1. Path Determination Static Routes Dynamic Routing Protocols
Routing Information Protocol (RIP)
Interior Gateway Routing Protocol (IGRP)
Open Shortest Path First (OSPF)
Enhanced Interior Gateway Routing Protocol (EIGRP)
Border Gateway Protocol (BGP)

2. Routing Overview
In order to travel from one network to another, some device must know to transport that information
Routing is the process by which information gets routed from one location to another:
mail
telephone calls
trains

3. Router Information
A router (or entity performing routing) needs to know:
Destination Address – What is the destination address of the item to be routed?
Information Sources – From which source (i.e., other routers) can the router learn paths to a given destination
Possible Routes – What are the initial possible routes or paths to the intended destination
Best Routes – What is the best path to the intended destination
Routing Information Maintenance and Verification – A way to verify that known paths to destinations are current and valid

4. Connected Routes 10.120.2.0 E0 172.16.0.0 S0 Network Protocol
Destination Network
Exit Interface
Connected
Learned E0 S0

5. Table Construction
If destination is directly connected, router knows which port to use when forwarding packets
If destination networks are not directly attached, router must learn best route
Manually by network administrator
Dynamically by collecting information about processes running through the network

6. Forwarding Packets
Static Routes – Routes learned by router when administrator manually establishes route. The administrator must update these routes when topology changes occur
Dynamic Routes – Routes automatically learned by router after administrator configures a routing protocol that helps determine routes. Route knowledge is automatically updated whenever topology changes

7. Enabling Static Routes
Static routes are administratively defined routes that specify the explicit path packets must take to destination
They are administratively defined and allow very precise control over routing behavior
Important if Cisco IOS software cannot build a route to destination
Gateway of “last resort” – address a router would send a packet destined for a network not listed in the routing table

8. Stub Network
Static routes are commonly used when routing from a network to a stub network
Stub network (aka “leaf node”) is a network accessed by a single route
Stub Network
Network S0

9. End-to-End Connectivity
Static route is configured for connectivity to data link, not directly to router
End-to-end connectivity is configured in both directions

10. Configuring Static Routes
Enter ip route in global configuration mode.
Parameters for ip route further define the static route
Static route allows manual configuration of routing table
Entry will remain in routing table as long as path is active
Only exception is permanent option – route will remain in table even if path is not active

11. Static Route To Stub
Static route from Router A to stub network is configured as follows:
RouterA(config)#ip route
ip route – Identifies static route command
– Specifies static route to destination subnetwork
– Indicates subnet mask
– Specifies IP address of next-hop router in path to destination
Stub Network A B
Network S0

12. Default Route
Default route is special type of static route for situations in which
the route from source to destination is not known, or
it is infeasible for the routing table to store sufficient information about all possible routes
Default route is “gateway of last resort”

13. Static Route From Stub
To configure default route, you would enter following at router B:
RouterB(config)#ip route
ip route – Identifies static route command
– routes to non-existent subnet (with special mask, it denotes the default network)
– Specifies special mask indicating default route
– Specifies IP address of next-hop router to be used as default for packet forwarding
Stub Network A B
Network S0

14. Learning Routes Static routes are useful in some situations
It is not satisfactory that the network administrator reconfigure routers to accommodate change
Another method is to learn available routes automatically accommodating changes

15. Routing Protocols S0 E0 S1
Network Protocol
Destination Network
Exit Interface
Connected
RIP
IGRP E0 S0 S1

16. Routing Protocols
Dynamic routing relies on a routing protocol to dissiminate and gather knowledge
Routing protocol defines set of rules used to communicate with neighboring routers
Routing protocol is a network layer protocol that intercepts packets from other routers to learn and maintain a routing table

17. Routing Protocols (Cont)
Routing protocols describe the following information
How updates are conveyed
What knowledge is conveyed
When to convey knowledge
How to locate recipients of updates
Examples of routing protocols are:
RIP
IGRP

18. Routed Protocols
Routed protocols such as TCP/IP and IPX define the format and use fields within a packet to provide a transport mechanism for user traffic
As soon as routing protocol determines a valid path between routers, the router can route a routed protocol

