Ch.16/Mod.7 – Distance Vector Routing
Protocols
Part 2 of 2: Distance Vector Routing and
IGRP

IGRP Features
•
IGRP is a distance vector routing protocol developed by Cisco.
•
IGRP sends routing updates at 90 second intervals, advertising networks for a particular autonomous system.
•
Key design characteristics of IGRP are a follows:
– The versatility to automatically handle indefinite, complex topologies
– The flexibility needed to segment with different bandwidth and delay characteristics
– Scalability for functioning in very large networks

IGRP Features
•
By default, the IGRP routing protocol uses bandwidth and delay as metrics.
•
Additionally, IGRP can be configured to use a combination of variables to determine a composite metric.
•
Those variables include:
– Bandwidth
– Delay
– Load
– Reliability

IGRP Metrics

IGRP Metrics
•
The metrics that IGRP uses are:
– Bandwidth – The lowest bandwidth value in the path
– Delay – The cumulative interface delay along the path
– Reliability – The reliability on the link towards the destination as determined by the exchange of keepalives
– Load – The load on a link towards the destination based on bits per second
– NO… MTU – The Maximum Transmission Unit value of the path.
MTU has never been used by IGRP or EIGRP as a routing metric.
•
IGRP has an administrative distance of 100 , more “trustworthy” than
RIP at 120.
•
This means a Cisco router will prefer an IGRP learned route over a RIP learned route to the same network.

Administrative Distances
Cisco Default Administrative Distances
Route Source
Connected interface
Administrative Distance
0
Static Route 1
EIGRP summary route 5
External BGP
EIGRP
IGRP
20
90
100
OSPF
IS-IS
RIP
110
115
120
EGP
External EIGRP
Internal BGP
Unknown
140
170
200
255

IGRP Metrics

IGRP Routes
•
Interior
“Interior routes are routes between subnets of a network attached to a router interface. If the network attached to a router is not subnetted,
IGRP does not advertise interior routes.”
•
Clarification
•
IGRP also advertises three types of routes:
– interior, system, and exterior.
•
Interior routes are routes between subnets in the network attached to a router interface.
•
If the network attached to a router is not subnetted, IGRP does not advertise interior routes.

IGRP Routes
•
System
“System routes are routes to networks within an autonomous system.
The Cisco IOS software derives system routes from directly connected network interfaces and system route information provided by other
IGRP-speaking routers or access servers. System routes do not include subnet information.”

IGRP Routes
•
Exterior
“Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The Cisco
IOS software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway (router) of last resort if a better route is not found and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.”

IGRP Timers
•
IGRP has a number of features that are designed to enhance its stability, such as:
– Holddowns
– Split horizons
– Poison reverse updates

IGRP
Timers
Update timer
•
The update timer specifies how frequently routing update messages should be sent.
•
The IGRP default for this variable is 90 seconds .

IGRP
Timers
Invalid timer
•
The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid (unreachable), but still in the routing table .
•
The IGRP default for this variable is three times the update period or
270 seconds .
•
Then placed in the holddown state.
• “If I haven’t heard from you in 270 seconds, I am considering this route as unreachable, I will start the holddown timer, but I will keep it in the routing table until the flush timer expires.”

IGRP
Timers
Holddown timer
•
The holddown timer specifies the amount of time for which information about poorer routes are ignored.
• Zinin: “Holddown specifies the number of seconds that a route must spend in holddown state after expiration of the Invalid Timer.”
•
The IGRP default for this variable is three times the update timer period plus 10 seconds = 280 seconds .
•
The original route is still in the routing table but marked as unreachable, until the flush timer expires.

IGRP
Timers
Flush timer
•
Finally, the flush timer indicates how much time should pass before a route is flushed from the routing table.
•
The IGRP default is seven times the routing update timer or 630 seconds .
•
Zinin: “Flush specifies the number of seconds that a route must remain in the routing table in the garbage collection state after it exits the holddown state.”
•
Each time an update is received the invalid and flush timers are reset .
•
If the invalid timer expires before another update is heard, the route is marked as unreachable , but remains in the routing table.
•
If the flush timer then expires before another update is heard, the route will be deleted from the routing table .

IGRP
Timers
•
All timers begin at the same time.
– Update timer = 90 seconds
– Invalid timer = 270 seconds
– Holddown timer = 280 seconds
– Flush timer = 630 seconds
•
Today, IGRP is showing its age, it lacks support for variable length subnet masks (VLSM).
•
Enhanced IGRP (EIGRP) supports VLSM.

