1. Chapter 5 IP Routing Routing Protocol vs. Routed Protocol

2. Topics Review Routed and Routing Protocols Routing Protocol Activity
Internetworking
Path determination
Router
IP Address
Routed and Routing Protocols
Network protocols
Routed
Routing
Interior Protocols vs. Exterior Protocols
Routing Protocol Activity

3. Review Router and IP Address

4. Internetworking

5. Path determination
Path determination is the process that the router uses to choose the next hop in the path for the packet to travel to its destination based on the link bandwidth, hop, delay ...

6. Router
A router is a type of internetworking device that passes data packets between networks, based on Layer 3 addresses.
A router has the ability to make intelligent decisions regarding the best path for delivery of data on the network.

7. IP addresses
IP addresses are implemented in software, and refer to the network on which a device is located.
IP addressing scheme, according to their geographical location, department, or floor within a building.
Because they are implemented in software, IP addresses are fairly easy to change.

8. Router and Bridge

9. Router connections
Routers connect two or more networks, each of which must have a unique network number in order for routing to be successful.
The unique network number is incorporated into the IP address that is assigned to each device attached to that network.

10. Router Interface

11. Router function

12. Router function (cont.1)
Strips off the data link header, carried by the frame.
(The data link header contains the MAC addresses of the source and destination.)

13. Router function (cont.2)
Examines the network layer address to determine the destination network.

14. Router function (cont.3)
Consults its routing tables to determine which of its interfaces it will use to send the data, in order for it to reach its destination network.

15. Router function (cont.4)
Send the data out interface B1, the router would encapsulate the data in the appropriate data link frame.

16. Router Interface example
Interface is a router’s attachment to a network, it may also be referred to as a port. In IP routing.
Each interface must have a separate, unique network address.

17. ROUTED AND ROUTING PROTOCOLS

18. Network protocols
In order to allow two host communicate together through internetwork, they need a same network protocol.
Protocols are like languages.
IP is a network layer protocol.

19. Network protocol operation

20. Routed protocol
Protocols that provide support for the network layer are called routed or routable protocols.
IP is a network layer protocol, and because of that, it can be routed over an internetwork.

21. Protocol addressing variations

22. Three important routed protocols
TCP/IP: 04 bytes
Class A: 1 byte network + 3 bytes host
Class B: 2 bytes network + 2 bytes host
Class C: 3 bytes network + 1 byte host
IPX/SPX: 10 bytes
4 bytes network + 6 bytes host
AppleTalk: 03 bytes
2 bytes network + 1 byte host

23. Non-routable protocol
Non-routable protocols are protocols that do not support Layer 3.
The most common of these non-routable protocols is NetBEUI.
NetBEUI is a small, fast, and efficient protocol that is limited to running on one segment.

24. Addressing of a routable protocol

25. Routing table E0 E1 E2

26. Multi-protocol routing

27. Classification #1: Static and Dynamic
Static routes:
The network administrator manually enter the routing information in the router.
Dynamic routes:
Routers can learn the information from each other on the fly.
Using routing protocol to update routing information.
RIP, IGRP, EIGRP, OSPF …

28. Static routes

29. Dynamic routes

30. Static vs. dynamic routes
Static routes:
For hiding parts of an internetwork.
To test a particular link in a network.
For maintaining routing tables whenever there is only one path to a destination network.
Dynamic routes:
Maintenance of routing table.
Timely distribution of information in the form of routing updates.
Relies on routing protocol to share knowledge.
Routers can adjust to changing network conditions.

31. Routing protocol
Routing protocols determine the paths that routed protocols follow to their destinations.
Routing protocols enable routers that are connected to create a map, internally, of other routers in the network or on the Internet.

32. Routed vs. Routing protocol
Routing protocols
determine how routed
protocols are routed

33. Classification #2: IGP and EGP
Dynamic routes.
Interior Gateway Protocols (RIP, IGRP, EIGRP, OSPF):
Be used within an autonomous system, a network of routers under one administration, like a corporate network, a school district's network, or a government agency's network.
Exterior Gateway Protocols (EGP, BGP):
Be used to route packets between autonomous systems.

34. IGP vs. EGP
IGP
EGP

35. Classification #3: DVP and LSP
Distance-Vector Protocols (RIP, IGRP):
View network topology from neighbor’s perspective.
Add distance vectors from router to router.
Frequent, periodic updates.
Pass copy of routing tables to neighbor routers.
Link State Protocols (OSPF):
Gets common view of entire network topology.
Calculates the shortest path to other routers.
Event-triggered updates.
Passes link state routing updates to other routers.

