1. CCNA version 1.0 Rick Graziani Cabrillo College
Ch. 7 – Distance Vector Routing Protocols Part 2 of 2: Distance Vector Routing and IGRP
CCNA version 1.0
Rick Graziani
Cabrillo College

2. Note to instructors
If you have downloaded this presentation from the Cisco Networking Academy Community FTP Center, this may not be my latest version of this PowerPoint.
For the latest PowerPoints for all my CCNA, CCNP, and Wireless classes, please go to my web site:
The username is cisco and the password is perlman for all of my materials.
If you have any questions on any of my materials or the curriculum, please feel free to me at (I really don’t mind helping.) Also, if you run across any typos or errors in my presentations, please let me know.
I will add “(Updated – date)” next to each presentation on my web site that has been updated since these have been uploaded to the FTP center.
Thanks! Rick
Rick Graziani

3. 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
Rick Graziani

4. 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
Rick Graziani

5. IGRP Metrics
Rick Graziani

6. 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.
Rick Graziani

7. Administrative Distances
Rick Graziani

8. IGRP Metrics
Rick Graziani

9. 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.
Rick Graziani

10. 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.”
Rick Graziani

11. 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.”
Rick Graziani

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

13. 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.
A random jitter variable of 20% is subtracted from each update time to prevent update timer synchronization.
IGRP updates will vary from 72 to 90 seconds.
Rick Graziani

14. 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.”
Rick Graziani

15. 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.
Rick Graziani

16. 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.
Rick Graziani

17. IGRP Timers Update 90 secs – Update and Invalid timers are then reset.
270 secs - Invalid timer expires, route now “unreachable” but still in routing table until flush timer expires. Holddown timer of 280 sec begins.
Holddown
280 secs – Holddown timer expires, will now accept a poorer route to same network. Still in routing table
Flush
630 secs – Route will now be removed from the routing table.
My testing shows that the flush timer starts after the first 90 second update is missed.
Update timer: how frequently routing update messages should be sent
Invalid timer: 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
Holddown timer: specifies the amount of time for which information about poorer routes are ignored.
Flush timer: how much time should pass before a route is flushed from the routing table
Rick Graziani

18. 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.
Rick Graziani

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

20. Configuring IGRP timers basic (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.
Rick Graziani

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

22. Verifying IGRP
Rick Graziani

23. Verifying IGRP
Rick Graziani

24. Verifying IGRP
Rick Graziani

25. Verifying IGRP
Rick Graziani

26. Verifying IGRP
Rick Graziani

27. Troubleshooting IGRP
Rick Graziani

28. Troubleshooting IGRP
Rick Graziani

29. Troubleshooting IGRP
Rick Graziani

30. Domains… Rick’s extra information on autonomous systems…(FYI only!)
Two types of autonomous systems:
1. Process domain
2. Routing domain
Rick Graziani

31. Domains… Process domain
A single IGP (Interior Gateway Protocol) process that is autonomous from other IGP processes.
IGRP autonomous systems are also known as a process domains.
Redistribution is used to route between these types of autonomous systems.
Rick Graziani

32. Domains… Routing domain
A system of one or more IGPs (Interior Gateway Protocols) that is autonomous from other IGP systems.
An EGP (Exterior Gateway Protocol) like BGP is used to route between these types of autonomous systems.
Rick Graziani

33. Rick Graziani graziani@cabrillo.edu

34. Summary
But there is still more!
Rick Graziani

35. IGRP Metric Information (and for EIGRP as well!)

36. Metric Calculation
The metrics used by IGRP in making routing decisions are (lower the metric the better):
bandwidth
delay
load
reliability
By default, IGRP uses only:
Bandwidth
Delay
Analogies:
Think of bandwidth as the width of the pipe
and
delay as the length of the pipe.
Bandwidth is a the carrying capacity
Delay is the end-to-end travel time.
Rick Graziani

37. Metric Calculation If these are the default: bandwidth (default)
delay (default)
When are these used?
load
reliability
Only when configured by the network administrator to do so!
IGRP also tracks (but does not use in its metric calculation):
MTU (Maximum Transmission Unit)
Hop Count
Use show interface command to view the metrics used on a specific interface that is routing EIGRP.
These are the raw values.
Rick Graziani