19. Types of Routing Protocols
Interior Gateway protocols (IGP) – Used to exchange routing information within an autonomous system. Examples:
RIP
IGRP
Exterior Gateway Protocols (EGP) – Used to exchange routing information between autonomous systems. Example:
BGP
EGPs are not discussed in this book

20. IGP Vs EGP IGP: RIP, IGRP EGP: BGP Autonomous System 100

21. Autonomous System
Collection of networks under a common administrative domain
Internet Assigned Numbers Authority (IANA) allocates autonomous system numbers
Using IANA-assigned autonomous system number is necessary only if organization plans to use EGP public network such as the internet

22. Administrative Distance
Multiple routing protocols and static routes may be used at the same time
If several routing sources provide common routing information, an administrative distance value is used to rate trustworthiness of each routing source
Allows Cisco IOS software to discriminate between sources of routing information
For each network learned, IOS selects route from routing source with lowest administrative distance
It is a number between 0 and 255.
Routing protocol with lowest administrative distance has most likelihood of being used

23. Administrative Distance (Cont)
Send packet from router A
to network E by best route
IGRP
Administrative
Distance = 100
Router A
Router B
RIP
Administrative
Distance = 120 E
Router C
Router D

24. Default Values Route Source Default Distance Connected interface
Static route address 1
EIGRP 90
IGRP
100
OSPF
110
RIP
120
External EIGRP
170
Unknown/Unbelievable
255 (Will not be used to pass traffic)

25. Non-Default Values
Non-default values may be necessary when redistributing routes
Network administrator can use Cisco IOS to configure administrative distance values on a per-router, per-route basis
See ACRC coursebook available from CISCO press

26. Classes of Routing Protocols
Distance Vector – Determines direction (vector) and distance of any link in the internetwork. Examples include RIP and IGRP
Link-state – (also called shortest path first) recreates exact topology of entire internetwork for route computation (or at least component where router is located). Examples include OSPF and NLSP
Balanced Hybrid – Combines aspects of link-state and distance vector algorithms. Example is EIGRP

27. Comparison
There is no single best routing algorithm for all internetworks
All routing protocols provide information differently

28. Distance Vector Protocols
Also known as Bellman-Ford-Fullerton algorithms
Pass periodic copies of routing table from router to router and accumulate distance vectors
Distance means how far
Vector means which direction
Regular updates between routers communicate topology changes
Each router receives routing table from its direct neighbor

29. Table Updates B A C D C B A Routing Table Routing Table Routing Table

30. Algorithm Activities Identify sources of information Discover routes
Select best route
Maintain routing information

31. Information Exchange 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 E0 S0 S0 S1B C
Routing Table
Routing Table
Routing Table E0 S0 1 2 S0 S1 1 S0 E0 1 2

32. Multiple Paths
There might be multiple paths to any given destination network
When table is updated, primary objective is to determine best path
Each distance vector routing protocol uses a different routing algorithm to determine best route
Algorithms generate a number called metric value for each path through the network
Smaller the metric, the better the path

33. Metrics Hop count – Number of routers through which packet will pass
Ticks – Delay on a data link using IBM PC clock ticks ( 55 milliseconds)
Cost – Arbitrary value, usually based on bandwidth, dollar expense, or another measurement assigned by network administrator
Bandwidth – Data capacity of link. 10 Mbps Ethernet is better than 64Kbps leased line
Delay – Length of time to move from source to destination
Load – Amount of activity on a network resource such as router or link
Reliability – Usually refers to bit-error rate of each network link
MTU – Maximum transmission unit. Maximum frame length in octets that is acceptable to all links on path

34. Transmission from A to B56
IGRP
Bandwidth
Delay
Load
Reliability
MTU
RIP
Hop Count T1 56 C T1
IPX
Ticks, Hop Count B

35. Methods
IGRP – Bases decision on combined characteristics, such as bandwidth, delay, reliability, and MTU. Emphasis is on bandwidth and delay, so it would choose T1 lines
RIP – Hop counts are equal, so it would load balance between paths

