ccna 1 module 10 routing fundamentals and subnets

CCNA 1 Module 10

Routing Fundamentals and Subnets

Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton

Today’s Topics

Routed Protocols

IP Routing Protocols

The Mechanics of Subnetting


Routable Protocols

What is a protocol

Routable protocol must have network addressing and host addressing

IPX uses network address + MAC address

IP uses subnet masks to indicate network and host parts of an address


IP Protocol

IP is a connectionless, unreliable, best-effort delivery protocol

Connectionless: no fixed route, routers forward traffic based on routing protocol rules

Unreliable/best effort: don’t mean IP isn’t very good, instead they mean that IP doesn’t check that data is delivered, that’s left to higher layers


Encapsulation/de-encapsulation


Connections?

Connectionless protocol – e.g. Postal Service

Connection-oriented – e.g. Telephone Call

Internet is large connectionless network

Routes vary due to variety of factors including congestion, route cost, available bandwidth


IP Packet Format


Routing

Routing is an OSI layer 3 (network layer) function

Routers maintain routing tables with information about other networks available on each interface

Routers make decisions about forwarding packets based on layer 3 addresses

IP most common routable protocol, also IPX/SPX and Appletalk but not NetBEUI


Comparison of Routers and Switches

Routers operate at layer 3, switches at layer 2

Compare to telephone exchange

Switch maintains switching table based on MAC addresses which are essentially random

Router maintains routing table based on IP addresses which are hierarchical


Routing vs Routed

Routed or Routable Protocols– Operate at Layer 3 (network layer)– Includes network layer address that allows a router to

forward towards destination– E.g. IP, IPX, AppleTalk and others

Routing Protocols– Communication between routers– Build up picture of network to allow routing decisions– E.g. RIP, OSPF, BGP and others


Path determination

Router builds a table of static and dynamic routes

Contains Network addresses and interface #

Router works down the table matching network portion of destination address with table (using netmask)

If a match, packet is forwarded through interface, if not the router moves on to the next entry.


Routing Tables

Routing protocols determine contents of routing table based on periodic communication with other routers

Table contains– Protocol type– Next hop (whether directly connected or not)– Routing Metric– Outbound Interface

Routing metrics are calculated in different ways by protocols


Routing Algorithms

Process varies greatly due to design goals– Optimisation - Capacity to select best route– Simplicity and Low Overhead - for scalability– Robustness and Stability - cope with load and failures– Flexibility - adapt quickly to network changes e.g. BW– Rapid Convergence - calculate best routes quickly

Depending on circumstances and priorities, users choose appropriate algorithm


Routing Metrics

Commonly used metrics include– Bandwidth– Delay– Load– Reliability (error rate)– Hop Count– Ticks– Cost (assigned)


IGP and EGP

Autonomous System (AS): network(s) under the common administrative control e.g. bolton.ac.uk

Interior Gateway Protocols (IGP) route within AS– RIP and RIP v2– IGRP and EIGRP– OSPF– IS-IS

Exterior Gateway Protocols (EGP) route between A– BGP


Distance Vector Protocols

Routers use distance and direction (vector) to decide routing

Routers exchange complete routing tables regularly to learn network structure

Metric can be hop count or a more complex calculation

Protocols: RIP, IGRP, EIGRP


Link State Protocols

Respond quickly to network changes

Generate a Link State Advertisement (LSA) and send to adjacent routers

LSAs propagated to other routers on the network

Algorithms include IS-IS and OSPF


RIP Protocol

Distance vector protocol

Uses hop count as metric

Max 15 hops

Fastest route may not be shortest

Requires common subnet mask


RIP v2 Protocol

Can support multiple subnets on a network

Known as classless routing

Using different subnet masks is known as Variable Length Subnet Masking (VLSM)


IGRP

Cisco proprietary protocol

Distance vector protocol

Uses performance metrics such as bandwidth and delay rather than hop count


OSPF

Open Shortest Path First

Industry standard protocol

Link state algorithm

Copes well with large networks


EIGRP

Cisco proprietary protocol

Advanced algorithm with low overheads compared to IGRP

Distance vector protocol with some link state features (hybrid protocol)


Border Gateway Protocol (BGP)

Exchanges information between AS

Principal protocol used between major Internet outfits and ISPs

Decisions based on policies rather than metrics

Mechanics of subnetting


Introduction

Extend the network + host address model to network + subnet + host

Some host bits used to indicate subnetwork instead

Advantages– Contain network broadcasts– Low level security– Traffic between subnets must pass through router


Establishing Subnet Masks

Need to know how max hosts on each subnet and map to nearest power of 2

Tells us how many host bits we need

Remained can be used for subnet addresses

n 2n - 2

8 254

7 126

6 62

5 30

4 14

3 6

2 2


Subnet Mask Calculation

Subnet mask uses binary 1s to represent network portion of address and 0s for host portion

11111111.11111111.11111111.11000000

Or 255.255.255.192

Often represented using slash notation /26 meaning 26 network bits e.g. 192.168.224.0/26


Building a subnet table

192.168.10.0/27 3 bits used for subnet, 5 for host


Network classesClass Network bits Host bits

A 8 24

B 16 16

C 24 8

Have scope for more subnets, or larger subnets with class A and class B addresses

On a class B network, if we need 2000 subnets containing 25 hosts what is the netmask?


Calculating subnets using AND

Address: 194.252.190.92/29

11000010.11111100.10111110.01011100

11111111.11111111.11111111.11111000

11000010.11111100.10111110.01011000

Subnetwork Address: 194.252.190.88

ccna 1 module 10 routing fundamentals and subnets

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