WXES2106Network Technology

Semester 1 2004/2005

Chapter 6

Routing Protocol

CCNA1: 10.1, 10.2, CCNA2: Module 6 and 7

Contents

Introduction Routed Protocol Routing Protocol Static Route Dynamic Route RIP IGRP

Introduction

A protocol is a set of rules that determines how computers communicate with each other across networks.

A protocol describes the following: The format that a message must conform to The way in which computers must exchange a

message within the context of a particular activity

Routed Protocol A routed protocol allows the router to forward data

between nodes on different networks. Internet Protocol (IP) is the routed protocol of the Internet.

IPX/SPX, Appletalk are types of routed protocol. IP is a connectionless, unreliable, best-effort delivery

protocol. It determines the most efficient route for data based on the routing protocol.

As a packet travels through an internetwork to its final destination, the Layer 2 frame headers and trailers are removed and replaced at every Layer 3 device.

This is because layer 2 data units, frames, are for local addressing while layer 3 data units, packets, are for end-to-end addressing.

Routed Protocol

Routed Protocol

As a frame is received at a router interface, the destination MAC address is extracted.

The address is checked to see if the frame is directly addressed to the router interface, or if it is a broadcast.

In either of these two cases, the frame is accepted. Otherwise, the frame is discarded

The packet is then checked to see if it is actually destined for the router, or if it is to be routed to another device in the internetwork.

If the destination IP address matches one of the router ports, the Layer 3 header is removed and the data is passed up to the Layer 4.

Routed Protocol

If the packet is to be routed, the destination IP address will be compared to the routing table.

If a match is found or there is a default route, the packet will be sent to the interface specified in the matched routing table statement.

When the packet is switched to the outgoing interface, a new CRC value is added as a frame trailer, and the proper frame header is added to the packet.

The frame is then transmitted to the final destination

Routed Protocol

Two types of delivery services are connectionless and connection-oriented.

In a connectionless system, the destination is not contacted before a packet is sent.

Connectionless network processes are often referred to as packet switched processes

In connection-oriented systems, a connection is established between the sender and the recipient before any data is transferred.

Connection-oriented network processes are often referred to as circuit switched processes.

Routing Protocol Routing is an OSI Layer 3 function. Routing is a hierarchical organizational scheme that allows

individual addresses to be grouped together. Routing is the process of finding the most efficient path from

one device to another. The primary device that performs the routing process is the

router. Two key functions of a router:

Maintain routing tables and make sure other routers know of changes in the network topology.

The router switches the packets to the appropriate interface, adds the necessary framing information for the interface, and then transmits the frame.

Routing Protocol

Routing protocols use various combinations of metrics for determining the best path for data.

Routed protocols transport data across a network. Routing protocols allow routers to choose the best path for data from source to destination.

Routers use routing protocols to exchange routing tables and share routing information. It enable routers to route routed protocols.

Main functions: Provides processes for sharing route information Allows routers to communicate with other routers to

update and maintain the routing tables

Routing Protocol

Routing Protocol

Example, Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and Enhanced IGRP (EIGRP).

Path Determination enables a router to compare the destination address to

the available routes in its routing table, and to select the best path

decide which port an incoming packet should be sent out of to travel on to its destination.

It lets the router to decide which outbound port the packet should be sent.

Routing Protocol

Path Determination Process The destination address is obtained from the packet. The mask of the first entry in the routing table is

applied to the destination address. The masked destination and the routing table entry

are compared. If there is a match, the packet is forwarded to the

port that is associated with that table entry. If there is not a match, the next entry in the table is

checked.

Routing Protocol

If the packet does not match any entries in the table, the router checks to see if a default route has been set.

If a default route has been set, the packet is forwarded to the associated port. A default route is the route to use if there are no matches in the routing table.

If there is no default route, the packet is discarded. Usually a message is sent back to the sending device indicating that the destination was unreachable.

