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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