Designing a Network Topology Mod

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Top-Down Network Design

Chapter Five

Designing a Network Topology

Copyright 2010 Cisco Press & Priscilla OppenheimerModified by Stylianos Drakatos for TCN 6210


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Network Topology Design

Themes Hierarchy Redundancy Modularity Well-defined entries and exits Protected perimeters


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Summary

Use a systematic, top-down approach Plan the logical design before the physical

design Topology design should feature hierarchy,

redundancy, modularity, and security


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Why Use a Hierarchical Model?Just think of the telephone number (xxx)xxx-xxx

Reduces workload on network devices Avoids devices having to communicate with too

many other devices (reduces CPU

adjacencies) Constrains broadcast domains Enhances simplicity and understanding

Facilitates changes Facilitates scaling to a larger size


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Hierarchical Network Design

Enterprise WANBackbone

Campus A Campus B

Campus C

Building C-1 Building C-2

Campus C Backbone

Core Layer

DistributionLayer

Access Layer


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Ciscos Hierarchical DesignModel

A core layer of high-end routers andswitches that are optimized for availability

and speed A distribution layer of routers and switches

that implement policies and segment traffic

An access layer that connects users viahubs, switches, and other devices


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Flat Versus Hierarchy

Flat Loop Topology

Headquarters inMedford

Grants PassBranch Office

AshlandBranchOffice

Klamath FallsBranch Office

Headquarters in

Medford

AshlandBranchOffice

Klamath FallsBranch Office

Grants PassBranchOffice

White CityBranch Office

Hierarchical Redundant Topology


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Mesh

Designs

Partial-Mesh Topology

Full-Mesh Topology


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A Partial-Mesh Hierarchical Design

Headquarters

(Core Layer)

Branch Offices (Access

Layer)

RegionalOffices

(DistributionLayer)


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A Hub-and-Spoke Hierarchical Topology

CorporateHeadquarters

Branch Office Branch OfficeHome Office


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How Do You Know When YouHave a Good Design?

When you already know how to add a newbuilding, floor, WAN link, remote site, e-commerceservice, and so on

When new additions cause only local change, to thedirectly-connected devices

When your network can double or triple in sizewithout major design changes

When troubleshooting is easy because there are nocomplex protocol interactions to wrap your brainaround


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Campus Topology Design

Use a hierarchical, modular approach Minimize the size of bandwidth domains Minimize the size of broadcast domains Provide redundancy

Mirrored servers

Multiple ways for workstations to reach a routerfor off-net communications


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Virtual LANs (VLANs)

An emulation of a standard LAN thatallows data transfer to take place withoutthe traditional physical restraints placed ona network

A set of devices that belong to anadministrative group

Designers use VLANs to constrainbroadcast traffic


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VLANs versus Real LANs

Switc

h A

Station

A1

Station

A2

Station

A3

Networ

k A

Switc

h B

Station

B1

Station

B2

Station

B3

Networ

k B


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A Switch with VLANs

Station

A1

Station

A2

Station

A3

VLA

N A

Station

B1

Station

B2

Station

B3

VLA

N B


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VLANs Span Switches

Switc

h A

Station

B1

Station

B2

Station

B3

Switc

h B

Station

B4

Station

B5

Station

B6

Station

A1

Station

A2

Station

A3

Station

A4

Station

A5

Station

A6

VLA

N B

VLA

N A

VLA

N B

VLA

N A


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Workstation-to-RouterCommunication

Proxy ARP (not a good idea) Listen for route advertisements (not a great

idea either) ICMP router solicitations (not widely used) Default gateway provided by DHCP (better

idea but no redundancy) Use Hot Standby Router Protocol (HSRP) for

redundancy


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HSRP

Active Router

Standby Router

Virtual Router

Workstation

EnterpriseInternetwork


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Multihoming the Internet Connection

Enterprise

Enterprise

Enterprise

ISP 1

ISP 1 ISP 2

ISP 1

ISP 1 ISP 2

Enterprise

Option A

Option B

Option C

Option D

Paris NY

Paris NY


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Security Topologies

Enterprise

Network

DMZ

Web, File, DNS, Mail Servers

Internet


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IPsec

A firewall protecting an internal network


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Virtual Private Networks

Topology as seen from the inside


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Security Topologies

Internet

EnterpriseNetwork

DMZ

Web, File, DNS, Mail Servers

Firewall


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The Network Layer Principles (1)

1. Make sure it works

2. Keep it simple

3. Make clear choices4. Exploit modularity

5. Expect heterogeneity

. . .


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The Network Layer Principles (2)

. . .

6. Avoid static options and parameters

7. Look for good design (not perfect)8. Strict sending, tolerant receiving

9. Think about scalability

10. Consider performance and cost


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The Network Layer in the Internet(1)

The IP Version 4 Protocol IP Addresses IP Version 6 Internet Control Protocols Label Switching and MPLS OSPFAn Interior Gateway Routing Protocol BGPThe Exterior Gateway Routing Protocol Internet Multicasting Mobile IP


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The Network Layer in the Internet (2)

The Internet is an interconnected collection of many networks.


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The IP Version 4 Protocol (1)

The IPv4 (Internet Protocol) header.


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IP Addresses (1)

An IP prefix.


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IP Addresses (3)

A set of IP address assignments


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IP Addresses (6)

IP address formats


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IP Addresses (8)

Placement and operation of a NAT box.


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IP Version 6 Goals

Support billions of hosts Reduce routing table size Simplify protocol Better security Attention to type of service Aid multicasting

Roaming host without changing address Allow future protocol evolution Permit coexistence of old, new protocols. . .


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IP Version 6 (1)

The IPv6 fixed header (required).


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IP Version 6 (2)

IPv6 extension headers


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Internet Control Protocols (1)

The principal ICMP message types.


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Internet Control Protocols (2)

Two switched Ethernet LANs joined by a router


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Label Switching and MPLS (1)

Transmitting a TCP segment using IP, MPLS,

and PPP.


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Label Switching and MPLS (2)

Forwarding an IP packet through an MPLS network

OSPF An Interior Gateway Routing Protocol


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OSPFAn Interior Gateway Routing Protocol

An autonomous system


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OSPFAn Interior Gateway Routing Protocol (2)

The relation between ASes, backbones, and

areas in OSPF.


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OSPFAn Interior Gateway Routing Protocol (3)

The five types of OSPF messages


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BGPThe Exterior GatewayRouting Protocol (1)

Examples of routing constraints:

1.

No commercial traffic for educat. network2. Never put Iraq on route starting at Pentagon

3. Choose cheaper network

4.

Choose better performing network5. Dont go from Apple to Google to Apple

BGP The Exterior Gateway


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BGPThe Exterior GatewayRouting Protocol (2)

Routing policies between four Autonomous

Systems

BGP Th E t i G t R ti P t l


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BGPThe Exterior Gateway Routing Protocol(3)

Propagation of BGP route advertisements

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Designing a Network Topology Mod