Introduction to Routing & Routing Protocol

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Agenda

– - Router Operations– - Static Route– - Default Route– - Dynamic Route– - Class of Dynamic Routing Protocol– Administrative Distance– - Best Route Selection– - Distance Vector Protocol– - Link State Routing Protocol– - RIP V1 & V2– - RIP Configuration – - Structured approach for Troubleshooting

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Router Operations :- Routing is the process by which items get from one location to another. In networking, a router is the device used to route traffic. Routers can forward packets over static routes or dynamic routes, based on the router

configuration.

A router needs to do the following:– Know the destination address.– Identify the sources from which the router can learn.– Discover possible routes to the intended destination.– Select the best route.– Maintain and verify routing information.

Routers must learn destinations that are not directly connected.

Router Operations (Cont.)

Static Routes :- Static routers use a route that a network administrator enters into the router manually to reach the next hop.

Configure unidirectional static routes to and from a stub network to allow communications to occur.

Static Route Configuration

– Defines a path to an IP destination network or subnet or host– Address = IP address of the next hop router– Interface = outbound interface of the local router

RouterX(config)# ip route network [mask] {address | interface}[distance] [permanent]

Static Route Example

This is a unidirectional route. You must have a route configured in the opposite direction.

RouterA(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.1

RouterA(config)#ip route 172.16.1.0 255.255.255.0 s0/0/0

or

Verifying the Static Route Configuration

RouterA# show 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

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

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

 

10.0.0.0/8 is subnetted, 1 subnets

C 10.1.1.0 is directly connected, Fastethernet 0/0

S* 0.0.0.0/0 is directly connected, Serial0/0/0

Configuring a Default Route• The ip default-network command establishes a default route in networks

using dynamic routing protocols.– Router(config-router)#ip default-network network-number

• Creating an ip route to 0.0.0.0/0 is another way to configure a default route.– Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hop-ip-address | exit-

interface]

Default Routes

This route allows the stub network to reach all known networks beyond Router A.

Configuring Default Route

By default, routers learn paths to destinations in three different ways:

• Static routes: manually defines the static routes as next hop to destination.

• Default routes: manually defines default routes as path when there is no known route to destination.

• Dynamic routes: router learns the path by receiving periodic updates from other routers.

The ip default-network command is usually configured on the routers that connect to a router with a static default route.

HongKong1(config)#ip route 0.0.0.0 0.0.0.0 192.168.20.2

Static vs. Dynamic Routes

• Static Route– Uses a route that a

network administrator enters into the router manually

• Dynamic Route–Uses a route that a

network routing protocol adjusts automatically for topology or traffic changes

What Is a Dynamic Routing Protocol?

Routing protocols are used between routers to determine paths to remote networks and maintain those networks in the routing tables.

After the path is determined, a router can route a routed protocol to the learned networks.

An autonomous system is a collection of networks within a common administrative domain.

Interior gateway protocols operate within an autonomous system.

Exterior gateway protocols connect different autonomous systems.

Autonomous Systems: Interior and Exterior Routing Protocols

Classes of Routing Protocols

Classful Routing Protocol

– Classful routing protocols do not include the subnet mask with the route advertisement.

– Within the same network, consistency of the subnet masks is assumed.– Summary routes are exchanged between foreign networks.– These are examples of classful routing protocols:

• RIPv1• IGRP

Classless Routing Protocol

– Classless routing protocols include the subnet mask with the route advertisement.

– Classless routing protocols support a variable-length subnet mask (VLSM).

– Summary routes can be manually controlled within the network.

– These are examples of classless routing protocols:• RIPv2• EIGRP• OSPF• IS-IS

Administrative Distance :- is used to rate the trustworthiness of the routing protocol.

- Administrative distance is the feature that routers use in order to select the best path when there are multiple routes to the same destination from

different routing protocols.

- Administrative distance defines the reliability of a routing protocol.

- Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.Lower the AD

vlaue higher is the reliability of that routing protocol.

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Default Distance Value Table

Route Source Default Distance Values

Connected interface 0

Static route 1

Enhanced Interior Gateway Routing Protocol (EIGRP) summary route

5

External Border Gateway Protocol (BGP)

20

Internal EIGRP 90

IGRP 100

OSPF 110

Intermediate System-to-Intermediate System (IS-IS)

115

Routing Information Protocol (RIP)

120

Exterior Gateway Protocol (EGP) 140

On Demand Routing (ODR) 160

External EIGRP 170

Internal BGP 200

Unknown* 255

Administrative Distance: Ranking Routing Sources

Routers choose the routing source with the best administrative distance:

OSPF has an administrative distance of 110.