36. Distance Vector Events
The following occurs step-by-step from processor to processor
Topology change
Network discovery process
Topology change updates
The entire routing table is sent to each adjacent or directly connected neighbor
Routing table contains information about total path cost (defined by metric) and logical address of the first router on the path to each network it knows about
Updates are compared to own routing table
Router adds cost of reaching neighbor to path cost reported by neighbor
Finding a better route results in update of routing table

37. Maintaining Routes Process to update this routing table Process to
Topology
change
causes
routing
table
update
Router A sends
out updated routing
table at the end of
next period A B

38. “Converged” Network 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 E0 S0 S0 S1 S0A B C
Routing Table
Routing Table
Routing Table E0 S0 1 2 S0 S1 1 S0 E0 1 2

39. “Slow” Convergence
If network fails, the routing tables should change so that the network slowly re-converges

40. Start of Counting to Infinity X E0 S0 S0 S1 S0 E0 A B C
Routing Table
Routing Table
Routing Table E0 S0 1 2 S0 S1 1 S0 E0
down 1 2

41. Router C Update X E0 S0 S0 S1 S0 E0 A B C
Routing Table
Routing Table
Routing Table E0 S0 1 2 S0 S1 1 S0 2 1

42. Router A and B Updates X 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 E0 S0 S0C
Routing Table
Routing Table
Routing Table E0 S0 1 4 S0 S1 3 1 S0 2 1

43. Next Iteration X 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 E0 S0 S0 S1 S0 E0B C
Routing Table
Routing Table
Routing Table E0 S0 1 6 S0 S1 5 1 S0 4 1 2

44. Troubleshooting
IP distance vector routing algorithms have inherent limits via Time To Live (TTL) value in IP header
Router reduces TTL by 1 each time it gets a packet
When 0, router discards packet
However, routing loop might count to infinity first

45. Maximum Metric X 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 E0 S0 S0 S1 S0 E0B C
Routing Table
Routing Table
Routing Table E0 S0 1 16 S0 S1 16 1 S0 16 1 2

46. Maximum Metric Setting
When value reaches maximum, network is considered unreachable

47. Split Horizon
It is never useful to send information about a route back in the direction from which the original update came

48. Split Horizon Example
Router B has access to network through Router C, so it makes no sense for Router B to announce to Router C that it has access to through Router C
Router B announced network to Router A, so it makes no sense for Router A to announce its distance to Router B
Having no alternate path to , Router B concludes is inaccessible

49. Split Horizon Routing Tables X E0 S0 S0 S1 S0 X E0 X A B C
Routing Table
Routing Table
Routing Table E0 S0 1
down S0 S1
down 1 S0
down 1 2

50. Route Poisoning Route Poisoning is another form of Split Horizon
Attempts to eliminate routing loops caused by inconsistent updates
Router sets a table entry that keeps network state consistent while other routers gradually converge
Frequently used with “holddown timers (described in next section)

51. Poisoning Example When 10.4.0.0 goes down,
Router C poisons its link to network by entering infinite cost (indicating network is unreachable)
By poisoning route to network, Router C is not susceptible to other incorrect updates about

52. Poisoned Route X 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 E0 S0 S0 S1 S0 E0A B C
Routing Table
Routing Table
Routing Table E0 S0 1 2 S0 S1 1 S0 E0
infinity 1 2

53. Router B Actions Router B notices that 10.4.0.0 jumps to infinity
Router B sends update called poison reverse back to Router C
Poison reverse overrides Split Horizon direction
Poison reverse serves as acknowledgement that poison message was received

54. Poison Reverse X 10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0 E0 S0 S0 S1 S0 E0A B C
Poison Reverse
Routing Table
Routing Table
Routing Table E0 S0 1 2 S0 S1
possibly
down 1 S0 E0
infinity 1 2

55. Holddown Timers
Holddown times are used to prevent regular update messages from inappropriately reinstating a bad route
Tell routers to hold any changes that might affect routes for some period of time
Holddown period is usually just grater than time necessary to update entire network with a routing change

