RIP

What is the UNIX implementation of RIP? Many UNIX distributions are based on the popular 4.2 BSD (Berkeley Software Distribution) and implement RIP in either routed or gated daemons.

RIP Operation Port (UDP 520)

From which port does RIP operate? The RIP process operates from UDP port 520. All RIP messages are encapsulated in a UDP segment with both the Source and the Destination port fields set to 520.

RIP Metric: Hop Count Values

What is the metric used by RIP and what are its values? RIP uses hop count, with the following significance:

• 1: Signifies a directly connected network of the advertising router.
• 16: Signifies an unreachable network (infinity metric).

RIP Startup Process on a Router

How does the RIP process operate for the first time (on start-up) on a given router? On start-up, a RIP-enabled router broadcasts a packet carrying a RIP Request on each RIP-enabled interface. The RIP process then enters a loop, listening for RIP Request or RIP Response messages from other routers.

Handling Higher Advertised Hop Counts (Holddown)

What happens if a router continuously receives an advertised hop count higher than the recorded hop count, and the update was originated by the same recorded next-hop router? The router marks the network as unreachable for a specified holddown period. If this time expires and the same neighbor is still advertising the higher hop count, the new metric will be accepted.

RIP Update Destination Addresses

What is the destination address of the RIP update messages, is it always the same or not? The destination address is the all-hosts broadcast 255.255.255.255, unless there is no broadcast media. In this case, updates are sent to the directly connected neighbor on point-to-point links.

RIP Update Periodicity and Synchronization Avoidance

By using Ethereal software try to capture RIP update messages from a Cisco Router and identify their periodicity? Why is this timer used? IP adds a small random variable to the update timer at each reset to help avoid route table synchronization. The RIP updates from Cisco Routers typically vary between 25.5 and 30 seconds.

Key RIP Timers and Default Values

What are the main RIP timers? The main RIP timers are:

• Update timer: (Between 25 to 35 seconds)
• Expiration time (Timeout or Invalid timer): (180 seconds)
• Garbage Collection or Flush timer: (Set to 240 seconds)
• Cisco specific Holddown timer: (180 seconds)

Modifying RIP Timers Across a Domain

Is it feasible to change the timing of one router in a RIP domain without changing the timers of all the routers? By which Cisco command? The RIP timers should generally not be changed from their default values. If the timing is changed in one router, the timing of all other routers must be changed for consistency. The command used is:

timers basic update invalid holddown flush

RIP Silent Host Definition and Activation

What is a silent host in RIP? How to activate that in a UNIX host? It is possible that some hosts employ RIP in a silent mode. They do not generate RIP updates, but they listen for them and update their internal route table accordingly. The command routed -q activates the silent mode on a UNIX host.

Defining the passive-interface Command

What is a passive-interface command? It is an IP command that makes a router a silent host on the specified data link.

Maximum Route Entries in a RIP Message

How many route entries does an RIP message contain? A RIP message can contain up to 25 routes.

Subnet Masks Used by RIP Routers

Which subnet masks does a RIP router use to determine the exit routes? A RIP router uses the subnet mask of the connecting interfaces. All subnet masks within a major, class-level network must be consistent.

RIP Interpretation of Unattached Major Network Subnets

How does a RIP process interpret the subnet of a major network (Class A, B, or C) if it has no interfaces attached to that network? In such a case, the router needs only a single route entry pointing toward a router that is directly attached.

RIP Boundary Router Behavior and Summarization

Does a RIP router at the boundary of two major networks (such as a Network with Class A, and another of Class C), advertise the subnets of one network to routers within the other network? The boundary router does not send details of the subnets of one major network into the other major network. It automatically performs summarization.

Configuring RIP Unicast Updates

How to configure unicast updates between two adjacent routers? Unicast updates are configured by adding a neighbor command under the RIP process (using the IP address of the attached neighbor router's interface) in conjunction with the passive-interface command to prevent broadcast updates.

Manipulating RIP Route Metrics

How can a route metric be manipulated? The route metrics can be manipulated with the Cisco offset-list command.

Minimizing RIP Updates on Low Bandwidth Links

How to minimize the routing updates if you are paying for traffic traversing a low bandwidth link? This is achieved by using the interface command ip rip triggered on the two interfaces of the neighbor routers (e.g., a serial link). Updates are then exchanged only when changes to the route tables occur.