EIGRP has an administrative distance of 90.

Administrative Distance: Ranking Routes

Selecting the Best Route Using Metrics

Routing protocols use metrics to determine the best route to a destination if there are multiple route to that destination from same Routing Protocol.

Determining the Route Next Hop

• Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router.

Distance Vector Routing Protocols

Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.

Sources of Information and Discovering Routes

Routers discover the best path to destinations from each neighbor.

Maintaining Routing Information

Updates proceed step by step from router to router.

Inconsistent Routing Entries:Counting to Infinity and Routing

Loops

Each node maintains the distance from itself to each possible destination network.

Counting to Infinity

Slow convergence produces inconsistent routing.

Counting to Infinity (Cont.)

Router C concludes that the best path to network 10.4.0.0 is through router B.

Counting to Infinity (Cont.)

Router A updates its table to reflect the new but erroneous hop count.

Counting to Infinity (Cont.)

The hop count for network 10.4.0.0 counts to infinity.

Solution to Counting to Infinity:Defining a Maximum

A limit is set on the number of hops to prevent infinite loops.

Routing Loops

Packets for network 10.4.0.0 bounce (loop) between routers B and C.

Solution to Routing Loops: Split Horizon

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

Solution to Routing Loops:Route Poisoning and Poison

Reverse

Routers advertise the distance of routes that have gone down to infinity.

Solution to Routing Loops:Route Poisoning and Poison Reverse (Cont.)

Poison reverse overrides split horizon.

Solution to Routing Loops: Hold-Down Timers

The router keeps an entry for the “possibly down” state in the network, allowing time for other routers to recompute for this topology change.

Triggered Updates

The router sends updates when a change in its routing table occurs.

Eliminating Routing Loops

Eliminating Routing Loops (Cont.)

Eliminating Routing Loops (Cont.)

Link-State Routing Protocols

After an initial flood of LSAs, link-state routers pass small, event-triggered link-state updates to all other routers.

OSPF Hierarchical Routing

Consists of areas and autonomous systems

Minimizes routing update traffic

Link-State Routing Protocol Algorithms

Benefits and Drawbacks of Link-State Routing – Benefits of link-state routing:

• Fast convergence: – Changes are reported immediately by the affected source

• Robustness against routing loops:– Routers know the topology– Link-state packets are sequenced and acknowledged

• Hierarchical network design enables optimization of resources.

– Drawbacks of link-state routing:• Significant demands for resources:

– Memory (three tables: adjacency, topology, forwarding)– CPU (Dijkstra’s algorithm can be intensive, especially when there are many

instabilities)• Requires very strict network design • Configuration can be complex when tuning various parameters and

when design is complex

RIP Overview– is one of the oldest distance-vector routing protocols– Use Hop-count as routing metric selects the path.– prevents routing loops by implementing a limit on the number of hops allowed in a

path from the source to a destination.– The maximum number of hops allowed for RIP is 15.Hop 16 is consider unreachable .– So this max hop count limit the size of network that RIP can support.– RIP implements the split horizon, route poisoning and hold-down mechanisms to

prevent incorrect routing information from being propagated– Routes update every 30 seconds , Routing Information Protocol send complete routing

table every 30 sec. to all its neighboring devices .– This cause a lot of bandwidth utilize on these update flooding . Even If there is no

change in routing information even then these updates are flooded on the network.– As the network size grow there would be massive traffic burst every 30 sec.– This cause high latency & slow convergence of Routing Table .– AD value is 120 , which is least preferred among Dynamic Routing Protocol.

• Split Horizon :- split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned. Thus when a device that participates in such route advertisements receives an update from an interface, it (the device) does not forward updates through the same interface. By doing so, routing loops are prevented.

• Route Poisoning :- Distance-vector routing protocols in computer networks use route poisoning to indicate to other routers that a route is no longer reachable and should not be considered from their routing tables. RIP, use a maximum hop count to determine how many routers the traffic must go through to reach the destination. Each route has a hop count number assigned to it which is incremented as the routing information is passed from router to router. A route is considered unreachable if the hop count exceeds the maximum allowed. Max limit is 15 hop , 16 is consider unreachable.

