VMware ESXi and vSphere Cluster Management

RIP (Routing Information Protocol) Overview

Learn how RIP distance-vector routing works, how hop count and administrative distance affect route selection, and how RIPv1, RIPv2, and RIPng differ.

RIP, or Routing Information Protocol, is a legacy dynamic interior gateway routing protocol. It helps routers exchange reachability information inside an organization or other autonomous system.

RIP is mainly useful for learning routing fundamentals and supporting limited legacy deployments. It is generally not preferred for modern, large, or complex networks because its hop-count limit, periodic full-table updates, and basic path-selection model limit scalability.

What RIP Does

RIP is a distance-vector routing protocol. In this model, a router learns about destination networks from neighboring routers. Each neighbor advertises a distance to destinations, along with the next-hop direction used to reach them.

Unlike a link-state protocol, RIP does not maintain a complete map of the network topology. Instead, each router builds routing knowledge from the information received from its neighbors and advertises its own routing information onward.

RIP was created in the 1980s. Its simple design made it useful in early IP networks and still makes it valuable in introductory networking labs. However, newer protocols such as OSPF and EIGRP provide capabilities that are better suited to larger or more demanding routed networks.

Distance-Vector Routing Fundamentals

A router using RIP performs three basic tasks:

1. It sends reachability information to neighboring routers.
2. It receives route advertisements from those neighbors.
3. It compares candidate routes and places the preferred route in its routing table.

The word distance refers to the protocol's measure of how far away a destination is. The word vector refers to the direction or next-hop router through which the destination can be reached.

RIP evaluates competing RIP routes using a metric. A metric is a value used by a routing protocol to compare candidate paths. RIP's metric is hop count.

Hop Count and RIP Route Selection

Hop count is the number of routers traversed to reach a destination network. RIP selects the available RIP route with the lowest hop count.

For example, if R1 reaches a destination through one router, that path has a RIP metric of 1. If another path requires two routers, that path has a metric of 2. Assuming both routes are learned from RIP, R1 prefers the metric-1 path.

RIP supports a maximum usable metric of 15 hops. A metric of 16 means that the destination is unreachable. This convention is sometimes called an unreachable metric or route poisoning value.

RIP Characteristics at a Glance

Administrative Distance Versus Metric

Administrative distance is a trust ranking used when a router learns routes to the same destination from different sources. A lower administrative distance is normally preferred. RIP has a default administrative distance of 120.

The RIP metric is different. It is used to compare paths learned through RIP. For example, a static route, an OSPF route, and a RIP route to the same destination are first compared by administrative distance. If the router is comparing two RIP routes, their hop counts determine which RIP path is preferred.

RIP Routing Updates and Convergence

By default, RIP sends its full routing table to its neighbors every 30 seconds. These periodic advertisements allow routers to learn new routes and refresh information about routes they already know.

Convergence is the process by which routers reach a consistent view of routing information after a network change. Periodic updates help RIP converge, but the 30-second interval and full-table behavior can make convergence slower and less efficient than in many newer protocols.

Sending the entire routing table repeatedly can also consume bandwidth, especially on constrained links or in networks with many routes. RIP's update behavior is simple, but it is one reason RIP is not a strong choice for modern, complex network designs.

RIPv1: Classful IPv4 RIP

RIPv1 is the original IPv4 version of RIP. It is classful, meaning that its routing updates do not include subnet mask information.

Without a subnet mask, a receiving router cannot fully represent arbitrary classless prefixes in the update. This creates problems for networks using VLSM, or Variable-Length Subnet Masking, and for discontiguous networks where portions of the same major network are separated by different networks.

RIPv1 sends its routing updates as IPv4 broadcasts. A broadcast is delivered to all hosts on the local IPv4 broadcast domain, even though only routers running RIP need the information.

RIPv2: Classless IPv4 RIP

RIPv2 is the classless IPv4 version of RIP. Its route updates include subnet mask information, allowing routers to advertise and learn classless prefixes.

Because RIPv2 carries the mask, it supports classless addressing and VLSM. It is therefore more appropriate than RIPv1 when an IPv4 design uses non-default subnet masks or multiple subnet sizes.

RIPv2 sends updates to the IPv4 multicast address 224.0.0.9. Multicast delivers a packet to members of a multicast group, so RIPv2 updates target routers interested in RIP rather than broadcasting the update to every local host.

RIPng: RIP for IPv6

RIPng is the RIP variant designed for IPv6 networks. It provides RIP-style distance-vector routing for IPv6 rather than IPv4.

