Download IGRP Timers

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
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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
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•
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
20
EIGRP
90
IGRP
100
OSPF
110
IS-IS
115
RIP
120
EGP
140
External EIGRP
170
Internal BGP
200
Unknown
255
IGRP Metrics
IGRP Routes
• Interior
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•
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“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
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•
•
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
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•
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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
Router(config-router)#network
Router(config-router)#exit
Router(config)#router igrp 10
Router(config-router)#network
Router(config-router)#network
Router(config-router)#exit
Router(config)#no router rip
•
•
172.16.0.0
192.168.1.0
172.16.0.0
192.168.1.0
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
BGP
Router
Router
Router
Router
Router
Router
Router
IGRP 30
Router
IGRP 40
AS 90
Routing Domain
AS 10
Routing Domain
Summary
But there is still more!
IGRP Metric Information
Metric Calculation
Router> show interfaces s1/0
Serial1/0 is up, line protocol is up
delay
bandwidth
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>
reliability
load
Displaying Interface Values
Router> show interface s0/0
Serial0/0 is up, line protocol is up
Hardware is QUICC Serial
Bandwidth
Delay
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>
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.
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/bandwidth)
• delay
= delay/10
IGRP Metrics
Values displayed in show interface
commands and sent in routing updates.
Media
100M ATM
Fast Ethernet
FDDI
HSSI
16M Token Ring
Ethernet
T1 (Serial Default)
512K
DS0
56K
Bandwidth
K= kilobits
100,000K
100,000K
100,000
45,045K
16,000K
10,000K
1,544K
512K
64K
56K
BWIGRP
10,000,000/Bandwidth
100
100
100
222
625
1,000
6,476
19,531
156,250
178,571
DLYIGRP
Delay
100 S
100 S
100 S
20,000 S
630 S
1,000 S
20,000 S
20,000 S
20,000 S
20,000 S
Delay/10
10
10
10
2,000
63
100
2,000
2,000
2,000
2,000
BWIGRP and DLYIGRP display values as sent in IGRP updates and used in calculating
the IGRP metric.
Metric Calculation
Router> show interfaces s1/0
Serial1/0 is up, line protocol is up
delay
bandwidth
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>
reliability
load
From Casablanca to 172.20.40.0/24
Using the Calculated Values
1,000
100
1,000
100
172.20.4.0/24
1,000
100
6,476
2,000
6,476
2,000
172.25.1.0/24
Casablanca
Bandwidth (lowest or minimum)
slowest bandwidth: Quebec
= 19,531
Delay (sum of outgoing interfaces)
= 100+2,000+2,000+100
= 4,200
Teheran
Legend:
Bandwidth
Delay
172.20.2.0/24
Quebec
19,531
2,000
172.20.20.0/24
19,531
2,000
172.20.40.0/24
Yalta
Metric = 19,531 + 4,200
= 23,731
1,000
100
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 Metric
Using the
Raw Values:
Bandwidth and Delay
From Casablanca to 172.20.40.0/24
Using the Raw Values
10,000K
1,000uS
10,000K
1,000uS
172.20.4.0/24
10,000K
1,000uS
1,544K
20,000uS
1,544K
20,000uS
172.25.1.0/24
Casablanca
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
Teheran
Legend:
Bandwidth
Delay
172.20.2.0/24
Quebec
512K
20,000uS
172.20.20.0/24
512K
20,000uS
172.20.40.0/24
Yalta
10,000K
1,000uS
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
Calculating Delay
So how is Delay, DLYIGRP, 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 DLYIGRP, 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
...
•
Not to be redundant, but if we were using RIP, the
RIP metric would be 3 hops.
Ch.16/Mod. 7 – Distance Vector Routing
Protocols
Part 2 of 2: Distance Vector Routing and
IGRP