56. Holddown Timer Operation
Step 1 – When router receives update indicating a network is inaccessible, the router marks the route as inaccessible and starts holddown timer
Step 2 – If update arrives from neighboring router with better metric than originally recorded, the router marks the network as accessible and removes holddown timer
Step 3 – If a poorer metric update is received from a neighboring router at any time before the holddown timer expires, the update is ignored. Ignoring poorer updates allows more time for knowledge of the change to propagate
Step 4 – During holddown period, routes appear in routing table as “possibly down”

57. Holddown Example X Update after holddown time Network 10.4.0.0
is unreachable X E0 S0 S0 S1 S0 E0 A B C
Update after holddown time

58. Triggered Updates
If routers wait for regularly scheduled updates, before notifying neighbors of catastrophes, the following serious problems can occur
Loops
Dropped traffic
A triggered update is sent immediately
Detecting router immediately sends messages to adjacent routers which in turn notify their neighbors

59. Triggered Updates X Network 10.4.0.0 is unreachable Network 10.4.0.0 X E0 S0 S0 S1 S0 E0 A B C

60. Triggered Update Problems
Packets containing update message can be dropped or corrupted by some link in the network
Triggered updates do not occur instantaneously
It is possible that a router issue a regular update just before it is about to receive a triggered update, causing a bad route to be reinserted into a neighbor who already received the triggered update
Therefore, we couple triggered updates with holddowns

61. Holddown Timers with Triggered Updates
Holddown rule says that when a route is invalid, no new route with the same or worse metric will be accepted at the destination for holddown time
Therefore, triggered updates have time to propagate throughout the network

62. Multiple Solutions X Routers have multiple routes to each otherD C X E B A
Routers have multiple routes to each other
Routers A and D receive triggered update
Router B removes its route to network

63. Route Fails X Routers A and D receive triggered update
Holddown
Holddown C X E B A
Holddown
Routers A and D receive triggered update
Set their own holddown timers
Routers A and D in turn send triggered updates to Router E

64. Route Holddown X Routers A and D send poison reverse to Router B
Holddown C X E B
Poison Reverse
Poison Reverse A
Holddown
Routers A and D send poison reverse to Router B
Router E sends poison reverse to Routers A and D

65. Holddown Duration Routers A, D and E remain in holddown until
one of following occurs:
Holddown timer expires
New route with better metric is received
A flush timer (the time a route is held before being removed) removes the route from the routing table

66. Packets During Holddown
Holddown C X E B
Packet for
Packet for A
Holddown
Router E sends message to
Router B will drop packet and send ICMP “network unreachable” message

67. 10.4.0.0 Returns to Operation Network 10.4.0.0 returns to operationD C E B
Link Up! A
Network returns to operation
Router B sends a trigger update to Routers A and D notifying them that the link is active
After holddown timer expires, Routers A and D add back to their routing table

68. Network Converges D C E B
Link Up! A
Routers A and D send Router E a routing update stating that network is up
Router E updates its routing table after hoddown timer expires

69. Link-State and Hybrid Routing Protocols
Focus of this chapter has been “distance vector” routing
Link-State and Hybrid are alternative routing protocols

70. Link-State Protocol Diagram
TopologicalDatabase
SPF
Algorithm
Link State Packets
Routing Table
Shortest-Path-First Tree

71. Link-State Protocol
Link-state protocols build routing tables based on a topology database
Topology database is built from link-state packets that are passed between all routers to describe the state of a network
Database is used by the shortest-path-first algorithm to build the routing table

72. Shortest-Path Algorithms
Link-state algorithms (also known as shortest-path algorithms) maintain a complex database of topology information
Distance vector algorithm has non-specific information about distant networks and no knowledge of distant routers
Link-state maintains full knowledge of distant routers and how they interconnect
Link-state routing uses Link-State Packets (LSPs), a topological database, the SPF algorithm, the resulting SPF tree, and the routing table of paths and ports to each network