Configuring IGRP
•
Same network commands as RIP.
• IGRP “AS” number must be the same on all routers.

Configuring IGRP
Router(config-router)# router igrp 100
Router(config-router)# timers basic update invalid holddown
flush [sleeptime]
Router(config-router)# no timers basic timers basic (IGRP)
•
To adjust Interior Gateway Routing Protocol (IGRP) network timers, use the timers basic router configuration command. To restore the default timers, use the no form of this command.

Migrating from RIP to IGRP
Router(config)# router rip
Router(config-router)# network 172.16.0.0
Router(config-router)# network 192.168.1.0
Router(config-router)# exit
Router(config)# router igrp 10
Router(config-router)# network 172.16.0.0
Router(config-router)# network 192.168.1.0
Router(config-router)# exit
Router(config)# no router rip
•
Enable IGRP
•
Suggestion: Remove RIP configuration from routers even though the administrative distance will prefer RIP

Verifying IGRP

Verifying IGRP

Verifying IGRP

Verifying IGRP

Verifying IGRP

Troubleshooting IGRP

Troubleshooting IGRP

Troubleshooting IGRP

Two Types of Autonomous Systems:
Process Domains and Routing Domains
Process Domain
Process Domain
Router
Router
Router
IGRP 30
Router
AS 10
Router
BGP
Router
IGRP 40
Router
Routing Domain
Routing Domain
Router
Router
Router
Router
AS 90

Summary
But there is still more!

IGRP Metric Information

Metric Calculation
Router> show interfaces s1/0
Serial1/0 is up, line protocol is up
Hardware is QUICC Serial bandwidth
Description: Out to VERIO
Internet address is 207.21.113.186/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 246/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
<output omitted> delay reliability load

Displaying Interface Values
Router> show interface s0/0
Serial0/0 is up, line protocol is up
Hardware is QUICC Serial
Description: Out to VERIO
Bandwidth
Load
Delay
Internet address is 207.21.113.186/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 246/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
<output omitted> Reliability
Routing Table Metric
•
Default: Slowest of bandwidth plus the sum of the delays of all outgoing interfaces from “this router” to the destination network.

Metric Calculation
Bandwidth
•
Expressed in kilobits ( show interface )
•
This is a static number and used for metric calculations only.
•
Does not necessarily reflect the actual bandwidth of the link.
•
It is an information parameter only .
•
You cannot adjust the actual bandwidth on an interface with this command.
•
Use the show interface command to display the raw value
The default values:
•
Default bandwidth of a Cisco interface depends on the type of interface.
•
Default bandwidth of a Cisco serial interface is 1544 kilobits or 1,544,000 bps (T1), whether that interface is attached to a T1 line (1.544 Mbps) or a 56K line.
•
IGRP metric uses the slowest bandwidth of all of the outbound interfaces to the destination network.

Metric Calculation
Changing the bandwidth informational parameter:
The bandwidth can be changed using:
Router(config-if)# bandwidth kilobits
To restore the default value:
Router(config-if)# no bandwidth

Metric Calculation
Delay
•
Like bandwidth, delay it is a static number .
•
Expressed in microseconds , millionths of a second
•
(Uses the Greek letter mu with an S,
 S, NOT “ms” which is millisecond or thousandths of a second )
•
Use the show interface command to display the raw value
•
It is an information parameter only .
The default values:
•
The default delay value of a Cisco interface depends upon the type of interface .
•
Default delay of a Cisco serial interface is 20,000 microseconds , that of a T1 line.
•
IGRP metric uses the sum of all of the delays of all of the outbound interfaces to the destination network.

Metric Calculation
Changing the delay informational parameter:
The delay can be changed using:
Router(config-if)# delay tens-of-  S
(microseconds)
Example of changing the delay on a serial interface to 30,000 microseconds :
Router(config-if)# delay 3000
To restore the 20,000 microsecond default value:
Router(config-if)# no delay

Metric Calculation
IGRP
bandwidth = (10,000,000/
)
delay =
/10

Values displayed in show interface commands and sent in routing updates.
FDDI
HSSI
512K
DS0
56K
Media
100M ATM
Fast Ethernet
16M Token Ring
Ethernet
T1 (Serial Default)
Bandwidth
K= kilobits
100,000K
100,000K
100,000
45,045K
16,000K
10,000K
1,544K
512K
64K
56K
BW
IGRP
10,000,000/Bandwidth
Delay
100
100
625
1,000
100

