Home > Articles > Cisco Certification > CCDA > CCDA Self-Study: RIP, IGRP, and EIGRP Characteristics and Design

CCDA Self-Study: RIP, IGRP, and EIGRP Characteristics and Design

Chapter Description

This sample chapter reviews distance-vector routing protocols, covers both versions of the Routing Information Protocol (RIP), and discusses the Cisco Interior Gateway Routing Protocol (IGRP) and Enhanced Interior Gateway Routing Protocol (EIGRP) — important knowledge for the CCDA.

RIPv1

RFC 1058 from June 1988 defines RIPv1. RIP is a distance-vector routing protocol that uses router hop count as the metric. RIP is a classful routing protocol that does not support VLSMs or CIDR.

There is no method for authenticating route updates. A RIP router sends a copy of its routing table to its neighbors every 30 seconds. RIP uses split horizon with poison reverse; therefore, route updates are sent out an interface with an infinite metric for routes learned (received) from the same interface.

The RIP standard was based on the popular routed program used in UNIX systems since the 1980s. The Cisco implementation of RIP adds support for load balancing. RIP will load-balance traffic if there are several paths with the same metric (equal-cost load balancing) to a destination. Also, RIP sends triggered updates when the metric of a route changes. Triggered updates can help the network converge faster rather than wait for the periodic update. RIP has an administrative distance of 120. Chapter 11, "Routing Protocol Selection Criteria," covers administrative distance.

RIP summarizes to IP network values at network boundaries. A network boundary occurs at a router that has one or more interfaces that do not participate in the specified IP network. The IP address assigned to the interface determines participation. IP class determines the network value. For example, an IP network that uses 24-bit subnetworks from 180.100.50.0/24 to 180.100.120.0/24 is summarized to 180.100.0.0/16 at a network boundary.

RIPv1 Forwarding Information Base

The RIPv1 protocol keeps the following information about each destination:

  • IP address—IP address of the destination host or network

  • Gateway—The first gateway along the path to the destination

  • Interface—The physical network that must be used to reach the destination

  • Metric—A number indicating the number of hops to the destination

  • Timer—The amount of time since the entry was last updated

The database is updated with the route updates received from neighboring routers. As shown in Example 12-1, the show ip rip database command shows the RIP private database of a router.

Example 12-1 show ip rip database Command

router9# show ip rip database
172.16.0.0/16  auto-summary
172.16.1.0/24  directly connected, Ethernet0
172.16.2.0/24
  [1] via 172.16.4.2, 00:00:06, Serial0
172.16.3.0/24
  [1] via 172.16.1.2, 00:00:02, Ethernet0
172.16.4.0/24  directly connected, Serial0

RIPv1 Message Format

As described in RFC 1058, the RIPv1 message format appears in Figure 12-1. The RIP messages are encapsulated using User Datagram Protocol (UDP). RIP uses the well-known UDP port 520.

Figure 1Figure 12-1 RIPv1 Message FormatÅ@

The following is a description of each field:

  • Command—Describes the purpose of the packet. The RFC describes five commands, two of which are obsolete and one of which is reserved. The two used commands are

    • request—Requests all or part of the responding router's routing table.

    • response—Contains all or part of the sender's routing table. This message might be a response to a request, or it might be an update message generated by the sender.

  • Version—Set to the value of 1 for RIPv1.

  • Address Family Identifier (AFI)—Set to a value of 2 for IP.

  • IP address—The destination route. It might be a network address, subnet, or host route. Special route 0.0.0.0 is used for the default route.

  • Metric—A field that is 32 bits in length. It contains a value between 1 and 15 inclusive, specifying the current metric for the destination. The metric is set to 16 to indicate that a destination is not reachable.

Because RIP has a maximum hop count, it implements counting to infinity. For RIP, infinity is 16 hops. Notice in the RIP message that there are no subnet masks accompanying each route. Five 32-bit words are repeated for each route entry: AFI (16 bits); unused, which is 0 (16 bits); IP address; two more 32-bit unused fields; and the 32-bit metric. Five 32-bit words equals 20 bytes for each route entry. Up to 25 routes are allowed in each RIP message. The maximum datagram size is limited to 512 bytes, not including the IP header. Calculating 25 routes by 20 bytes each, plus the RIP header (4 bytes), plus an 8-byte UDP header, you get 512 bytes.

RIPv1 Timers

The Cisco implementation of RIP uses four timers:

  • Update

  • Invalid

  • Flush

  • Holddown

RIP sends its full routing table out all configured interfaces. The table is sent periodically as a broadcast (255.255.255.255) to all hosts.

Update Timer

The update timer specifies the frequency of the periodic broadcasts. By default, the update timer is set to 30 seconds. Each route has a timeout value associated with it. The timeout gets reset every time the router receives a routing update containing the route.

Invalid Timer

When the timeout value expires, the route is marked as unreachable because it is marked invalid. The router marks the route invalid by setting the metric to 16. The route is retained in the routing table. By default, the invalid timer is 180 seconds, or six updates periods (30 x 6 = 180).

Flush Timer

A route entry marked as invalid is retained in the routing table until the flush timer expires. By default, the flush timer is 240 seconds, which is 60 seconds longer than the invalid timer.

Holddown Timer

Cisco implements an additional timer for RIP, the holddown timer. The holddown timer stabilizes routes by setting an allowed time for which routing information regarding different paths is suppressed. After the metric for a route entry changes, the router accepts no updates for the route until the holddown timer expires. By default, the holddown timer is 180 seconds.

The output of the show ip protocol command, as shown in Example 12-2, shows the timers for RIP, unchanged from the defaults.

Example 12-2 RIP Timers Verified with show ip protocol

router9> show ip protocol
Routing Protocol is "rip"
Sending updates every 30 seconds, next due in 3 seconds
 Invalid after 180 seconds, hold down 180, flushed after 240

 Outgoing update filter list for all interfaces is
 Incoming update filter list for all interfaces is
 Redistributing: rip
 Default version control: send version 1, receive any version
  Interface       Send Recv Triggered RIP Key-chain
  Ethernet0       1   1 2
  Serial0        1   1 2
 Automatic network summarization is in effect
 Routing for Networks:
  172.16.0.0
 Routing Information Sources:
  Gateway     Distance   Last Update
  172.16.4.2      120   00:00:00
  172.16.1.2      120   00:00:07
 Distance: (default is 120)

RIPv1 Design

Things to remember in designing a network with RIPv1 include that it does not support VLSM and CIDR. The IP addressing scheme with RIPv1 requires the same subnet mask for the entire IP network, a flat IP network. RIPv1 is limited to 15 hops; therefore, the network diameter cannot exceed this limit. RIPv1 also broadcasts its routing table every 30 seconds. RIPv1 is usually limited to accessing networks where it can interoperate with servers running routed or with non-Cisco routers. RIP also appears at the edge of larger networks.

As shown in Figure 12-2, when you use RIPv1, all segments must have the same subnet mask.

Figure 2Figure 12-2 RIPv1 Design

RIPv1 Summary

The characteristics of RIPv1 follow:

  • Distance-vector protocol.

  • Uses UDP port 520.

  • Classful protocol (no support for VLSM or CIDR).

  • Metric is router hop count.

  • Maximum hop count is 15; unreachable routes have a metric of 16.

  • Periodic route updates broadcast every 30 seconds.

  • 25 routes per RIP message.

  • Implements split horizon with poison reverse.

  • Implements triggered updates.

  • No support for authentication.

  • Administrative distance for RIP is 120.

  • Used in small, flat networks or at the edge of larger networks.