73. Link-State Advantages
As networks become larger in scale, link-state
becomes more attractive because:
Link-state protocols only send updates of a topology change
Periodic updates are more infrequent for distance vector protocols
Networks running link-state can be segmented into area hierarchies, limiting the scope of route changes
Networks running link-state support classless addressing
Networks running link-state support summarization

74. Balanced Hybrid
A third protocol, called “Balanced Hybrid,” combines distance vector and link-state protocols
Balanced hybrid uses distance vectores with more accurate metrics to determine the best paths to destination networks
However, it uses topology changes to trigger routing database updates as opposed to periodic updates
Balanced hybrid converges more rapidly, like link-state but emphasizes economy in the use of resources such as bandwidth, memory and processor overhead
CISCO’s Enhanced Interior Gateway Routing Protocol is a balanced hybrid protocol

75. Configuring Dynamic Routing Protocols
To enable dynamic routing protocol, perform the following
Select a routing protocol, such a RIP or IGRP
Select IP networks to be routed
Dynamic routing uses broadcasts and multicasts to communicate with other routers
When information from other routers is received, it uses routing metric to find the best path to each network or subnet.

76. Use of IGRP and RIP at Same Router
RIP
IGRP
RIP

77. Router Command The router command starts the routing process
router(config)#router protocol [keyword]
Protocol is RIP, IGRP, OSPF or EIGRP
Keyword refers to an autonomous system in protocols that require an autonomous system such as IGRP

78. Network Command
The network command allows the routing process to determine which interfaces it will participate in the sending and receiving of routing updates
The network command starts the routing protocol on all of a router’s interfaces that have IP addresses within the specified network scope
The network command also allows router to advertise that network to other routers
router(config-router) #network network-number
The network-number parameter specifies a directly connected network number
For RIP and IGRP, network-number must be based on major-class network numbers, not subnet numbers

79. Enabled Protocols
After the protocol is enabled and a networks path is chosen, the router begins to dynamically learn the networks and paths available in the internetwork

80. RIP Route Choice
19.2 kbps T1 T1 C T1
The above shows how RIP would choose routes based on hop count

81. Versions 1 and 2 The book describes RIP version 1
Version 1 is described in RFC 1058
We are using RIP version 2 in our project
Version 2 is described in RFCs 1721 and 1722

82. General Characteristics of RIP
It is a distance vector protocol
Hop count is the metric for path selection
Maximum allowable hop count is 15
Entire routing table is broadcast every 30 seconds by default
Can load balance over as many as six equal-cost paths (four paths is the default)

83. RIP-1 vs RIP-2
RIP-1 requires that only one network mask can be used per network number for each major classful network being advertised
RIP-2 permits variable-length subnet masks (VLSM) on the internetwork
Standard RIP-2 supports triggered updates
Standard RIP-1 does NOT support triggered updates

84. RIP Configuration Example
router rip
network A B C S2 E0 S3 S2 E0 S3
router rip
network
network
router rip
network
network

85. Router A router rip selects RIP as the routing protocol
network specifies a directly connected network
network specifies a directly connected network
Router A interfaces connected to networks and will send and receive RIP updates
These interfaces will also be advertised to neighboring routers
The updates allow the router to learn new topologies

86. Show IP Protocols
Displays values associated with routing timers and network information associated with the entire router

87. Show IP Protocols RouterA#sh ip protocols Routing Protocol is “rip”
Sending updates every 30 seconds, next due in 0 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Redistributing: rip
default version control: send version 1, receive any version
interface Send Recv Key-Chain
Ethernet
Serial
Routing for Networks:
Routing Information Sources;
Gateway Distance last Update
:00:10
Distance: (Default is 120)

88. Show IP Protocols Analysis
Router A sends updated routing table information every 30 seconds
If router running RIP does not receive an update for 180 seconds, marks route as invalid
Holddown timer is set to 180 seconds – update to a route that returns to up will not be made for 180 seconds
With no update, routing table entry is discarded after 240 seconds
It has been 10 seconds since Router A received an update from Router B
Advertised routes are listed after Routing for Networks line
Administrative distance default is 120

