Chapter 5 IP Routing Routing Protocol vs. Routed Protocol

Chapter 5 IP Routing Routing Protocol vs. Routed Protocol

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

Review Router and IP Address

Internetworking

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 ...

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.

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.

Router and Bridge

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.

Router Interface

Router function

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.)

Examines the network layer address to determine the destination network.

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.

Send the data out interface B1, the router would encapsulate the data in the appropriate data link frame.

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.

ROUTED AND ROUTING PROTOCOLS

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.

Network protocol operation

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.

Protocol addressing variations

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

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.

Addressing of a routable protocol

Routing table E0 E1 E2

Multi-protocol routing

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 …

Static routes

Dynamic routes

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.

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.

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

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.

IGP vs. EGP IGP EGP

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.

Distance vector routing

Link state routing

Part II

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

Distance Vector Routing Protocols
Dynamic routing protocols help the network administrator overcome the time-consuming and exacting process of configuring and maintaining static routes. Examples of Distance Vector routing protocols: Routing Information Protocol (RIP) RFC 1058. Hop count is used as the metric for path selection. If the hop count for a network is greater than 15, RIP cannot supply a route to that network. Routing updates are broadcast or multicast every 30 seconds, by default. Interior Gateway Routing Protocol (IGRP) proprietary protocol developed by Cisco. Bandwidth, delay, load and reliability are used to create a composite metric. Routing updates are broadcast every 90 seconds, by default. IGRP is the predecessor of EIGRP and is now obsolete. Enhanced Interior Gateway Routing Protocol (EIGRP) Cisco proprietary distance vector routing protocol. It can perform unequal cost load balancing. It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path. There are no periodic updates as with RIP and IGRP. Routing updates are sent only when there is a change in the topology.

The Meaning of Distance Vector: A router using distance vector routing protocols knows 2 things: Distance to final destination The distance or how far it is to the destination network Vector, or direction, traffic should be directed The direction or interface in which packets should be forwarded For example, in the figure, R1 knows that the distance to reach network /24 is 1 hop and that the direction is out the interface S0/0/0 toward R2.

Characteristics of Distance Vector routing protocols: Periodic updates Periodic Updates sent at regular intervals (30 seconds for RIP). Even if the topology has not changed in several days, Neighbors The router is only aware of the network addresses of its own interfaces and the remote network addresses it can reach through its neighbors. It has no broader knowledge of the network topology Broadcast updates Broadcast Updates are sent to Some distance vector routing protocols use multicast addresses instead of broadcast addresses. Entire routing table is included with routing update Entire Routing Table Updates are sent, with some exceptions to be discussed later, periodically to all neighbors. Neighbors receiving these updates must process the entire update to find pertinent information and discard the rest. Some distance vector routing protocols like EIGRP do not send periodic routing table updates.

Routing Protocol Algorithm: The algorithm is used to calculate the best paths and then send that information to the neighbors. Different routing protocols use different algorithms to install routes in the routing table, send updates to neighbors, and make path determination decisions.

Routing Protocol Characteristics Criteria used to compare routing protocols includes Time to convergence Time to convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. The faster the convergence, the more preferable the protocol. Scalability Scalability defines how large a network can become based on the routing protocol that is deployed. The larger the network is, the more scalable the routing protocol needs to be. Resource usage Resource usage includes the requirements of a routing protocol such as memory space, CPU utilization, and link bandwidth utilization. Higher resource requirements necessitate more powerful hardware to support the routing protocol operation Classless (Use of VLSM) or Classful Classless routing protocols include the subnet mask in the updates. This feature supports the use of Variable Length Subnet Masking (VLSM) and better route summarization. Implementation & maintenance Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Routing Loops Routing loops can eliminate Defining a maximum metric to prevent count to infinity Holddown timers Split horizon Route poisoning or poison reverse Triggered updates Note: The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.

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.

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.

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.

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.

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.

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.

Link-State Routing Link state routing protocols
-Also known as shortest path first algorithms -These protocols built around Dijkstra’s SPF OSPF will be discussed in Chapter 11, and IS-IS will be discussed in CCNP.

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.

Link-State Routing Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm This algorithm accumulates costs along each path, from source to destination.