S
100

S
100 100

S
222 20,000

S
630

S
1,000

S
6,476 20,000

S
19,531 20,000

S
156,250 20,000

S
178,571 20,000

S
DLY
IGRP
Delay/10
10
10
10
2,000
100
2,000
2,000
2,000
2,000
BW
IGRP
and DLY
IGRP
display values as sent in IGRP updates and used in calculating the IGRP metric.
63

Metric Calculation
Router> show interfaces s1/0
Serial1/0 is up, line protocol is up
Hardware is QUICC Serial bandwidth
Description: Out to VERIO
Internet address is 207.21.113.186/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 246/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
<output omitted> delay reliability load

From Casablanca to 172.20.40.0/24
1,000
100
1,000
100
1,000
100
6,476
2,000
6,476
2,000
172.20.4.0/24
Casablanca
172.25.1.0/24
Teheran
Legend :
Bandwidth
Delay
172.20.2.0/24 Quebec
172.20.20.0/24
Bandwidth (lowest or minimum)
slowest bandwidth: Quebec
= 19,531
Delay (sum of outgoing interfaces)
= 100+2,000+2,000+100
= 4,200
Metric = 19,531 + 4,200
= 23,731
172.20.40.0/24
1,000
100
Yalta
19,531
2,000
19,531
2,000

From Casablanca to 172.20.40.0/24
Using BW
IGRP and DLY
IGRP to calculate the IGRP metric:
The slowest bandwidth has the highest BW
IGRP value.
IGRP metric
= highest BW
IGRP
+ total of the DLY
IGRP
= 19,531 + (100 + 2,000 + 2,000 + 100)
= 19,531 + 4,200
= 23,731

Calculating the IGRP Metric
Using the
Raw Values:
Bandwidth and Delay

From Casablanca to 172.20.40.0/24
10,000K
1,000uS
Using the Raw Values
10,000K
1,000uS
10,000K
1,000uS
1,544K
20,000uS
1,544K
20,000uS
172.20.4.0/24
Casablanca
172.25.1.0/24
Teheran
Legend :
Bandwidth
Delay
172.20.2.0/24 Quebec
172.20.20.0/24 Bandwidth (lowest or minimum)
lowest bandwidth = 512
= 10,000,000/512
= 19,531
Delay (sum of outgoing interfaces)
= (1,000/10) + (20,000/10) +
(20,000/10) + (1,000/10)
= 42,000/10
= 4,200
Metric = 19,531 + 4,200
= 23,731
172.20.40.0/24
10,000K
1,000uS
Yalta
512K
20,000uS
512K
20,000uS

Calculating Bandwidth
So how is Bandwidth, BW
IGRP
, calculated?
•
The bandwidth metric is calculated by taking 10 7 (10,000,000) and dividing it by the slowest bandwidth metric along the route to the destination.
•
This is known as taking the inverse of the bandwidth scaled by a factor of 10 7 (10,000,000)
The lowest bandwidth on the route is 512K or 512 (measured in kilobits), the outgoing interface of the Quebec router.
Divide 10,000,000 by 512 and you get the bandwidth!
Bandwidth = 10,000,000/512
= 19,531
Which is the lowest BW
IGRP along the route

Calculating Delay
So how is Delay, DLY
IGRP
, calculated?
•
Delay is the total sum of delays on the outgoing interfaces, in 10microsecond units
•
The sum of the delays on each of the outgoing interfaces between
Casablanca and Yalta, from 172.20.1.0/24 through 172.20.40.0/24 is:
•
1,000 (Casablanca) + 20,000 (Teheran) + 20,000 (Quebec) + 1,000
(Yalta) = 42,000
We need this in 10-microsecond units:
= (1,000/10)+(20,000/10) + (20,000/10) + (1,000/10)
= 100 + 2,000 + 2,000 + 100 or
= (1,000 + 20,000 + 20,000 +1,000) / 10
In either case the total sum is:
Delay = 4,200
Which is the total of the DLY
IGRP
, the total Delays along the route!

Slowest Bandwidth + Sum of Delays
IGRP metric = Bandwidth + Delay
IGRP metric = 19,531 + 4,200
= 23,731
IF we were using RIP, the RIP metric would be 3 hops .

show ip route 172.20.40.0
Casablanca# show ip route 172.20.40.0
Known via igrp 1, distance 100, metric 23,731
…
172.20.1.2, from 172.20.1.2 on Ethernet 0
Route metric is 23,731
Total delay is 42,000 microseconds, minimum bandwidth is 512 Kbit
...

Ch.16/Mod. 7 – Distance Vector Routing
Protocols
Part 2 of 2: Distance Vector Routing and
IGRP