Routing and Routing Protocols

PJC CCNA Semester 2 Ver. 3.0byWilliam Kelly

Routing Basics Routing is the process of forwarding packets

from a source to a destination based on the destination IP address of the packet

Routing occurs at layer 3 of the OSI model (network layer)

Routes can be entered by the administrator (static) or learned from other routers (dynamic)

The network layer provides a “best-effort” delivery system

The router uses a routing table to send packets from a source to a destination

How Routers Route Packets Consistent end-to-end Network

addressing is the key to routing Broadcasts are minimized and

bandwidth is conserved

The 2 Functions of a Router Best Path Determination – The router

determines the best path to a destination by examining the Network portion of an address and consulting it’s routing tables (a map of the network contained in each router)

Switching – A router accepts a packet on one interface and forwards it to another interface

Routed vs. Routing Protocols Routed protocols are network

protocols that provide enough information for a packet to be forwarded from one host to another

Routing protocols allow routers to communicate information about network paths and to maintain tables

Routed vs. Routing Protocols Routed protocols are network

protocols that provide enough information for a packet to be forwarded from one host to another

Routed protocols allow routers to communicate information about network paths and to maintain tables

Common Routed and Routing Protocols

Routed Protocols IP IPX Appletalk

Routing Protocols RIP IGRP OSPF EIGRP BGP

Static Routing Routing instructions manually entered

by an administrator Routes must be changed manually

when the network topology changes Static routing is labor intensive Static routes are used to enhance

security and to set up routes accessible by only one path

Network Layer Protocols

Application

Presentation

Session

Transport

Network

Data Link

Physical1

2

3

4

5

6

7

Network

Data Link

Physical1

2

3 Network

Data Link

Physical1

2

3

Application

Presentation

Session

Transport

Network

Data Link

Physical1

2

3

4

5

6

7

Host X Host Y

RouterA

RouterB

Configuring a Static RouteSyntax for entering a static route using the

interface:Router(config)# ip route 192.168.2.0 255.255.255.0 S1

Syntax for entering a static route using the next hop address:Router(config)# ip route 192.168.2.0 255.255.255.0 192.168.1.2

Syntax for entering a default route (sometimes called a quad route):Router(config)# ip route 0.0.0.0 0.0.0.0 S1

Syntax for entering a quad route with an administrative distance

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.2 4

Syntax for entering a static route using the interface:

Router(config)# ip route 192.168.2.0 255.255.255.0 192.168.1.2

Examining Static Route SyntaxSyntax for entering a static route using the interface:

Router(config)# ip route 192.168.2.0 255.255.255.0 S1

destination network address

next hop interface

next hop addressdestination network address

Common Administrative Distances

Application of Administrative distances in Static Routes

When using a next hop address the default administrative distance is 1

When using an outgoing interface the default administrative distance is 0

Dynamic Routing Dynamic routing allows routers to

automatically share routing (topology) information with other routers

Routing Protocols RIP – distance vector protocol, metric is hop

count, max hop count is 15, updates every 30 seconds

IGRP – distance vector protocol, metric is bandwidth, delay, load, and reliability, highly scalable for large networks, updates every 90 seconds

OSPF – link state protocol, uses triggered updates, VLSM compliant

EIGRP – Cisco proprietary hybrid protocol, VLSM compliant

BGP – exterior routing protocol that was designed to work between ISP’s

Commonly Used Metrics Bandwidth – data capacity of a link Delay – length of time for a packet to

travel through links Load – amount of network activity Reliability – error rate Hop Count – number of routers a

packet must traverse Ticks – each tick is 1/18 of a second Cost

Link State Basics Link State Protocols use the Link State

Algorithm (also called the Dijkstras algorithm)1. Link State Advertisements (LSA’s) are

transferred between routers to provide topology information

2. Each router maintains a topology database of LSA information

3. The Dijkstras algorithm computes the shortest paths to different networks

4. The routing table is then populated with the best paths

Link State Concerns Processor Overhead Memory requirements Bandwidth consumption – The initial

discovery process is bandwidth intensive; thereafter, minimal bandwidth is required

Hybrid Routing Protocols Similar to distance vector protocols in

that metrics are used but the metrics are more accurate.

Similar to Link State protocols in that triggered updates are used instead of period updates

Examples are IS-IS and EIGRP

Configuring a Router to Use Routing ProtocolsStarting routing

Router(config)# router protocol {options}ex: Router(config)# router rip

Entering Networks

Router(config)# network network-number ex: Router(config)# network 192.168.1.0

Note: A separate network line must be used to enter every network whose interface will participate in the sending and receiving of routing updates

Autonomous Systems Interior Gateway Protocols (IGP) were

designed to be used in a network designed to be administered by a single organization

Exterior Gateway Protocols (EGP) were designed to be used between networks that are controlled by two organizations. EGP’s such as BGP are used between ISP’s

Every autonomous system is assigned a 16 bit identifying number by the American Registry of Internet Numbers (ARIN)