Link-State Routing: Step 1 – Learn about directly connected Networks
This is an interface on a router Link state This is the information about the state of the links

Link-State Routing: step 2 - Sending Hello Packets to Neighbors
Link state routing protocols use a hello protocol Purpose of a hello protocol: -To discover neighbors (that use the same link state routing protocol) on its link

Link-State Routing: step 2 - Sending Hello Packets to Neighbors
Connected interfaces that are using the same link state routing protocols will exchange hello packets. Once routers learn it has neighbors they form an adjacency 2 adjacent neighbors will exchange hello packets These packets will serve as a keep alive function

Link-State Routing: step 3 - Building the Link State Packet (LSP)
Contents of LSP: State of each directly connected link Includes information about neighbors such as neighbor ID, link type, & bandwidth. A simplified version of the LSPs from R1 is: 1. R1; Ethernet network /16; Cost 2 2. R1 -> R2; Serial point-to-point network; /16; Cost 20 3. R1 -> R3; Serial point-to-point network; /16; Cost 5 4. R1 -> R4; Serial point-to-point network; /16; Cost 20

Link-State Routing: step 4 - Flooding LSPs to Neighbors
Once LSP are created they are forwarded out to neighbors. Each router floods its link-state information to all other link-state routers in the routing area. Whenever a router receives an LSP from a neighboring router, it immediately sends that LSP out all other interfaces except the interface that received the LSP. This process creates a flooding effect of LSPs from all routers throughout the routing area.

Link-State Routing: step 4 - Flooding LSPs to Neighbors
LSPs are sent out under the following conditions Initial router start up or routing process When there is a change in topology including a link going down or coming up, or a neighbor adjacency being established or broken

Link-State Routing: step 5 - Constructing a link state data base
Routers use a database to construct a topology map of the network After each router has propagated its own LSPs using the link-state flooding process, each router will then have an LSP from every link-state router in the routing area. These LSPs are stored in the link-state database. Each router in the routing area can now use the SPF algorithm to construct the SPF trees that you saw earlier.

Link-State Routing: step 5 - Constructing a link state data base
router R1 has learned the link-state information for each router in its routing area. With a complete link-state database, R1 can now use the database and the shortest path first (SPF) algorithm to calculate the preferred path or shortest path to each network.

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

How Routing Information Is Maintained

Link-State Routing Protocol Algorithms

Advantages of a Link-State Routing Protocol Routing protocol Builds Topological map Router can independently determine the shortest path to every network. Convergence Event driven routing updates Use of LSP Distance vector No Slow Generally No Link State Yes Fast Generally Yes

There are several advantages of link-state routing protocols compared to distance vector routing protocols. Builds a Topological Map Link-state routing protocols create a topological map, or SPF tree of the network topology. Using the SPF tree, each router can independently determine the shortest path to every network. Distance vector routing protocols do not have a topological map of the network. Routers implementing a distance vector routing protocol only have a list of networks, which includes the cost (distance) and next-hop routers (direction) to those networks. Fast Convergence When receiving a Link-state Packet (LSP), link-state routing protocols immediately flood the LSP out all interfaces except for the interface from which the LSP was received. A router using a distance vector routing protocol needs to process each routing update and update its routing table before flooding them out other interfaces, even with triggered updates. Event-driven Updates After the initial flooding of LSPs, link-state routing protocols only send out an LSP when there is a change in the topology. The LSP contains only the information regarding the affected link. Unlike some distance vector routing protocols, link-state routing protocols do not send periodic updates. Hierarchical Design Link-state routing protocols such as OSPF and IS-IS use the concept of areas. Multiple areas create a hierarchical design to networks, allowing for better route aggregation (summarization) and the isolation of routing issues within an area.

Requirements for using a link state routing protocol Memory requirements Typically link state routing protocols use more memory Processing Requirements More CPU processing is required of link state routing protocols Bandwidth Requirements Initial startup of link state routing protocols can consume lots of bandwidth This should only occur during initial startup of routers, but can also be an issue on unstable networks.

Modern link-state routing protocols are designed to minimize the effects on memory, CPU, and bandwidth. The use and configuration of multiple areas can reduce the size of the link-state databases. Multiple areas can also limit the amount of link-state information flooding in a routing domain and send LSPs only to those routers that need them. For example, when there is a change in the topology, only those routers in the affected area receive the LSP and run the SPF algorithm. This can help isolate an unstable link to a specific area in the routing domain. In the figure, If a network in Area 51 goes down, the LSP with the information about this downed link is only flooded to other routers in that area. Routers in other areas will learn that this route is down, but this will be done with a type of link-state packet that does not cause them to rerun their SPF algorithm. Note: Multiple areas with OSPF and IS-IS are discussed in CCNP

2 link state routing protocols used for routing IP -Open Shortest Path First (OSPF) -Intermediate System-Intermediate System (IS-IS)

Routing Protocol

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.

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.

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

End Chapter V

Chapter 5 IP Routing Routing Protocol vs. Routed Protocol

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