89. Show IP Route Show ip route displays routing table information
The routing table contains entries for all known networks and subnetworks

90. Show IP Route Example RouterA#sh ip route
Codes: C – connected, S – static, I – IGRP, 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, E – EGP
I – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, * - candidate default
U – per-user static route, o – ODR
T – traffic engineered route
Gateway of last resort is not set
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
/24 is subnetted, 2 subnets
R [120/1] via , 00:00:07, Serial2
C is directly connected Serial 2
R /24 [120/2] via , 00:00:07, Serial2

91. Show IP Route Fields Output Description R or C
Identifies source of the route – C: directly connected – R: RIP
Route’s address of the destination network
[120/1]
[administrative distance/number of hops]
via
Address of next hop router to reach the remote network
00:00:07
Time since the route was updated – hours:minutes:seconds
Serial2
Interface through which the specified network can be reached

92. Routing Table Problems
If show ip route shows no entries that were learned, routing information is not being exchanged
Use show running-config or show ip protocols to check for possible misconfigurations

93. Debug IP RIP
The debug ip rip command displays RIP routing updates as they are sent and received

94. Debug Router RIP Example
RouterA#debug ip rip
RIP protocol debugging is on
RouterA#
00:06:24: RIP: received v1 update from on Serial2
00:06:24: in 1 hops
00:06:24: in 2 hops
00:06:33: RIP: sending v1 update to via Ethernet0 ( )
00:06:34: network , metric 1
00:06:34: network , metric 3
00:06:34: RIP: sending v1 update to via Serial2 ( )
00:06:24: network
To disable debugging, use no debug ip rip or no debug all

95. No Debug All no debug all turns off all debugging
Debugging output can be overwhelming
It is often useful to turn off all debugging

96. IGRP
Interior Gateway Routing Protocol (IGRP) is an advanced distance vector routing protocol developed by Cisco in the mid-1989s

97. IGRP Features
Increased Scalability – Provides improved routing for larger networks as compared with RIP
HOP count
RIP 15
IGRP default 100
IGRP max 255
Sophisticated Metric
Default uses internetwork delay and bandwidth
Optionally, reliability, load and MTU can be included
Multiple Path Support
IGRP can maintain up to 6 unequal cost pats between network source and destination
(Unlike RIP) Paths do not mandate equal costs
Multiple paths can be used to increase bandwidth or increase route redundancy

98. IGRP Applicability
IGRP should be used in IP networks that require a simple, robust, and more scalable router protocol than RIP
IGRP performs triggered updates, which gives it an advantage over RIP-1

99. IGRP Metrics
IGRP’s composite routing metric provides greater accuracy than RIP’s hop-count
Path with smallest metric is best
By default, IGRP metrics are weighted with constants K1 through K5
Constants convert IGRP metric vector into a scalar quantity

100. IGRP Metric Components
Bandwidth – The lowest bandwidth value in the path
Delay – The cumulative interface delay along the path
Reliability – Determined by exchange of keepalives
Load – Load on a link between source and destination based on bits per second
MTU – Maximum Transfer Unit value of the path

101. IGRP Metric Notes Default Metrics
Bandwidth (values between 1200bps and 10gbps)
Delay (values between 1 and 2x1023)
Reliability and load metrics are unitless and can take values between 0 and 255

102. IGRP Route Selection
Assume two routes are available, one through 19.2kbps lines, and the other through 10mbps lines
IGRP will select the routes with 10mbps lines, because it has higher bandwidth

103. Multiple Paths
IGRP supports multiple paths between source and destination
Dual equal-bandwidth lines can run a single stream of traffic in a round-robin fashion
Switchover is automatic if one line goes down

104. Unequal Paths
Multiple paths can be used even if metrics for the paths are different
If metric for one path is three times better than for another path, it will be used three times more often
Only routes with metrics in a certain range are used as multiple paths

105. Unequal-Cost Load Balancing
Allows traffic to be distributed among up to six unequal paths to provide greater overall throughput and reliability
Following rules apply
IGRP will accept up to six paths for a given destination (four by default)
Next-hop router in any of the paths must be closer to the destination than the local router is by its best path
Alternative path metric must be within specified variance metric