36. Distance vector routing

37. Link state routing

38. Part II

39. Distance Vector Routing
© 2004 Cisco Systems, Inc. All rights reserved.
ICND v2.2—3-39

40. Outline Overview Distance Vector Route Selection
Routing Information Maintenance
How Routing Inconsistencies Occur with Distance Vector Routing Protocols
Count to Infinity Prevention
Techniques to Eliminate Routing Loops
Implementation of Techniques to Eliminate Routing Loops
Summary
Slide 1 of 2
Purpose: This slide states the chapter objectives.
Emphasize: Read or state each objective so that each student has a clear understanding of the chapter objectives.
Note: Catalyst switches have different CLIs. The Catalyst 2900xl and the Catalyst 1900 has a Cisco IOS CLI. The Cisco IOS CLI commands available on the 2900xl is different from the The Catalyst 5000 family has no Cisco IOS CLI, and use the set commands instead. This class only covers the configuration on the Catalyst 1900 switch.

41. Distance Vector Routing Protocols
Purpose: This figure introduces the distance vector routing algorithm, the first of the classes of routing protocols, and outlines how it operates.
Emphasize: Distance vector algorithms do not allow a router to know the exact topology of an internetwork.
This information is somewhat analogous to the information found on signs at a highway intersection. A sign points toward a road leading away from the intersection and indicates the distance to the destination.
Further down the highway, another sign also points toward the destination, but now the distance to the destination is shorter.
As long as each successive point on the path shows that the distance to the destination is successively shorter, the traffic is following the best path.
Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.

42. Sources of Information and Discovering Routes
Layer 3 of 3
Emphasize: Layer 3 adds the final entries received some time later that have distances of 2 from routers A and C.
Routers discover the best path to destinations from each neighbor.

43. Selecting the Best Route with Metrics
Emphasize: How the routing algorithm defines “best” determines the most important characteristics of each routing algorithm.
Hop count—Some routing protocols use hop count as their metric. Hop count refers to the number of routers a packet must go through to reach a destination. The lower the hop count, the better the path. Path length is used to indicate the sum of the hops to a destination. As indicated in the figure, RIP uses hop count for its metric.
Ticks—Metric used with Novell IPX to reflect delay. Each tick is 1/18th of a second.
Cost—Factor used by some routing protocols to determine the best path to a destination; the lower the cost, the better the path. Path cost is the sum of the costs associated with each link to a destination.
Bandwidth—Although bandwidth is the rating of a link’s maximum throughput, routing through links with greater bandwidth does not always provide the best routes. For example, if a high-speed link is busy, sending a packet through a slower link might be faster. As indicated in the figure with highlighting, delay and bandwidth comprise the default metric for IGRP.
Delay—Depends on many factors, including the bandwidth of network links, the length of queues at each router in the path, network congestion on links, and the physical distance to be traveled. A conglomeration of variables that change with internetwork conditions, delay is a common and useful metric. As indicated in the figure with highlighting, delay and bandwidth comprise the default metric for IGRP.
Load—Dynamic factor can be based on a variety of measures, including CPU use and packets processed per second. Monitoring these parameters on a continual basis can itself be resource intensive.

44. Maintaining Routing Information
Layer 3 of 3
Layer 3 adds router B, which receives the updated routing table from router A. In turn, router B will perform its own process to update its routing table given this new topology update from router A.
Distance vector updates occur step by step.
Typically, a router sends updates by multicasting its table on each configured port, but other methods, such as sending the table only to preconfigured neighbors, are employed by some routing algorithms.
Multicast is used by the RIP2, OSPF, and EIGRP routing protocols. RIP and IGRP use broadcast.
The routing table can be sent routinely and periodically, or whenever a change in the topology is discovered. Updates sent when changes occur are called triggered updates.
Updates proceed step by step from router to router.

45. Inconsistent Routing Entries
Slide 1 of 4
Purpose: This figure describes the first of the general problems that a distance vector protocol could face without the corrective influence of some countermeasure.
Emphasize: Layer 1 shows the original state of the network and routing tables. All routers have consistent knowledge and correct routing tables. In this example, the cost function is hop count, so the cost of each link is 1. Router C is directly connected to network with a distance of 0. Router A’s path to network is through router B, with a hop count of 2.
Each node maintains the distance from itself to each possible destination network.

