ITEC 275 Computer Networks – Switching, Routing, and WANs

ITEC 275 Computer Networks – Switching, Routing, and

WANs

Week 5Robert D’Andrea

Some slides provide by Priscilla Oppenheimer and used with permission

Agenda• Learning Activities

– Network Design Document, logical design, and top-down network design methodology.

– Hierarchical Network Design, network topology consisting of many interrelated components. This task might be easier to divide and conquer the problem and develop it.

– Spanning Tree Protocol, fast convergence network routers.– VLANs, small bandwidths to switches rather than

broadcasting.– Redundancy, provides availability, performance, and

scalability.– VPNs, use a third party communication media securring data.

Documenting Your Design• If you are given a request for proposal (RFP),

respond to the request in the exact format that the RFP specifies

• If no RFP, you should still write a design document– Describe your customer’s requirements and how your

design meets those requirements– Document the budget for the project– Explain plans for implementing the design

Typical RFP Response Topics• A network topology for the new design• Information on the protocols, technologies, and

products that form the design• An implementation plan• A training plan• Support and service information and plan• Prices and payment options• Qualifications of the responding vendor or supplier• Recommendations from other customers • Legal contractual terms and conditions

Contents of a Network Design Document

• Executive summary• Project goal• Project scope• Design requirements• Current state of the network• New logical and physical design• Results of network design testing• Implementation plan• Project budget

Design Requirements

• Business goals explain the role the network design will play in helping an organization succeed

• Technical goals include scalability, performance, security, manageability, usability, adaptability, and affordability

Logical and Physical Design• Logical design

– Topology– Models for addressing and naming– Switching and routing protocols– Security strategies– Network management strategies

• Physical design– Actual technologies and devices

Implementation Plan• Recommendations for deploying the network design• Project schedule

– Including any dates and times for service provider installations

• Any plans for outsourcing• Training• Risks• A fallback plan if the implementation should fail• A plan for evolving the design as new requirements

arise

Possible Appendixes

• Detailed topology maps• Device configurations• Addressing and naming details• Network design testing results• Contact information• Pricing and payment options• More information about the company that is

presenting the design– Annual reports, product catalogs, press releases

• Legal contractual terms and conditions

Topology• A branch of mathematics concerned with those

properties of geometric configurations that are unaltered by elastic deformations such as stretching or twisting

• A term used in the computer networking field to describe the structure of a network

What is a Topology?

Definition of TopologyA topology is a map of an internetwork

that indicates network, segments, interconnection points, and user communities. The purpose of the map is to show the geometry of the network, not the physical geography or technical implementation.

External Network Topology

Internal Network Topology

Detail Description of External Network Topology

What is Convergence?

Definition of ConvergenceThe speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an un-internetwork after a change in the topology.

Convergence is Voice, Data, and Video

Scope of Convergence

Network Topology Design Themes

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

Why Use a Hierarchical Model?• 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

Hierarchical Network DesignEnterprise WAN

BackboneCampus A Campus B

Campus C

Building C-1 Building C-2

Campus C Backbone

Core Layer

Distribution Layer

Access Layer

Cisco’s Hierarchical Design Model• A core layer of high-end routers and switches that

are optimized for availability and speed. Avoid connecting packet filters or network monitors at this layer.

• A distribution layer of routers and switches that implement policies and segment traffic. This is a demarcation point between access and core layer of the network.

Cisco’s Hierarchical Design Model• An access layer that connects users via hubs,

switches, routers, and other devices. Switches are usually implemented at the access layer in campus networks to divide up bandwidth domains to meet the demands of applications that need a lot of bandwidth or cannot handle the delay associated with sharing a bandwidth.

A network design guideline would be to design the access layer first, then the distribution, and core layer.

Cisco’s Hierarchical Design Model• Controlling a Network Diameter

Provides low and predictable latency.