RIPng should be kept separate in terminology and configuration from RIPv1 and RIPv2. RIPv1 and RIPv2 are IPv4 RIP versions; RIPng is the IPv6 version. Its configuration is performed in an IPv6 routing context and is enabled on IPv6 interfaces.

RIPv1, RIPv2, and RIPng Comparison

Example: Choosing the Lowest-Hop Route

Suppose R1 must reach the network 10.0.0.0/24. It learns two routes through RIP:

• R1 reaches the network through R2. This path crosses one intermediate router and has a RIP metric of 1.
• R1 reaches the network through R3 and then R4. This path crosses two intermediate routers and has a RIP metric of 2.

Both candidate routes come from RIP, so both have the same default administrative distance of 120. The RIP metric therefore decides the result. R1 installs and uses the path through R2 because metric 1 is lower than metric 2.

Basic Cisco IOS RIPv2 Configuration Context

The following is an introductory template, not a universal configuration. Replace the placeholder with a connected IPv4 network that should participate in RIP.

router rip
 version 2
 network <connected-IPv4-network>
 no auto-summary

• router rip enters the RIP configuration context.
• version 2 selects RIPv2 operation.
• network <connected-IPv4-network> enables RIP on interfaces matching the specified connected network.
• no auto-summary is commonly used when a classless lab topology must advertise specific subnet prefixes instead of automatically summarized major networks.

Verifying IPv4 RIP

show ip route
show ip route rip
show ip protocols
show running-config | section router rip

Use show ip route to inspect the routing table. show ip route rip focuses on RIP-learned routes and their metrics. show ip protocols can show the RIP version, advertised networks, timers, and routing information sources. The running-configuration command helps confirm the configured RIP settings.

RIPng Configuration Context

RIPng is configured separately from IPv4 RIP. A basic Cisco IOS context looks like this:

ipv6 unicast-routing
ipv6 router rip <process-name>
interface <interface-id>
 ipv6 rip <process-name> enable

IPv6 forwarding is enabled first. A named RIPng process is then created, and RIPng is enabled on the required IPv6 interfaces.

RIP Limitations

• 15-hop ceiling: A route requiring more than 15 hops cannot be used through RIP. Metric 16 represents unreachable.
• Periodic full-table updates: Sending the complete routing table every 30 seconds can waste bandwidth and contribute to slower convergence.
• Simple metric: Hop count does not account for bandwidth, delay, reliability, or link cost. A path with fewer routers is not necessarily the fastest path.
• Limited scalability: The hop limit and update behavior make RIP unsuitable for many large or complex routed networks.
• Fewer modern capabilities: RIP lacks many capabilities associated with newer routing protocols, including more sophisticated topology awareness and path calculations.

OSPF and EIGRP are examples of more advanced interior routing protocols. Choosing a protocol depends on the network's size, design, operational requirements, platform support, and administrative goals.

Troubleshooting RIP

A Subnetted Route Is Missing or Summarized

Likely causes include using RIPv1 in a classless topology or allowing automatic summarization to affect RIPv2 advertisements.

• Verify the configured RIP version.
• Check the route prefix and mask in the routing table.
• Review the RIP configuration and protocol status.
• Use RIPv2 for classless IPv4 routing and align the configuration with the addressing design.

A Destination More Than 15 Routers Away Is Unreachable

Inspect the RIP metric. If it is 16, RIP considers the destination unreachable. RIP cannot support a usable route requiring more than 15 hops, so a different network design or a more scalable routing protocol is needed.

The Router Chooses an Unexpected Path

First determine whether the competing routes come from the same routing source. Compare RIP metrics for all RIP candidates, and check the administrative distance and route source shown in the routing table. A lower hop count wins among RIP routes, but a route from another source may be preferred because of administrative distance.

RIP Updates Produce Unnecessary Traffic

Confirm the default 30-second update interval and consider the size of the routing table and the capacity of the link. RIP's full-table periodic update behavior can be inefficient on constrained networks. This is a protocol limitation to recognize when evaluating a more scalable routing approach.

Exam-Relevant Summary

• RIP means Routing Information Protocol.
• RIP is a legacy dynamic interior gateway protocol and a distance-vector protocol.
• RIP uses hop count as its metric.
• The lowest-hop RIP route is preferred.
• RIP supports usable metrics from 1 through 15; metric 16 means unreachable.
• RIP's default administrative distance is 120.
• RIP sends the full routing table every 30 seconds by default.
• RIPv1 is classful, omits subnet masks, and uses broadcasts.
• RIPv2 is classless, includes subnet masks, and uses multicast address 224.0.0.9.
• RIPng provides RIP-style routing for IPv6.

For a related reference, see RIP Routing Information Protocol Overview.