Page 1 Copyright 2004 Cisco Press & Priscilla Oppenheimer Top-Down Network Design, Ch. 5: Designing a Network Topology 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.
� 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.
Top-Down Network Design, Ch. 5: Designing a Network Topology
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
Top-Down Network Design, Ch. 5: Designing a Network Topology
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 fourthlayer. 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.
Top-Down Network Design, Ch. 5: Designing a Network Topology
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
Top-Down Network Design, Ch. 5: Designing a Network Topology
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.
Top-Down Network Design, Ch. 5: Designing a Network 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.
Top-Down Network Design, Ch. 5: Designing a Network Topology
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)