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

39. Metric Calculation (Review)
k1 for bandwidth
k2 for load
k3 for delay
k4 and k5 for Reliability
Router(config-router)# metric weights tos k1 k2 k3 k4 k5
EIGRP
bandwidth is in kbps
Rick Graziani

40. IGRP Viva la difference! EIGRP
Calculated values (cumulative) displayed in routing table (show ip route).
EIGRP values are 256 times greater.
Rick Graziani

41. Displaying Interface Values
Router> show interface s0/0
Serial0/0 is up, line protocol is up
Hardware is QUICC Serial
Description: Out to VERIO
Internet address is /30
MTU 1500 bytes, BW 1544 Kbit, DLY usec,
rely 255/255, load 246/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
<output omitted>
Bandwidth
Delay
Reliability
Load
shows reliability as a fraction of 255, for example (higher is better):
rely 190/255 (or 74% reliability)
rely 234/255 (or 92% reliability)
rely 255/255 (or 100% reliability)
shows load as a fraction of 255, for example (lower is better):
load 10/255 (or 3% loaded link)
load 40/255 (or 16% loaded link)
load 255/255 (or 100% loaded link)
Rick Graziani

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

43. 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.
Rick Graziani

44. 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
Rick Graziani

45. 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.
Rick Graziani

46. 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
Rick Graziani

47. Metric Calculation IGRP bandwidth = (10,000,000/bandwidth)
delay = delay/10
Note: EIGRP
bandwidth = (10,000,000/bandwidth) * 256
delay = (delay/10) * 256
Note: The reference-bandwidth
For both IGRP and EIGRP: 107, (10,000,000/bandwidth kbps), whereas with OSPF it was 108 (100,000,000/bandwidth)
The difference:
IGRP metric is 24 bits long
EIGRP metric is 32 bits long
EIGRP metric is 256 times greater for the same route
EIGRP allows for finer comparison of potential routes
Rick Graziani

48. IGRP Metrics
Values displayed in show interface commands and sent in routing updates.
Calculated values (cumulative) displayed in routing table (show ip route). EIGRP values are 256 times greater.
Rick Graziani

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

50. From Casablanca to /24
Rick Graziani

51. From Casablanca to /24
Using BWIGRP and DLYIGRP to calculate the IGRP metric:
The slowest bandwidth has the highest BWIGRP value.
IGRP metric
= highest BWIGRP + total of the DLYIGRP
= 19,531 + ( , , )
= 19, ,200
= 23,731
Rick Graziani

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

53. From Casablanca to /24
Rick Graziani

54. Calculating Bandwidth
So how is Bandwidth, BWIGRP, calculated?
The bandwidth metric is calculated by taking 107 (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 107 (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 BWIGRP along the route
Rick Graziani

55. Calculating Delay So how is Delay, DLYIGRP, calculated?
Delay is the total sum of delays on the outgoing interfaces, in 10-microsecond units
The sum of the delays on each of the outgoing interfaces between Casablanca and Yalta, from /24 through /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)
= , , or
= (1, , ,000 +1,000) / 10
In either case the total sum is:
Delay = 4,200
Which is the total of the DLYIGRP, the total Delays along the route!
Rick Graziani

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

57. show ip route
Casablanca# show ip route
Known via igrp 1, distance 100, metric 23,731 …
, from on Ethernet 0
Route metric is 23,731
Total delay is 42,000 microseconds,
minimum bandwidth is 512 Kbit
...
Not to be redundant, but if we were using RIP, the RIP metric would be 3 hops.
Rick Graziani

58. So, what about Reliability and Load?

59. Reliability and Load
The metrics used by EIGRP in making routing decisions are (lower the metric the better):
bandwidth
delay
load
reliability
By default, EIGRP uses only:
Bandwidth
Delay
Rick Graziani

