Module 7 – Distance Vector Routing Protocols

Ch. 7 – Distance Vector Routing ProtocolsPart 2 of 2: Distance Vector Routing and

IGRP

CCNA version 1.0

Rick Graziani

Cabrillo College

Rick Graziani [email protected] 2

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Thanks! Rick


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 Administrative Distance Connected interface

Static Route

EIGRP summary route

External BGP

EIGRP

IGRP

OSPF

IS-IS

RIP

EGP

External EIGRP

Internal BGP

Unknown

0

1

5

20

90

100

110

115

120

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

• 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.

Update timer


IGRP Timers

• 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.”

Invalid timer


IGRP Timers

• 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.

Holddown timer


IGRP Timers

• 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.

Flush timer


IGRP Timers

• 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

Update 90 secs – Update and Invalid timers are then reset.

Invalid 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.


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

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.

Router(config-router)#router igrp 100

Router(config-router)#timers basic update invalid holddown flush [sleeptime]

Router(config-router)# no timers basic


Migrating from RIP to IGRP

Router(config)#router rip

Router(config-router)#network 172.16.0.0


Router(config-router)#exit

Router(config)#router igrp 10



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






Rick’s extra information on autonomous systems…(FYI only!)

Two types of autonomous systems:

1. Process domain

2. Routing domain

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.

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.

Domains…


Router Router

Router

Router

RouterRouter

Router

Router Router

Router

Router

BGP

AS 90

AS 10

IGRP 40IGRP 30

Process Domain

Process Domain

Routing Dom ain

Routing Dom ain

Tw o Types of Autonomous Systems:P rocess D om ains and R outing D om ains


Summary

But there is still more!

IGRP Metric Information (and for EIGRP as well!)


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 anddelay as the length of the pipe.

• Bandwidth is a the carrying capacity• Delay is the end-to-end travel time.

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.

Metric Calculation


Router> show interfaces s1/0

Serial1/0 is up, line protocol is up

Hardware is QUICC Serial

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>

bandwidth delay

reliability load

Metric Calculation

Rick Graziani [email protected]

Metric Calculation (Review)

EIGRP

– k1 for bandwidth– k2 for load– k3 for delay– k4 and k5 for Reliability

Router(config-router)# metric weights tos k1 k2 k3 k4 k5bandwidth is in kbps


Media

Bandwidth K= kilobits

BWIGRP

10,000,000/Bandwidth

Delay DLYIGRP

Delay/10

100M ATM 100,000K 100 100 S 10 Fast Ethernet 100,000K 100 100 S 10 FDDI 100,000 100 100 S 10 HSSI 45,045K 222 20,000 S 2,000 16M Token Ring 16,000K 625 630 S 63 Ethernet 10,000K 1,000 1,000 S 100 T1 (Serial Default) 1,544K 6,476 20,000 S 2,000 512K 512K 19,531 20,000 S 2,000 DS0 64K 156,250 20,000 S 2,000 56K 56K 178,571 20,000 S 2,000 BWIGRP and DLYIGRP display values as sent in IGRP updates and used in calculating the IGRP metric.

Media


BWEIGRP


*256

Delay

DLYEIGRP Delay/10

*256

100M ATM 100,000K 25,600 100 S 2,560 Fast Ethernet 100,000K 25,600 100 S 2,560 FDDI 100,000K 25,600 100 S 2,560 HSSI 45,045K 56,832 20,000 S 512,000 16M Token Ring 16,000K 160,000 630 S 16,128 Ethernet 10,000K 256,000 1,000 S 25,600 T1 (Serial Default)

1,544K 1,657,856 20,000 S 512,000

512K 512K 4,999,936 20,000 S 512,000 DS0 64K 40,000,000 20,000 S 512,000 56K 56K 45,714,176 20,000 S 512,000 BWEIGRP and DLYEIGRP display values as sent in EIGRP updates and used in calculating the EIGRP metric.