106. Configuration Commands
router(config-router)#router igrp autonomous system
router(config-router)#network network-number

107. IGRP Configuration Example
Autonomous System = 100 A B C S2 E0 S3 S2 E0 S3
router igrp 100
network
network
router igrp 100
network
network
router igrp 100
network

108. RouterA Analysis
router IGRP 100 – enables IGRP routing process for autonomous system 100
network – associates network and its interfaces with IGRP routing process
network – associates network and its interfaces with IGRP routing process

109. IGRP Updates
IGRP sends updates out interfaces in networks and
It also advertises directly connected networks and , as well as other networks it learns about through IGRP ( )

110. IGRP Load Balancing
The variance router configuration command controls IGRP load balancing
router(config-router)#variance multiplier
Multiplier parameter specifies the range of metric values acceptable for load balancing
Range is from lowest (best) metric value to the lowest multiplied times the variance value
Acceptable values are nonzero, positive integers
Default value is 1, which implies equal-cost load balancing

111. IGRP Traffic-Share
The traffic-share {balanced | min} command is used to control how traffic is distributed among IGRP load sharing routes
router(config-router)#traffic-share {balanced|min}
Balanced option destributes traffic proportional to the ratios of the metrics
Min option specifies using routes with the minimum cost

112. IGRP Show IP Protocols
The show ip protocols command displays parameters, filters and network information about the entire router

113. IGRP Show IP Protocols Example
RouterA#sh ip protocols
Routing Protocol is “igrp 100”
Sending updates every 90 seconds, next due in 21 seconds
Invalid after 270 seconds, hold down 280, flushed after 630
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
IGRP maximum hopcount 100
IGRP maximum metric variance 1
Redistributing: igrp 100
Routing for Networks:
Routing Information Sources:
Gateway Distance Last Update
:01:01
Distance: (default is 100)

114. Show IP Protocols Fields
Output
Description
Routing Protocol
Routing protocol and autonomous system
Update
Rate at which updates are sent
Invalid
Number of seconds after which a route is declared invalid – Should be at least 3 times update value
Hold-Down
Number of seconds worst path routing information is surpressed – Should be at least 3 times value of update
Flushed
Number of seconds that must pass before route is removed from table – Should be equal to or greater than sum of invalid and holddown values

115. IGRP Show IP Route RouterA#sh ip route
Codes: C – connected, S – static, I – IGRP, 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, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, * - candidate default
U – per-user static route, o – ODR
T – traffic engineered route
Gateway of last resort is not set
/24 is subnetted, 1 subnets
C is directly connected, Ethernet0
/24 is subnetted, 2 subnets
I [100/90956] via , 00:00:23, Serial2
C is directly connected Serial 2
I /24 [100/90956] via , 00:00:23, Serial2

116. Debug IP IGRP – Transactions
RouterA#debug ip igrp transactions
IGRP protocol debugging is on
RouterA#
00:21:06: IGRP: sending update to via Ethernet0 ( )
00:21:06: network , metric=88956
00:21:06: network , metric=91056
00:21:07: IGRP: sending update to via Serial 2 ( )
00:21:07: network , metric=1100
00:21:16: IGRP: received update from on Serial2
00:21:16: subnet , metric (neighbor 88956)
00:21:16: network , metric (neighbor 89056)
Disable debugging with no debug igrp transactions or no debug all

117. Debug IP IGRP – Events RouterA#debug ip igrp events
IGRP event debugging is on
RouterA#
00:23:44: IGRP: sending update to via Ethernet0 ( )
00:23:44: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes.
00:23:44: IGRP: Total routes in update: 2
00:23:44: IGRP: sending update to via Serial2 ( )
00:23:45: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.
00:23:45: IGRP: Total routes in update: 1
00:23:48: IGRP: received update from on Serial 2
00:23:48: IGRP Update contains 1 interior, 1 system, and 0 exterior routes
00:23:48: IGRP Total routes in update: 2