• Hold Time :- A router will wait for a particular amount of time before considering a route unreachable , That time is called Hold time for which router wait before flush the route , for RIP Holdtimer is 180 second .

RIPv1 has the following limitations :-

– It does not send subnet mask information in its updates.

– It sends updates as broadcasts on 255.255.255.255.

– It does not support authentication. – It is not able to support VLSM or classless

interdomain routing (CIDR).

RIPv2 Features

RIPv1 and RIPv2 Comparison

RIPv1 RIPv2

Routing protocol Classful Classless

Supports variable-length subnet mask? No Yes

Sends the subnet mask along with the routing update? No Yes

Addressing type BroadcastMulticast Routing

updates over 224.0.0.9

Defined in … RFC 1058RFCs 1721, 1722,

and 2453

Supports manual route summarization? No Yes

Authentication support? No Yes

IP Routing Configuration Tasks

–Router configuration– Select routing protocols– Specify networks or interfaces

RIP Configuration

–Starts the RIP routing process

RouterX(config)# router rip

RouterX(config-router)# network network-number

Selects participating attached networks

Requires a major classful network number

Enables RIP version 2

RouterX(config-router)# version 2

RIP Configuration Example

Verifying the RIP Configuration

Routing Protocol is "rip"Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 6 secondsSending updates every 30 seconds, next due in 6 seconds

Invalid after 180 seconds, hold down 180, flushed after 240Invalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: rip

Default version control: send version 2, receive version 2send version 2, receive version 2

Interface Send Recv Triggered RIP Key-chain

FastEthernet0/0 2 2

Serial0/0/2 2 2

Automatic network summarization is in effect

Maximum path: 4

Routing for Networks:

10.0.0.0

172.16.0.0

Routing Information Sources:

Gateway Distance Last Update

10.1.1.2 120 00:00:25

Distance: (default is 120)Distance: (default is 120)

RouterA#

Displaying the IP Routing Table

Troubleshooting RIPv2

The debug ip rip Command

Summary

– Routing is the process by which items get from one location to another.

– Dynamic routing protocols determine how updates are conveyed, what knowledge is conveyed, when to convey knowledge, and how to locate recipients of the updates.

– A routing protocol that has a lower administrative value is more trustworthy than a protocol that has a higher administrative value.

– There are three classes of routing protocols: distance vector, link-state, and balanced hybrid.

– The ip classless command can be used to prevent a router from dropping a packet that is destined for an unknown subnetwork of a directly attached network if a default route is configured.

Summary (Cont.)

– RIP is a distance vector routing protocol that uses hop count as the matrix for route selection and broadcasts updates every 30 seconds.

– RIPv1 uses classful routing protocol; RIPv2 uses classless routing protocol. RIPv2 supports VLSM, manual route summarization, and authentication; RIPv1 does not support these activities.

– To enable a dynamic routing protocol, first a routing protocol is selected, then IP network numbers are assigned without values being specified (except OSPF).

– The router command starts the routing process. The network command allows the routing process to determine which interfaces will participate in sending and receiving the routing updates.

Summary (Cont.)

– The router RIP command selects RIP as the routing protocol. The network command identifies a participating attached network.

– The show ip command displays information about routing protocols and the routing table.

– The debug ip rip command displays information on RIP routing transactions.

Structured Approach to Troubleshooting

Typical Layer 1 Errors

• Broken cables • Disconnected cables • Cables connected to the wrong ports • Intermittent cable connection • Wrong cables used for the task at hand • Transceiver problems • DCE cable problems • DTE cable problems • Devices turned off

Typical Layer 2 Errors

• Improperly configured serial interfaces

• Improperly configured Ethernet interfaces

• Improper encapsulation set • Improper clock rate settings on

serial interfaces • Network interface card (NIC)

problems

Typical Layer 3 Errors

• Routing protocol not enabled • Wrong routing protocol enabled • Incorrect IP addresses • Incorrect subnet masks

Layer 3 Troubleshooting Using Ping

Layer 7 Troubleshooting Using Telnet

Troubleshooting Layer 1 Using show interfaces

CommandThe show interfaces serial command

Troubleshooting Using show cdp neighbors Command

Troubleshooting Using show cdp neighbors detail Command

Troubleshooting Using traceroute Command

Troubleshooting Routing IssuesThe show ip route Command

Troubleshooting Routing IssuesThe show ip protocols Command

Troubleshooting Using show controllers serial

CommandThe show controllers serial Command

Introduction to debugDebug syntax