Viva la difference!

IGRP

EIGRP

Calculated values (cumulative) displayed in routing table (show ip route).

EIGRP values are 256 times greater.


Displaying Interface Values

shows reliability as a fraction of 255, for example (higher is better):

rely 190/255 (or 74% reliability)



Router> show interface s0/0






rely 255/255, load 246/255



<output omitted>

Bandwidth Delay

Reliability Load

shows load as a fraction of 255, for example (lower is better):

load 10/255 (or 3% loaded link)




Displaying Interface Values

Router> show interface s0/0






rely 255/255, load 246/255



<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.


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/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

Metric Calculation


Media


BWIGRP


Delay DLYIGRP

Delay/10

100M ATM 100,000K 100 100 S 10 Fast Ethernet 100,000K 100 100 S 10 FDDI 100,000 100 100 S 10 HSSI 45,045K 222 20,000 S 2,000 16M Token Ring 16,000K 625 630 S 63 Ethernet 10,000K 1,000 1,000 S 100 T1 (Serial Default) 1,544K 6,476 20,000 S 2,000 512K 512K 19,531 20,000 S 2,000 DS0 64K 156,250 20,000 S 2,000 56K 56K 178,571 20,000 S 2,000 BWIGRP and DLYIGRP display values as sent in IGRP updates and used in calculating the IGRP metric.

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.








rely 255/255, load 246/255



<output omitted>

bandwidth delay

reliability load

Metric Calculation


Casablanca Teheran Quebec

Yalta

172.25.1.0/24172.20.4.0/24172.20.2.0/24

172.20.40.0/24

Legend:BandwidthDelay

1,000100

1,000100

1,000100

6,4762,000

6,4762,000

19,5312,000

19,5312,000

1,000100

172.20.20.0/24Bandwidth (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

Using the Calculated Values

From Casablanca to 172.20.40.0/24


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 + (100 + 2,000 + 2,000 + 100)

= 19,531 + 4,200

= 23,731


Calculating the IGRP MetricUsing the

Raw Values: Bandwidth and Delay


Casablanca Teheran Quebec

Yalta

172.25.1.0/24172.20.4.0/24172.20.2.0/24

172.20.40.0/24

Legend :B andw id thD elay

10,000K1,000uS

10,000K1,000uS

10,000K1,000uS

1,544K20,000uS

1,544K20,000uS

512K20,000uS

512K20,000uS

10,000K1,000uS

172.20.20.0/24Bandw idth (lowest or m in im um ) lowest bandw idth = 512 = 10,000,000 /512 = 19,531

Delay (sum of outgo ing in terfaces) = (1 ,000/10) + (20 ,000/10) + (20,000/10) + (1 ,000/10) = 42,000/10 = 4 ,200

Metric = 19,531 + 4 ,200 = 23,731

Using the Raw Values



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

Calculating Bandwidth


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 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 + 100or= (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 DLYIGRP, the total Delays along the route!

Calculating Delay


IGRP metric = Bandwidth + Delay

IGRP metric = 19,531 + 4,200

= 23,731

IF we were using RIP, the RIP metric would be 3 hops.

Slowest Bandwidth + Sum of Delays


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

...

• Not to be redundant, but if we were using RIP, the RIP metric would be 3 hops.

show ip route 172.20.40.0

So, what about 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

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!



shows reliability as a fraction of 255, for example:










rely 255/255, load 246/255



<output omitted>

bandwidth delay

reliability 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 impactExpressed 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.



shows load as a fraction of 255, for example:










rely 255/255, load 246/255



<output omitted>

bandwidth delay

reliability load



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



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.



IGRP and EIGRP: A migration path

IGRP EIGRP

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.

Ch. 7 – Distance Vector Routing ProtocolsPart 2 of 2: Distance Vector Routing and

IGRP

CCNA version 1.0

Rick Graziani

Cabrillo College

Documents

Module 7 – Distance Vector Routing Protocols