RIP and IGRP

RIP and IGRP have many similarities in their general logic but several differences in the details of their implementation.

Feature RIP (Default) IGRP (Default)
Update Timer 30 sec 90 sec
Metric Hop Count Function of bandwidth and delay (the default). Can include reliability, load, and MTU.
Hold-down Timer 180 sec 280 sec
Flash (Triggered) Updates Yes Yes
Mask Sent in Update No No
Infinite-Metric Value 16 4 294 967 295

The IGRP metric provides a better measurement of how good a route is, as compared with RIP’s metric. IGRP’s metric is calculated using the bandwidth and delay settings on the interface on which the update was received. When bandwidth and delay are used, the metric is more meaningful than hop count; longer hop routes that go over faster links are considered better routes by IGRP.

RIP uses hop count as its metric. When an update is received, the metric for each subnet in the update signifies the number of routers between the router receiving the update and each subnet. Before sending an update, a router increments by 1 its metric for routes to each subnet.

Finally, the issue of whether the mask is sent is particularly important if variable-length subnet masks (VLSMs) in the same network are desired. Neither RIP or IGRP support VLSM.

The key to configuring RIP and IGRP is to master the use of the network command. Other than that, configuration is relatively easy. You should also know the more-popular show and debug commands, which help you examine and troubleshoot routing protocols.

The following table summarize the more popular commands used for RIP and IGRP configuration and verification.

Command Configuration Mode
router rip Global
router igrp as-number Global
network net-number Router subcommand
passive-interface [default] {interface-type interface-number} Router subcommand
maximum-paths number-paths Router subcommand
variance multiplier Router subcommand
traffic-share {balanced | min} Router subcommand
show ip route [ip-address [mask] [longer- prefixes]]| [protocol [process-id]] Shows the entire routing table, or a subset if parameters are entered.
show ip protocols Shows routing protocol parameters and current timer values.
debug ip rip Issues log messages for each RIP update.
debug ip igrp transactions [ip-address] Issues log messages with details of the IGRP updates.
debug ip igrp events [ip-address] Issues log messages for each IGRP packet.
ping [protocol | tag] {host-name |system-address} Sends and receives ICMP echo messages to verify connectivity.
trace [protocol] [destination] Sends a series of ICMP echoes with increasing TTL values to verify the current route to a host.

Each network command enables RIP or IGRP on a set of interfaces. This command “matches” one or more interfaces on a router. For each interface, the network command causes the router to do three things:

  • The router broadcasts or multicasts routing updates out an interface.
  • The router listens for incoming updates on that same interface.
  • The router, when sending an update, includes the subnet off that interface in the routing update.

The router matches interfaces with the network command by asking this simple question:

“Which of my interfaces have IP addresses with the same network number referenced in this network subcommand?”

For all interfaces that match the network command, the router does the three things just listed.

Here are the examples of configuring RIP and IGRP on the routers, shown on the following known picture:

Static Routes

RIP Configuration

Router R1:

...

!
router rip
network 10.0.0.0
!

R1#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 4 subnets
C       10.10.1.0 is directly connected, FastEthernet0/0
R       10.10.2.0 [120/1] via 10.10.1.2, 00:00:19, FastEthernet0/0
R       10.10.3.0 [120/2] via 10.10.1.2, 00:00:19, FastEthernet0/0
R       10.10.10.0 [120/2] via 10.10.1.2, 00:00:19, FastEthernet0/0
R1#
R1#ping 10.10.3.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 108/159/264 ms
R1#

Router R2:

...

!
router rip
network 10.0.0.0
!

R2#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 4 subnets
C       10.10.1.0 is directly connected, FastEthernet0/0
C       10.10.2.0 is directly connected, Serial1/0
R       10.10.3.0 [120/1] via 10.10.2.1, 00:00:14, Serial1/0
R       10.10.10.0 [120/1] via 10.10.2.1, 00:00:14, Serial1/0
R2#

Router R3:

...

!
router rip
network 10.0.0.0
!

R3#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 4 subnets
R       10.10.1.0 [120/2] via 10.10.3.1, 00:00:10, Serial1/0
R       10.10.2.0 [120/1] via 10.10.3.1, 00:00:10, Serial1/0
C       10.10.3.0 is directly connected, Serial1/0
R       10.10.10.0 [120/1] via 10.10.3.1, 00:00:10, Serial1/0
R3#

Router R:

...

!
router rip
network 10.0.0.0
!

R#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 4 subnets
R       10.10.1.0 [120/1] via 10.10.2.2, 00:00:01, Serial1/0
C       10.10.2.0 is directly connected, Serial1/0
C       10.10.3.0 is directly connected, Serial1/1
C       10.10.10.0 is directly connected, FastEthernet0/1
R#

IGRP Configuration

You configure IGRP just like RIP, except that the router igrp command has an additional parameter-the autonomous system (AS) number. The term autonomous system refers to a network that is within the control of a single company or organization. The term AS number refers to a number assigned to a single company or organization when it registers its connection to the Internet. However, for IGRP, you do not need a registered AS number. All that is needed for IGRP to work is for all the routers to use the same AS number.

IGRP configuration begins with the router igrp 1 global configuration command. Then four consecutive network commands match all the interfaces on the router so that IGRP is fully enabled. In fact, these network commands are identical to the network commands in the complete RIP configuration.

IGRP uses a composite metric. This metric is calculated as a function of bandwidth, delay, load, and reliability. By default, only bandwidth and delay are considered; the other parameters are considered only if they are enabled via configuration. Delay and bandwidth are not measured values but are set via the delay and bandwidth interface subcommands. (The same formula is used to calculate the metric for EIGRP, but with a scaling factor so that the actual metric values are larger, allowing more granularity in the metric.)

Administrative Distance

Many companies and organizations use a single routing protocol. However, in some cases, a company needs to use multiple routing protocols. For instance, if two companies connect their networks so that they can exchange information, they need to exchange some routing information. If one company uses RIP, and the other uses IGRP, on at least one router, both RIP and IGRP must be used. This is just one example, but it is not that unusual to need to run more than one routing protocol in a single router.

Depending on the network topology, two routing protocols might learn routes to the same subnets. When a single routing protocol learns multiple routes to the same subnet, the metric tells it which route is best. However, because different routing protocols use different metrics, the IOS cannot compare the metrics. For instance, RIP might learn a route to subnet 10.1.1.0 with metric 1, and IGRP might learn a route to 10.1.1.0 with metric 8729. There is no basis for comparison between the two metrics.

To decide which route to use, IOS uses a concept called administrative distance. Administrative distance is a number that denotes how believable an entire routing protocol is on a single router. The lower the number, the better, or more believable, the routing protocol. For instance, RIP has a default administrative distance of 120, and IGRP defaults to 100, making IGRP more believable than RIP. So, when both routing protocols learn routes to the same subnet, the router adds only the IGRP route to the routing table.

The administrative distance values are configured on a single router as follows and are not exchanged with other routers.

Route Type Administrative Distance (AD)
Connected 0
Static 1
EIGRP summary route 5
eBGP 20
EIGRP (internal) 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EIRP (external) 170
iBGP (external) 200

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