118. Network 172.16.0.0 Fails X S2 E0 S3 S2 E0 S3 172.16.1.0 192.168.1.0 AB C S2 X E0 S3 S2 E0 S3

119. Network 172.16.0.0 Fails – RouterA
RouterA#debug ip igrp trans
IGRP protocol debugging is on
RouterA#
00:31:15: %LINEPROTO – 5 - UPDOWN; Line protocol on interface Ethernet0
changed state to down
00:31:15: IGRP: edition is now 3
00:31:15: IGRP: sending update to via Serial2 ( )
00:31:16: network , metric= ROUTE DOWN
00:31:16: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.
00:31:16: IGRP: Total routes in update: 1
00:31:16: IGRP: broadcasting request on Serial2
00:31:16: IGRP: received update from on Serial2
00:31:16: subnet , metric (neighbor 88956)
00:31:16: network , metric (inaccessible) REVERSE POISON
00:31:16: network , metric (neighbor 89056)
00:31:16: IGRP: Update contains 1 interior, 2 system, and 0 exterior routes
00:31:16: IGRP: Total routes in update: 3

120. Network 172.16.0.0 Fails – RouterB
RouterB#debug ip igrp trans
IGRP protocol debugging is on
RouterB#
1d19h: IGRP: sending update to via Serial2 ( )
1d19h: subnet , metric=88956
1d19h: network , metric=89056
1d19h: IGRP: sending update to via Serial3 ( )
1d19h: subnet , metric=88956
1d19h: network , metric=89056
1d19h: IGRP: received update from on Serial2
1d19h: network , metric (inaccessible) POSSIBLY DOWN
1d19h: IGRP: edition is now 10
1d19h: subnet , metric 90956
1d19h: network , metric POISON
1d19h: network , metric 91056
1d19h: network , metric

121. Holddown State – Router B
RouterB#sh ip route
Codes: C – connected, S – static, I – IGRP, 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, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, * - candidate default
U – per-user static route, o – ODR
T – traffic engineered route
Gateway of last resort is not set
I /24 is possibly down, routing via , Serial2
/24 is subnetted, 2 subnets
C is directly connected, Serial3
C is directly connected, Serial3
I /24 [100/89056] via , 00:00:14, Serial3

122. Ping – From Router B RouterB#ping 172.16.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds: …
Success rate is 0 percent (0/5)
RouterB#

123. If Comes Back Up
RouterA sends another triggered update to Router B stating is accessible with metric 89056
Even Though RouterB receives the update, the route continues in a holddown state
RouterB will not remove route from holddown and update routing table until holddown timer expires
However, RouterB COULD successfully ping network and send traffic there

124. Unknown Subnet of a Directly Attached Network
Router assumes that all subnets of a directly attached network are present in the IP routing table
If a packet is received with a destination address within an unknown subnet of a directly attached network, the router assumes the subnet does not exist and drops the packet
This holds true even if the routing table contains a default route
This behavior can be changed with the ip classless global configuration command

125. IP Classless Default Route E0 S0 Network Protocol Destination Exit
Network
Protocol
Destination
Exit
Interface C
RIP
via E0 S0
Router(config)#ip classless

126. IP Classless
The middle router will forward a packet with as the destination address out of the default interface, E0, because ip classless is enabled

127. Routing Command Summary
Description
ip route network mask {address | interface} [distance] [permanent]
Defines a static route
router protocol [keyword]
Enables dynamic routing protocol
network network-number
Allows dynamic routing protocol to advertise a route and enables the protocol on the interfaces on that network
show ip protocols
Displays information about the dynamic routing protocols configured on the router
show ip route
Displays the IP routing table
debug ip rip
Enables the router to display RIP routing updates as they occur
variance multiplier
Enables IGRP to do unequal path load sharing
traffic-share {balanced | min}
Tells router how to load-balance traffic on load-sharing links
debug ip igrp transactions
Displays IGRP transaction info as it occurs
debug ip igrp events
Displays IGRP events as they occur
no debug all
Turns off all debugging displays
ip classless
Allows routing protocol to send traffic to a less specific route if one is available