Predict routing paths

Traffic flows

Capacity requirements

Flat Versus Hierarchy

Flat Loop Topology

Headquarters in Medford

Grants Pass Branch Office

Ashland Branch Office

Klamath Falls Branch Office

Headquarters in Medford

Ashland Branch Office

Klamath Falls Branch Office

Grants Pass Branch Office

White City Branch Office

Hierarchical Redundant Topology

Flat Network Topology

Mesh Versus Hierarchical-Mesh Topologies• Mesh Topologies

Full-mesh topology provides complete redundancy and good performance. There is only a single link delay between two sites. Costly to implement a full-mesh topology.

Partial-mesh topology has fewer connections between sites. To reach another switch or router, traffic flow would experience more traversing of intermediate links.

Mesh Designs

Partial-Mesh Topology

Full-Mesh Topology

A Partial-Mesh Hierarchical DesignHeadquarters (Core Layer)

Branch Offices (Access Layer)

Regional Offices

(Distribution Layer)

Company Structure• Small and Medium-Sized Companies

Recommend a hierarchical model that reflects a hub-and-spoke topology. Usually, corporate headquarters or a data center form the center hub. Links extended from the hub connect to remote offices and telecommuters’ locations.

See slide Hub-and-Spoke Hierarchical Topology

A Hub-and-Spoke Hierarchical TopologyCorporate

Headquarters

Branch Office Branch OfficeHome Office

Scope of Access

• Control Access Layer DiameterThe most likely place for network design violations to occur are at the access layer. Users and network administrators are more likely to add networks to the internetwork , and connect remote networks together. This is known as adding a chain.Avoid backdoors. A backdoor connection is a connection between devices in the same layer. A hub is considered a backdoor.

Avoid Chains and Backdoors

Core Layer

Distribution Layer

Access Layer

ChainBackdoor

How Do You Know When You Have a Good Design?

• When you already know how to add a new building, floor, WAN link, remote site, e-commerce service, and so on

• When new additions cause only local change, to the directly-connected devices

• When your network can double or triple in size without major design changes

• When troubleshooting is easy because there are no complex protocol interactions to wrap your brain around

Flat Network Use• A flat network topology is adequate for small

networks. Each network device functions the same, and the network is not divided into layers or modules. A flat network is easy to design.

Flat network designers are most difficult when there is network growth, and the lack of hierarchy makes trouble shooting more difficult.

Flat WAN Networks• Flat WAN Topologies

A WAN for a small company consists of a few sites connected in a loop. Each site has it’s own WAN router, routing protocols can converge quickly, and communication with any other site can recover when a link fails. Caveat: If only one link fails, recovery is possible. If two or more links fail, recovery is more difficult.

The flat loop topology goals are low cost and reasonably good availability. See slide -Flat verses Hierarchical.

Flat LAN Networks• Flat LAN Topologies

In the 1990s, a typical LAN configuration was to connect PCs and servers to one or more hubs. The PCs and servers implemented a media-access control process like token passing or carrier sense multiple access with collision detection (CSMA/CD) to control access to a shared bandwidth. This configuration had the potential to negatively affect delay and throughput for other devices. Today, designers recommend connecting PCs and servers to the data link layer (Layer 2) switches .

Layer 2 Configuration• Characterizing Layer 2 Network Traffic

Devices connected in a switched or bridged network are all in the same broadcast domain. Switches forward broadcasting frames out from every port. Routers on the other hand, separate segments into separate broadcast domains. The recommended limit for devices connected to one single broadcast domain is a couple hundred devices. Broadcasted traffic needs to be limited and watched closely on flat loop topologies, otherwise frames can be dropped or lost. Rule of Thumb – limit broadcast traffic to 20% of the traffic on each link.

Cisco’s SAFE Security Reference Architecture

Campus Topology Design• Use a hierarchical, modular approach• Minimize the size of bandwidth domains• Minimize the size of broadcast domains• Provide redundancy

– Backup paths– Mirrored servers– Mirror stored data– Multiple ways for workstations to reach a router

for off-net communications

Campus Topology Design

• Cisco SAFE Security Reference Architecture- Used to simplify the complexity of a large internetwork- SAFE is concerned with security- Defense-in-depth approach were multiple layers

of protection are strategically located through-out the network.