60. Reliability and Load Reliability Reliability is measure dynamically
Uses error rate for measurement
Reflects the total outgoing error rates of the interfaces along the route
Calculated on a five minute weighted average, so not to allow sudden peaks and valleys to make a significant impact
Expressed as an 8 bit number
255 is a 100% reliable link
1 is a minimally reliable link
Higher the better!
Rick Graziani

61. Reliability and Load bandwidth delay reliability load
Router> show interfaces s1/0
Serial1/0 is up, line protocol is up
Hardware is QUICC Serial
Description: Out to VERIO
Internet address is /30
MTU 1500 bytes, BW 1544 Kbit, DLY usec,
rely 255/255, load 246/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
<output omitted>
bandwidth
delay
reliability
load
shows reliability as a fraction of 255, for example:
rely 190/255 (or 74% reliability)
rely 234/255 (or 92% reliability)
rely 255/255 (or 100% reliability)
Rick Graziani

62. Reliability and Load Load Load is measure dynamically
Uses channel occupancy for measurement
Reflects the total outgoing load of the interfaces along the route
Calculated on a five minute weighted average, so not to allow sudden peaks and valleys to make a significant impact
Expressed as an 8 bit number
255 is a 100% loaded link
1 is a minimally loaded link
Lower the better!
Note: Even though load and reliability are dynamically changing values, EIGRP will not recalculate the route metric when these parameters change.
Rick Graziani

63. Reliability and Load bandwidth delay reliability load
Router> show interfaces s1/0
Serial1/0 is up, line protocol is up
Hardware is QUICC Serial
Description: Out to VERIO
Internet address is /30
MTU 1500 bytes, BW 1544 Kbit, DLY usec,
rely 255/255, load 246/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
<output omitted>
bandwidth
delay
reliability
load
shows load as a fraction of 255, for example:
load 10/255 (or 3% loaded link)
load 40/255 (or 16% loaded link)
load 255/255 (or 100% loaded link)
Rick Graziani

64. Reliability and Load k1 for bandwidth k2 for load k3 for delay
IGRP metric =
[k1* BWIGRP(minimum) +
(k2* BWIGRP(minimum))/(256-LOAD) +
k3* DLYIGRP(sum) ] *
[k5/RELIABILITY + k4)]
k2 metric effects LOAD
k4 and k5 effects RELIABILITY
Multiply Reliability only if > 0
Default:
k1=k3=1 and k2=k4=k5=0
You may change the k values to change what you want to give more or less weight to.
k1 for bandwidth
k2 for load
k3 for delay
k4 and k5 for Reliability
Higher the k value, the more that part of the metric is used to calculate the overall IGRP metric
Rick Graziani

65. Reliability and Load Turning the knobs:
We can use the other metrics of Reliability and Load by adjusting their k values to something greater than “0”
The command to adjust the k values is:
Router(config-router)# metric weights tos k1 k2 k3 k4 k5
Notes:
tos is always set to 0; at one time it was Cisco’s intent to use it, but it was never implemented
EIGRP neighbors must agree on K values to establish an adjacency and to avoid routing loops.
Caution!
Know what the impact will be before changing the defaults.
It can give you unexpected results if you do not know what you are doing!
If you modify the weights, you should configure all routers so they are all using the same weight values.
Rick Graziani

66. IGRP and EIGRP: A migration path
Classful Routing Protocol
Classless Routing Protocol
VLSM, CIDR
bandwidth = (10,000,000/bandwidth kbps)
delay = delay/10
24 bit metric for bandwidth and delay
bandwidth = (10,000,000/bandwidth kbps) * 256
delay = (delay/10) * 256
32 bit metric for bandwidth and delay
Maximum Hop Count = 255
Maximum Hop Count = 224
No differentiation between internal and external routes.
Outside routes (redistributed) are tagged as external routes.
Automatic redistribution between IGRP and EIGRP as long as “AS” numbers are the same.
Rick Graziani

67. CCNA version 1.0 Rick Graziani Cabrillo College
Ch. 7 – Distance Vector Routing Protocols Part 2 of 2: Distance Vector Routing and IGRP
CCNA version 1.0
Rick Graziani
Cabrillo College