46. Inconsistent Routing Entries (Cont.)
Slide 2 of 4
Emphasize: In Layer 2, router C has detected the failure of network and stops routing packets out its E0 interface. However, router A has not yet received notification of the failure and still believes it can access network through router B. Router A’s routing table still reflects a path to network with a distance of 2.
Slow convergence produces inconsistent routing.

47. Inconsistent Routing Entries (Cont.)
Slide 3 of 4
Emphasize: Because router B’s routing table indicates a path to network , router C believes it now has a viable path to through router B. Router C updates its routing table to reflect a path to network with a hop count of 2.
Router C concludes that the best path to network is through router B.

48. Inconsistent Routing Entries (Cont.)
Slide 4 of 4
Emphasize: In Layer 4, router A receives the new routing table from router B, detects the modified distance vector to network , and recalculates its own distance vector to network as 3.
If all routers in an internetwork do not have up-to-date, accurate information about the state of the internetwork, they might use incorrect routing information to make a routing decision.
The use of incorrect information might cause packets to take less-than-optimum paths or paths that return packets to routers that they have already visited.
Router A updates its table to reflect the new but erroneous hop count.

49. Count to Infinity
Purpose: This figure describes another of the general problems that a distance vector protocol could face without the corrective influence of some countermeasure.
Emphasize: Both routers conclude that the best path to network is through each other and continue to bounce packets destined for network between each other, incrementing the distance vector by 1 each time.
This condition, called count to infinity, continuously loops packets around the network, despite the fundamental fact that the destination network is down. While the routers are counting to infinity, the invalid information allows a routing loop to exist.
A related concept is the Time-to-Live (TTL) parameter. The TTL is a packet parameter that decreases each time a router processes the packet. When the TTL reaches zero, a router discards or drops the packet without forwarding it. A packet caught in a routing loop is removed from the internetwork when its TTL expires.
The hop count for network counts to infinity.

50. Defining a Maximum
Purpose: This figure describes a corrective measure that attempts to solve the routing loop problems that a distance vector protocol could face.
Emphasize: Routing loops occur only when routing knowledge being propagated has not yet reached the entire internetwork—when the internetwork has not converged after a change. Fast convergence minimizes the chance for a routing loop to occur, but even the smallest interval leaves the possibility open.
To avoid prolonging the count-to-infinity time span, distance vector protocols define infinity as some maximum number. This number refers to a routing metric, such as a hop count.
With this approach, the routing protocol permits the routing loop until the metric exceeds its maximum allowed value. This example shows this defined maximum as 16 hops. Once the metric value exceeds the maximum, network is considered unreachable.
A limit is set on the number of hops to prevent infinite loops.

51. Routing Loops
Slide 4 of 4
Emphasize: In Layer 4, router A receives the new routing table from router B, detects the modified distance vector to network , and recalculates its own distance vector to network as 3.
If all routers in an internetwork do not have up-to-date, accurate information about the state of the internetwork, they might use incorrect routing information to make a routing decision.
The use of incorrect information might cause packets to take less-than-optimum paths or paths that return packets to routers that they have already visited.
Packets for network bounce (loop) between routers B and C.

52. Split Horizon
Purpose: This figure introduces the corrective measure known as “split horizon.” The split horizon technique attempts to solve routing loops.
Emphasize: The split horizon technique attempts to eliminate routing loops and speed up convergence. The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original packet came. In the example:
Router C originally announced a route to network to router B. It makes no sense for router B to announce to router C that router B has access to network through router C.
Given that router B passed the announcement of its route to network to router A, it makes no sense for router A to announce its distance from network to router B.
Because router B has no alternative path to network , router B concludes that network is inaccessible.
In its basic form, the split-horizon technique simply omits from the message any information about destinations routed on the link. This strategy relies either on routes never being announced or on old announcements fading away through a timeout mechanism.
Split horizon also improves performance by eliminating unnecessary routing updates. Under normal circumstances, sending routing information back to the source of the information is unnecessary.
It is never useful to send information about a route back in the direction from which the original information came.

53. Route Poisoning
Purpose: This figure expands on the split-horizon technique by adding the concept of poisonous reverse updates.
Emphasize: Route poisoning closes the potential for longer routing loops. Fast convergence minimizes the chance for a routing loop to occur, but even the smallest interval leaves the possibility open. With a poison route in place, router B can maintain a steadfast entry that network is indeed down.
Routers advertise the distance of routes that have gone down to infinity.

