Top-Down Network Design
Chapter Five
Designing a Network Topology
Copyright 2010 Cisco Press & Priscilla Oppenheimer
Topology
A map of an internetwork that indicates network segments, interconnection points, and user communities.
A term used in the computer networking field to describe the structure of a network
During the topology design phase, you
identify networks and interconnection points, the size and scope of networks, and the types of internetworking devices that will be
required, but not the actual devices.
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
Hierarchical Network Design
Cisco’s Hierarchical Design Model
A core layer of high-end routers and switches 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 via hubs, switches, and other devices
◦ WAN: the access layer consists of the routers at the edge of the campus networks.
◦ Campus network: the access layer provides switches for end-user access
Flat Versus Hierarchy
A flat WAN for a small company can consist of a few
sites connected in a loop. Each site has a WAN
router that connects to two other adjacent sites via
point-to-point links
Mesh Designs
Mesh topology helps meet availability requirements
Partial-mesh network has fewer connections. Reach another router or switch might require traversing intermediate links
Full-mesh topology:
every router or switch is connected to every other router or switch.
A Partial-Mesh Hierarchical Design
A Hub-and-Spoke Hierarchical Topology
A topology that consists of one central network and a set of remote networks each with one connection to the central network and no direct connections to each other.
Traffic between remote networks goes through the hub network.
Avoid Chains and Backdoors
Connect the branch network to another branch, adding a fourth layer . This is a common network design mistake that is known as adding a chain.
A backdoor is a connection between devices in the same layer .
they cause unexpected routing and switching problems and
make network documentation and troubleshooting more
difficult.
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
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 router
Cisco’s Enterprise Composite Network Model
To scale the hierarchical model, Cisco developed the ECNM, which reduces the enterprise network into further physical, logical, and functional
boundaries. Hierarchy is embedded as required into each module.
Enterprise Campus Modules
Server farm
Network management module
Edge distribution module for connectivity to the rest of the world
Campus infrastructure module:
◦ Building access submodule
◦ Building distribution submodule
◦ Campus backbone
Redundant Network Design Topologies
Lets you meet network availability by
duplicating network links and interconnectivity devices.
Eliminates the possibility of having a single point of failure
Can be implemented in both campus and enterprise
◦
Campus goals for users accessing local services
◦
Enterprise goals for overall availability and performance
◦
Analyze business and technical goals of customer
Backup Paths
Consists of routers and switches and
individual backup links between routers and switches that duplicate devices and links on the primary path
Consider 2 aspects of backup path
◦
How much capacity does it support
◦
How quickly will the network begin using it
Common to have less capacity than a primary path
◦
Different technologies
◦
Expensive
Backup Paths (Cont)
Manual versus automatic
◦ Manual reconfigure users will notice disruption and for mission critical systems not acceptable
◦ Use redundant, partial-mesh network designs to speed automatic recovery time
They must be tested
Sometimes used for load balancing as well as
Load Balancing
Primary goal of redundancy is to meet availability
Secondary goal is to improve performance by load balancing across parallel links
Must be planned and in some cases configured
In ISDN environments can facilitate by configuring channel aggregation
◦
Channel aggregation means that a router can automatically bring up multiple ISDN B channel as bandwidth
requirements increase
Designing a Campus Network Design Topology
Should meet a customer’s goals for availability and performance by:
◦ featuring small broadcast domains,
◦ redundant distribution-layer segments,
◦ mirrored servers,
◦ and multiple ways for a workstation to reach a router for off-net communications
Designed using a hierarchical model for good
performance, maintainability and scalability.
Virtual LANs (VLANs)
An emulation of a standard LAN that allows data transfer to take place without the
traditional physical limits 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
A Switch with VLANs
VLANs Span Switches
The VLAN tag contains a VLAN ID that specifies to which
VLAN the frame belongs
Trunk
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
Campus Hierarchical Redundancy Topology
Workstation-to-Router Communication
Proxy ARP: router running proxy ARP can respond to the ARP
request with the router's data link layer address.
Listen for route advertisements: each router periodically
multicasts an ICMP router advertisement packet from each of its interfaces, announcing the IP address of that interface.
Workstations discover the addresses of their local routers simply by listening for advertisements
ICMP router solicitations: a workstation can multicast an
ICMP router solicitation packet to ask for immediate advertisements, rather than wait for the next periodic advertisement to arrive.
Default gateway provided by DHCP
◦
Use Hot Standby Router Protocol (HSRP) for redundancy
Hot Standby Router Protocol (HSRP)
HSRP works by creating a virtual router, also called a phantom router.
The virtual router has its own IP and MAC addresses. Each workstation is configured to use the virtual router as its default gateway. When a workstation broadcasts an ARP frame to find its default gateway, the active HSRP router responds with the virtual router's MAC address. If the active router goes offline, a standby router takes over as active router, continuing the delivery of the workstation's packets.
HSRP provides a way
for an IP workstation
to keep communicating
on an internetwork
even if its default
gateway becomes
unavailable.
Designing the
Enterprise Edge Topology
Redundant WAN Segments
Because Wan links can be critical, redundant (backup) WAN links are often included in the enterprise topology
Full-mesh topology provides complete redundancy
Full mesh is costly to implement, maintain,
Multihoming the Internet Connection
The generic meaning of multihomingis to "provide more than one connection for a system to access and offer network services."
Multihoming the Internet Connection
Virtual Private Networking
Enable a customer to use a public network to provide a secure connection among sites on the organization’s internetwork
Can also be used to connect an enterprise intranet to an extranet to reach outside parties
Gives the ability to connect geographically-dispersed offices via a service provider
Company data can be encrypted for routing
Firewalls and TCP/IP tunneling allow a customer to use a public network as a backbone for the
enterprise network
Meeting Security Goals with Firewall Topologies - DMZ
For the need to publish public data and protect private data,
the firewall topology can include a public LAN that hosts
Web, FTP, DNS, and SMTP servers. The public LAN referred
as the free-trade zone. Another term is demilitarized zone
(DMZ)
Security Topologies - Three-part firewall
An alternative topology is to use two routers as the firewalls
and place the DMZ between them.
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 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?