Routing Protocol

Routing Table Routing tables contain the information necessary to

forward data packets across connected networks. Protocol type

The type of routing protocol that created the routing table entry

Destination/next-hop associations Inform a router that a particular destination is either

directly connected to the router, or that it can be reached using another router called the “next-hop”

Routing Protocol

Routing metric Used to determine the desirability of a route. The

Routing Information Protocol (RIP) uses hop count as its only routing metric. Interior Gateway Routing Protocol (IGRP) uses a combination of bandwidth, load, delay, and reliability metrics

Outbound interfaces The interface that the data must be sent out on, in

order to reach the final destination.

Routing Protocol

Routing algorithms Different routing protocols use different algorithms to

decide which port an incoming packet should be sent to. Routing algorithms Design Goal

Optimization Describes the capability of the routing algorithm to

select the best route. The route will depend on the metrics and metric weightings used in the calculation.

Simplicity and low overhead The simpler the algorithm, the more efficiently it will

be processed by the CPU and memory in the router.

Routing Protocol

Robustness and stability A routing algorithm should perform correctly

Flexibility Should quickly adapt to a variety of network changes.

These changes include router availability, router memory, changes in bandwidth, and network delay.

Rapid convergence Convergence is the process of agreement by all

routers on available routes. When a network event causes changes in router availability, updates are needed to reestablish network connectivity.

Routing Protocol

Routing Metric Bandwidth

The data capacity of a link. Delay

The length of time required to move a packet along each link from source to destination.

Load The amount of activity on a network resource such as a

router or a link. Reliability

Usually a reference to the error rate of each network link.

Routing Protocol

Hop count The number of routers that a packet must travel

through before reaching its destination. Ticks

The delay on a data link using IBM PC clock ticks. One tick is approximately 1/18 second.

Cost An arbitrary value, usually based on bandwidth

Routing Protocol

Routing Protocol

An autonomous system is a network or set of networks under common administrative control, such as the cisco.com domain.

Two families of routing protocols are Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs).

EGPs route data between autonomous systems. An example of an EGP is Border Gateway Protocol

(BGP).

Routing Protocol

Routing Protocol

IGPs route data within an autonomous system. Routing Information Protocol (RIP) and (RIPv2) Interior Gateway Routing Protocol (IGRP) Enhanced Interior Gateway Routing Protocol (EIGRP) Open Shortest Path First (OSPF) Intermediate System-to-Intermediate System protocol

(IS-IS) IGPs can be further categorized as either distance-

vector or link-state protocols.

Routing Protocol

Routing Protocol

Distance Vector Determines the distance and direction, vector, to

any link in the internetwork. The distance may be the hop count to the link. Routers using distance-vector algorithms send all or

part of their routing table entries to adjacent routers on a periodic basis.

By receiving a routing update, a router can verify all the known routes and make changes to its routing table.

Also known as the Bellman-Ford algorithm Example, RIP, IGRP and EIGRP

Routing Protocol

Routing Protocol

Link State To overcome limitations of distance vector routing

protocols. Also known as Dijkstra's algorithm or as the shortest

path first (SPF) algorithm. protocols respond quickly to network changes sending

trigger updates only when a network change has occurred.

send periodic updates, known as link-state refreshes, at longer time intervals, such as every 30 minutes.

When a route or link changes, the device that detected the change creates a link-state advertisement (LSA) concerning that link.

Routing Protocol

The LSA is then transmitted to all neighboring devices. Each routing device takes a copy of the LSA, updates its link-state database, and forwards the LSA to all neighboring devices.

Topological database A collection of information gathered from LSAs

SPF algorithm A calculation performed on the database that results

in the SPF tree It computes network reachability.

Routing table A list of the known paths and interfaces

Routing Protocol

Three main concerns related to link-state protocols: Processor overhead Memory requirements Bandwidth consumption

Examples of link-state protocols include Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS).