- See page 134 for major design modulesSee Cisco SAFE high-level view slide

A Simple Campus Redundant DesignHost A

Host B

LAN X

LAN Y

Switch 1 Switch 2

Bridges and Switches use Spanning-Tree Protocol (STP) to Avoid Loops

X

Host A

Host B

LAN X

LAN Y

Switch 1 Switch 2

Bridges (Switches) Running STP• Participate with other bridges in the election of a single

bridge as the Root Bridge.• Calculate the distance of the shortest path to the Root

Bridge and choose a port (known as the Root Port) that provides the shortest path to the Root Bridge.

• For each LAN segment, elect a Designated Bridge and a Designated Port on that bridge. The Designated Port is a port on the LAN segment that is closest to the Root Bridge. (All ports on the Root Bridge are Designated Ports.)

• Select bridge ports to be included in the spanning tree. The ports selected are the Root Ports and Designated Ports. These ports forward traffic. Other ports block traffic.

Elect a Root

Bridge B Bridge C

Bridge A ID = 80.00.00.00.0C.AA.AA.AA

Bridge B ID = 80.00.00.00.0C.BB.BB.BB

Bridge C ID = 80.00.00.00.0C.CC.CC.CC

Port 1

Port 2

Port 1

Port 2

Port 1 Port 2

LAN Segment 2100-Mbps Ethernet

Cost = 19


Cost = 19


Cost = 19

RootBridge A

Lowest Bridge IDWins!

Determine Root Ports

Bridge B Bridge C

RootBridge A




Port 1

Port 2

Port 1

Port 2

Port 1 Port 2


Cost = 19


Cost = 19


Cost = 19

Root Port Root Port

Lowest CostWins!

Determine Designated Ports

Bridge B Bridge C

RootBridge A




Port 1

Port 2

Port 1

Port 2

Port 1 Port 2


Cost = 19


Cost = 19


Cost = 19

Root Port Root Port

Designated Port Designated Port

Designated Port Lowest Bridge IDWins!

Bridge B Bridge C

RootBridge A




Port 1

Port 2

Port 1

Port 2

Port 1 Port 2


Cost = 19


Cost = 19


Cost = 19

Root Port Root Port


Designated Port Blocked Port

X

Prune Topology into a Tree!

React to Changes

Bridge B Bridge C

RootBridge A




Port 1

Port 2

Port 1

Port 2

Port 1 Port 2

LAN Segment 2LAN Segment 1

LAN Segment 3

Root Port Root Port


Designated Port Becomes Disabled

Blocked Port Transitions to Forwarding State

Scaling the Spanning Tree Protocol

• Keep the switched network small– It shouldn’t span more than seven switches

• Use BPDU skew detection on Cisco switches• Use IEEE 802.1w

– Provides rapid reconfiguration of the spanning tree

– Also known as RSTP

Rapid Spanning Tree Protocol• Bridge port states

- Discarding a port that is neither learning MAC addresses nor forwarding user’s frames.

- Learning a port the is learning MAC addresses to populate the MAC address table but is not yet forwarding user frames

- Forwarding a port the is learning MAC addresses and forwarding user frames.

Rapid Spanning Tree Protocol• Converged switched network Bridge port roles

- Root port assigned on a non-root bridge, provides lowest cost path to the root bridge.

- Designated assigned on a port attached to a LAN, provides lowest cost path to the root

bridge.- Alternate assigned to a port that offers an

alternative path in the direction of the root bridge to that provided by the bridge’s root port. Considered a discarded port

Rapid Spanning Tree Protocol- Backup assigned to a port on a

designated bridge that acts as a backup for the path provided by a designated port in the direction of the leaves of the spanning tree.

- Disabled assigned to a port that is not operational or is excluded from the

active topology by network management. Considered a discarded port.

Rapid Spanning Tree ProtocolRSTP converges quicker than STP (50

seconds) to a tree topology where the lowest-cost paths are forwarding frames. RSTP archives rapid transition to the forwarding state on edge ports, root ports, and point-to-point links. Edge and root ports can transition to forwarding without transmitting or receiving messages from other bridges.