54. Poison Reverse Poison reverse overrides split horizon.
Purpose: This figure explains poison reverse.
Emphasize: Poison reverse overrides the split-horizon solution.
Poison reverse overrides split horizon.

55. Holddown Timers
Purpose: This figure describes how hold-down timers avoid the general problems that a routing protocol could face.
Emphasize: Hold-down timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad.
Hold-downs tell routers to hold any changes that might affect routes for some period of time.
The hold-down period is usually calculated to be just greater than the period of time necessary to update the entire network with a routing change.
Note: A network administrator can configure the hold-down timers on several routers to work together in tandem.
The router keeps an entry for the “possibly down state” in the network, allowing time for other routers to recompute for this topology change.

56. Triggered Updates
Purpose: This figure describes how triggered updates avoid the general problems that a routing protocol could face.
Emphasize: Normally, new routing tables are sent to neighboring routers on a regular basis. A triggered update is a new routing table that is sent immediately, in response to some change.
Each update triggers a routing table change in the adjacent routers, which, in turn, generate triggered updates notifying their adjacent neighbors of the change. This wave propagates throughout that portion of the network where routes went through the link.
Triggered updates would be sufficient if we could guarantee that the wave of updates reached every appropriate router immediately. However, there are two problems:
Packets containing the update message can be dropped or corrupted by some link in the network.
The triggered updates do not happen instantaneously. It is possible that a router that has not yet received the triggered update will issue a regular update at just the wrong time, causing the bad route to be reinserted in a neighbor that had already received the triggered update.
Coupling triggered updates with holddowns is designed to get around these problems. Because the hold-down rule says that when a route is removed, no new route will be accepted for the same destination for some period of time, the triggered update has time to propagate throughout the network.
The router sends updates when a change in its routing table occurs.

57. Eliminating Routing Loops
Slide 1 of 6
Purpose: This page begins a series of graphics that tie all the solutions together by showing how each solution works to prevent routing loops in a more complex network design.
Emphasize: Begin this series by describing that router B poisons its route to network

58. Eliminating Routing Loops (Cont.)
Slide 2 of 6
Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.
Emphasize: Describe that routers D and A both set their hold-down timers and send triggered updates to router E about the status of network Router E also sets its hold-down timer.

59. Eliminating Routing Loops (Cont.)
Slide 3 of 6
Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.
Emphasize: Router A and D send a poison reverse to router B. Router E sends a poison reverse to router B.

60. Eliminating Routing Loops (Cont.)
Slide 4 of 6
Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.
Emphasize: While in hold-down state, routers A, D, and E will still attempt to forward packets to network

61. Eliminating Routing Loops (Cont.)
Slide 5 of 6
Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.
Emphasize: When the link is back up, router B will send a triggered update to router A and router D, notifying them that network is active.

62. Eliminating Routing Loops (Cont.)
Slide 6 of 6
Purpose: This graphic continues to describe how the solution works to prevent routing loops in a more complex network design.
Emphasize: Router A and D, in turn, update router E that network is now up.

63. Summary
Distance vector routing protocols generate periodic routing updates addressed to directly connected routing devices. Routers running a distance vector routing protocol send periodic updates even if there are no changes in the network.
When a router receives an update from a neighbor’s router, the router compares the update with its own routing table. The router adds the cost of reaching the neighbor’s router to the path cost reported by the neighbor to establish a new metric.
Routing inconsistencies occur if slow internetwork convergence or a new configuration causes incorrect routing entries.
Purpose: This slide discuss the initial configurations on the routers and switches.
Note: There is no setup mode on the Catalyst 1900 switch.

64. Summary (Cont.)
Distance vector protocols define infinity as some maximum number. The routing protocol then permits the routing table update loop until the metric exceeds its maximum allowed value.
There are five techniques for eliminating routing loops on distance vector routing networks: split horizon, route poisoning, poison reverse, holddown timers, and triggered updates.
All five techniques can be used together to eliminate routing loops in area networks.
Purpose: This slide discuss the initial configurations on the routers and switches.
Note: There is no setup mode on the Catalyst 1900 switch.