Routing Protocol

Routing Protocol

RIP Distance vector routing protocol Uses hop count as its metric to determine the

direction and distance to any link in the internetwork. Cannot route a packet beyond 15 hops. Routing updates broadcast every 30 seconds

RIPV1 All devices in the network use the same subnet mask Classful routing

Routing Protocol

RIPV2 Provides prefix routing, and does send subnet mask

information in routing updates. Classless routing Different subnets within the same network can have different

subnet masks., variable-length subnet masking (VLSM). IGRP

Distance-vector routing protocol Based on delay, bandwidth, load, and reliability Classful routing Maximum hop 255 Routing updates broadcast every 90 seconds

Routing Protocol

OSPF Link-state routing protocol Address the needs of large, scalable internetworks Open standard routing protocol The SPF algorithm is used to calculate the lowest cost

to a destination. Routing updates are flooded as topology changes

occur. IS-IS

Link-state routing protocol Supports multiple routed protocols including IP

Routing Protocol

EIGRP Proprietary Cisco protocol Provides superior operating efficiency such as fast

convergence and low overhead bandwidth It uses load balancing. It uses a combination of distance vector and link-state

features. Hybrid routing protocol It uses Diffused Update Algorithm (DUAL) to

calculate the shortest path. Routing updates are multicast using 224.0.0.10

every 30 seconds or as triggered by topology changes.

Routing Protocol

BGP An example of an External Gateway Protocol (EGP) Exchanges routing information between autonomous

systems while guaranteeing loop-free path selection. BGP4 is the first version of BGP Supports classless interdomain routing (CIDR) and

route aggregation. Makes routing decisions based on network policies,

or rules using various BGP path attributes

Static Route

A network administrator configures information about remote networks manually.

Static routing is not as scalable as dynamic routing because of the extra administrative requirements.

Static route operations can be divided into these three parts: Network administrator configures the route Router installs the route in the routing table The static route is used to route packets.

Static Route

configure a static route enter global configuration mode Router(config)#ip route destination-network

subnet-mask outgoing-interface Router(config)#ip route destination-network

subnet-mask next-hop-ip-address Router(config)#copy running-config startup-config

save the active configuration to NVRAM. configure default route

Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]

Static Route

Router#show running-config Verify that the static route has been correctly entered.

Router#show ip route Verify that the route that was configured is in the

routing table. Use ping and tracert to troubleshoot the static route

configuration

Dynamic Routing

The routing protocol learns all available routes, places the best routes into the routing table, and removes routes when they are no longer valid.

The router uses the information in the routing table to forward routed protocol packets.

Whenever the topology of a network changes because of growth, reconfiguration, or failure, the network knowledgebase must also change.

The network knowledgebase needs to reflect an accurate view of the new topology.

Dynamic Routing

Configure Routing Protocol Router(config)#Router { rip | igrp | eigrp | ospf } option Router(config-router)# Network network-number

Network-number specifies the directly connected network

Dynamic Routing

Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

To reduce routing loops and counting to infinity, RIP uses the following techniques: Split horizon Poison reverse Holddown counters Triggered updates

Dynamic Routing

Example When Network 1 fails, Router E sends an update to

Router A. When Router A sends out its update, Routers B and D

stop routing to Network 1. However, Router C has not received an update. For Router C, Network 1 can still be reached through Router B.

Router C keep sending periodic update to Router D, which indicates a path to Network 1 by way of Router B. Router D changes its routing table to reflect this incorrect information, and sends it to Router A.

Dynamic Routing

Router A sends the information to Routers B and E, and the process continues. Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C.

Dynamic Routing

Split horizon can be used to avoid routing loops If a routing update about Network 1 arrives from Router

A, Router B or Router D cannot send information about Network 1 back to Router A.

Split horizon reduces incorrect routing information and routing overhead.

Dynamic Routing

Route poisoning is used by various distance vector protocols to overcome large routing loops and offer detailed information when a subnet or network is not accessible.

The hop count is usually set to one more than the maximum.

When Network 1 goes down, Router E will set a distance of 16 for Network 1 to poison the route.