Rapid Spanning Tree Protocol• Port Modes

Full-duplex mode port assumed to be point-to-point. Modern switched networks utilize this mode mostly.

Half-duplex mode port considered a shared port by default.

Rapid Spanning Tree Protocol• Root Bridge

High speed ReliableCentered in network topologyA switch with the lowest bridge ID

Priority fieldMAC address the lowest MAC address of a switch or bridge

Virtual LANs (VLANs)

• An emulation of a standard LAN that allows data transfer to take place without the traditional physical restraints placed on a network

• A set of devices that belong to an administrative group

• Designers use VLANs to constrain broadcast traffic

VLANs versus Real LANs

Switch A

Station A1 Station A2 Station A3

Network A

Switch B

Station B1 Station B2 Station B3

Network B

A Switch with VLANsStation A1 Station A2 Station A3

VLAN A


VLAN B

VLANs Span Switches

Switch A


Switch B


Station A1 Station A2 Station A3 Station A4 Station A5 Station A6

VLAN B

VLAN A

VLAN B

VLAN A

WLANs and VLANs• A wireless LAN (WLAN) is often implemented as a

VLAN• Facilitates roaming• Users remain in the same VLAN and IP subnet as

they roam, so there’s no need to change addressing information

• Also makes it easier to set up filters (access control lists) to protect the wired network from wireless users

Workstation-to-Router Communication

• 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

HSRPHot Standby Router Protocol

Active Router

Standby Router

Virtual Router

Workstation

Enterprise Internetwork

Week FiveDefinition of MultihomingMulti-homing is to provide more that one connection for a system to access and offer network services. In an enterprise network, multi-homing provides access to more than one entry into the Internet.Example: WAN backup and ISP redundancyIf a server has more than one network layer address

Multi-homing the Internet Connection

Enterprise

Enterprise

Enterprise

ISP 1

ISP 1 ISP 2

ISP 1

ISP 1 ISP 2

EnterpriseOption A

Option B

Option C

Option D

Paris NY

Paris NY

Security Topologies

EnterpriseNetwork

DMZ

Web, File, DNS, Mail Servers

Internet

Security Topologies

Internet

Enterprise NetworkDMZ

Web, File, DNS, Mail Servers

Firewall

Network SecurityDefinition of FirewallA firewall is a system or combination of systems that enforces a boundary between two or more networks.

Router with ACLFirewall should be placed within the

network topology so that all traffic from outside the protected network must pass through the firewall.

NAT (Network Address Translation)

Summary• When a customer provides an RFP, make sure to follow

the prescribed format• When not bound by an RFP, develop a design

document that describes requirements, the existing network, the logical and physical design, an implementation plan, and the budget

• Be sure to include an executive summary• In some cases, you should also include appendixes

with detailed information

Summary

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

design• Topology design should feature hierarchy,

redundancy, modularity, and security

Review Questions• Why is it important to document your

network design?• Why is it important to submit an RFP

proposal in the exact format prescribed?• What are the major topics in a design

document?• What are some possible appendixes for a

design document?

Review Questions

• Why are hierarchy and modularity important for network designs?

• What are the three layers of Cisco’s hierarchical network design?

• What are the major components of Cisco’s enterprise composite network model?

• What are the advantages and disadvantages of the various options for multihoming an Internet connection?

This Week’s Outcomes

• Network Design Document• Hierarchical Network Design• Spanning Tree Protocol• VLANs• Redundancy• VPNs

Due this week

• 4-2-1 – Simulator Tutorial and Basic IOS Command Exploration

Next week

• Read chapter 6 in Top-Down Network Design• Read chapter 6 in Designing Cisco

Internetwork Solutions• 5-1 – Concept questions 4• 1-5-1 – Network Design Project 1

– Switches

Q & A

• Questions, comments, concerns?

ITEC 275 Computer Networks – Switching, Routing, and WANs

Documents

hierarchical network

network design methodology

design documentdescribe

rfp specifiesif

proposal rfp

service information

customers requirements

payment optionsqualifications