65. Link-State and Balanced Hybrid Routing
© 2004 Cisco Systems, Inc. All rights reserved.
ICND v2.2—3-65

66. Outline Overview How Routing Information is Maintained with Link State
Link-State Routing Protocol Algorithms
Benefits and Limitations of Link-State Routing
When to Use Link-State Routing Protocols
Balanced Hybrid Routing
Summary
Slide 1 of 2
Purpose: This slide states the chapter objectives.
Emphasize: Read or state each objective so that each student has a clear understanding of the chapter objectives.
Note: Catalyst switches have different CLIs. The Catalyst 2900xl and the Catalyst 1900 has a Cisco IOS CLI. The Cisco IOS CLI commands available on the 2900xl is different from the The Catalyst 5000 family has no Cisco IOS CLI, and use the set commands instead. This class only covers the configuration on the Catalyst 1900 switch.

67. Link-State Routing Protocols
Purpose: This figure introduces the link-state routing algorithm, the second of the classes of routing protocols, and outlines how it operates.
Emphasize: In contrast with the analogy about the distance vector information being like individual road signs that show distance, link-state information is somewhat analogous to a road map with a “you are here” pointer showing the map reader’s current location. This larger perspective indicates the shortest path to the destination. Each router has its own map of the complete topology.
Link-state routing is not covered further in this course. Refer students interested in more details to the ACRC course.
After initial flood of LSAs, link-state routers pass small event-triggered link-state updates to all other routers.

68. Drawbacks to Link-State Routing Protocols
Initial discovery may cause flooding.
Link-state routing is memory- and processor-intensive.

69. How Routing Information Is Maintained

70. Link-State Routing Protocol Algorithms

71. Link-State Network Hierarchy Example
Summarizing is the consolidation of multiple routes into one single advertisement. Proper summarization requires contiguous addressing.
Route summarization directly affects the amount of bandwidth, CPU, and memory resources consumed by the OSPF process. With summarization, if a network link fails, the topology change will not be propagated into the backbone (and other areas by way of the backbone). As such, flooding outside the area will not occur, so routers outside of the area with the topology change will not have to run the SPF algorithm (also called the Dijkstra algorithm after the computer scientist who invented it). Running the SPF algorithm is a CPU-intensive activity.
There are two types of summarization:
Interarea route summarization—Interarea route summarization is done on ABRs and applies to routes from within the autonomous system. It does not apply to external routes injected into OSPF via redistribution. In order to take advantage of summarization, network numbers in areas should be assigned in a contiguous way so as to be able to consolidate these addresses into one range. This graphic illustrates interarea summarization.
External route summarization—External route summarization is specific to external routes that are injected into OSPF via redistribution. Here again, it is important to ensure that external address ranges that are being summarized are contiguous. Summarization overlapping ranges from two different routers could cause packets to be sent to the wrong destination.
Minimizes routing table entries
Localizes impact of a topology change within an area

72. Advantages and Disadvantages of Link-State Routing

73. Comparing Distance Vector and Link-State Routing

74. Balanced Hybrid Routing
Purpose: This figure introduces and describes a third routing protocol class, the balanced hybrid.
Emphasize: Indicate how balance hybrid protocols such as Enhanced IGRP operate with elements of both distance vector and link-state routing protocols.
The EIGRP balanced hybrid routing protocol is covered in the ACRC course, not in this course.
Shares attributes of both distance vector and link-state routing

75. Summary
Link-state routing protocols collect routing information from all other routers in the network. After all information is collected, each router calculates its own best path to all destinations in the network.
Link-state algorithms maintain a complex database of the network topology. Knowledge of the network routers and of how they interconnect is achieved through the exchange of LSAs with other routes in a network.
Using triggered, flooded updates, link-state protocols can immediately report changes in the network topology, leading to fast convergence times. In contrast, the use of many different databases can require a significant amount of memory.

76. Summary (Cont.)
To avoid an excessive use of memory, a strict hierarchical network design is required. The configuration of link-state networks should remain simple to avoid tuning.
Balanced hybrid routing protocols combine aspects of both distance vector and link-state protocols.

77. Routing Protocol

78. RIP Most popular. Interior Gateway Protocol. Distance Vector Protocol.
Only metric is number of hops.
Maximum number of hops is 15.
Updates every 30 seconds.
Doesn’t always select fastest path.
Generates lots of network traffic.

79. IGRP and EIGRP Cisco proprietary. Interior Gateway Protocol.
Distance Vector Protocol.
Metric is compose of bandwidth, load, delay and reliability.
Maximum number of hops is 255.
Updates every 90 seconds.
EIGRP is an advanced version of IGRP, that is hybrid routing protocol.

80. OSPF Open Shortest Path First. Interior Gateway Protocol.
Link State Protocol.
Metric is compose of cost, speed, traffic, reliability, and security.
Event-triggered updates.

81. End Chapter V