This indicates that the network is unreachable. After Router B receives a route poisoning from Router E, it

sends an update, which is called a poison reverse, back to Router E. This makes sure all routers on the segment have received the poisoned route information.

Dynamic Routing

Triggered updates A triggered update is sent immediately in response to

some change in the routing table. The router that detects a topology change immediately

sends an update message to adjacent routers. Ensure that all routers know of failed routes before

any holddown timers can expire.

Dynamic Routing

Holddown timers When a router receives an update from a neighbor,

which indicates a network fail, the router marks the route as inaccessible and starts a holddown timer.

Before the holddown timer expires, if an update is received from the same neighbor, which indicates that the network is accessible, the router marks the network as accessible and removes the holddown timer.

if an update arrives from a different neighbor router with a better metric for the network, the router marks the network as accessible and removes the holddown timer.

Dynamic Routing

If an update is received from a different router with a higher metric before the holddown timer expires, the update is ignored

This update is ignored to allow more time for the knowledge of a disruptive change to propagate through the entire network.

RIP

RIP has evolved Classful Routing Protocol, RIP Version 1 (RIP v1) to a Classless Routing Protocol, RIP Version 2 (RIP v2).

RIP v2 enhancements include: Ability to carry additional packet routing information Authentication mechanism to secure table updates Support for variable-length subnet mask (VLSM)

Configuring RIP

RIP

Optional task: Apply offsets to routing metrics Adjust timers Specify a RIP version Enable RIP authentication Configure route summarization on an interface Verify IP route summarization Disable automatic route summarization Run IGRP and RIP concurrently Disable the validation of source IP addresses Enable or disable split horizon Connect RIP to a WAN

RIP

Router(config)#ip classless forward these packets to the best supernet route. Example, if an enterprise uses the entire subnet

10.10.0.0 /16, then a supernet route for 10.10.10.0 /24 would be 10.10.0.0 /16

Router(config-if)#ip split-horizon enable split horizon (default)

Router(config-router)#timers basic update invalid holddown flush [sleeptime] change holddown timer

RIP

Router(config-router)#update-timer seconds Change update interval

Router(config-router)#passive-interface Fa0/0 disable routing updates on specified interfaces

Router(config-router)#neighbor ip-address exchange routing information with neighboring router

Router(config-router)#version { 1|2 } receive and send Version 1 and 2 packets by default send version 1 and 2 packets receive

Version 1 packets

RIP

Router(config-if)#ip rip {receive | send} version { 1|2 } receive or send Version 1 or 2 packets

Router#show ip protocols RIP routing is configured. The correct interfaces send and receive RIP updates. The router advertises the correct networks.

Router#debug ip rip displays RIP routing updates as they are sent and

received. RIP is capable of load balancing over as many as six equal-

cost paths. The default is four paths. RIP performs what is referred to as "round robin" load balancing.

RIP

Router(config-router)#maximum-paths [number] change the maximum number of parallel paths

Router(config)#ip route destination-network subnet-mask next-hop Administrative-Distance Create a static floating route if AD is higher than the

normal RIP route floating route take the place of the RIP route in the

event that the RIP routing process fails.

IGRP Key design of IGRP

Versatility Automatically handle indefinite, complex topologies

Flexibility Segment with different bandwidth and delay

characteristics Scalability

Functioning in very large networks IGRP Metrics

Bandwidth (K1) Delay (K3) Load Reliability

IGRP IGRP Route

Interior Route Routes between subnets of a network attached to a

router interface System routes

Routes to networks within an autonomous system. . Do not include subnet information.

Exterior routes Routes to networks outside the autonomous system

that are considered when a gateway of last resort is identified.

IGRP IGRP maintains many timers such as update timer, an

invalid timer, a holddown timer, and a flush timer. Invalid timer

Specifies how long a router should wait in the absence of routing-update messages about a route before it declares that route invalid.

Flush timer Indicates how much time should pass before a route is

flushed from the routing table Configuring IGRP

Router(config)#router igrp as-number Router(config-router)#network network-number

IGRP