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ITEC 275 Computer Networks – Switching, Routing, and WANs

ITEC 275 Computer Networks – Switching, Routing, and WANs. Week 5 Robert 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.

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ITEC 275 Computer Networks – Switching, Routing, and WANs

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  1. ITEC 275 Computer Networks – Switching, Routing, and WANs Week 5 Robert D’Andrea Some slides provide by Priscilla Oppenheimer and used with permission

  2. 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.

  3. Introduction Definition • Topology A 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.

  4. Introduction Definition • Convergence The speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an unternetwork after a change in the topology.

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Network Topology Design Themes • Hierarchy • Redundancy • Modularity • Well-defined entries and exits • Protected perimeters

  14. 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

  15. Hierarchical Network Design Enterprise WAN Backbone Core Layer Campus A Campus B Campus C Distribution Layer Campus C Backbone Access Layer Building C-1 Building C-2

  16. 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.

  17. 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.

  18. Cisco’s Hierarchical Design Model • Controlling a Network Diameter Provides low and predictable latency. Predict routing paths Traffic flows Capacity requirements

  19. Headquarters in Medford Headquarters in Medford Grants Pass Branch Office Klamath Falls Branch Office Ashland Branch Office Grants Pass Branch Office Klamath Falls Branch Office Ashland Branch Office White City Branch Office Flat Versus Hierarchy Flat Loop Topology Hierarchical Redundant Topology

  20. 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.

  21. Mesh Designs Full-Mesh Topology Partial-Mesh Topology

  22. A Partial-Mesh Hierarchical Design Headquarters (Core Layer) Regional Offices (Distribution Layer) Branch Offices (Access Layer)

  23. Week Five • 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

  24. A Hub-and-Spoke Hierarchical Topology Corporate Headquarters Branch Office Home Office Branch Office

  25. Week Five • Control Access Layer Diameter The 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.

  26. Avoid Chains and Backdoors Core Layer Distribution Layer Access Layer Backdoor Chain

  27. 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

  28. Week Five • 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.

  29. Week Five • 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.

  30. Week Five • 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 .

  31. Week Five • 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.

  32. Cisco’s SAFE Security Reference Architecture

  33. 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

  34. Campus Topology Design • Cisco SAFE Security Reference Archecture - 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 modules See Cisco SAFE high-level view slide

  35. A Simple Campus Redundant Design Host A LAN X Switch 1 Switch 2 LAN Y Host B

  36. Bridges and Switches use Spanning-Tree Protocol (STP) to Avoid Loops Host A LAN X X Switch 1 Switch 2 LAN Y Host B

  37. 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.

  38. Elect a Root Lowest Bridge ID Wins! Bridge A ID = 80.00.00.00.0C.AA.AA.AA Root Bridge A Port 1 Port 2 LAN Segment 1 100-Mbps Ethernet Cost = 19 LAN Segment 2 100-Mbps Ethernet Cost = 19 Port 1 Port 1 Bridge B Bridge C Port 2 Port 2 Bridge B ID = 80.00.00.00.0C.BB.BB.BB Bridge C ID = 80.00.00.00.0C.CC.CC.CC LAN Segment 3 100-Mbps Ethernet Cost = 19

  39. Determine Root Ports Bridge A ID = 80.00.00.00.0C.AA.AA.AA Root Bridge A Lowest Cost Wins! Port 1 Port 2 LAN Segment 1 100-Mbps Ethernet Cost = 19 LAN Segment 2 100-Mbps Ethernet Cost = 19 Root Port Root Port Port 1 Port 1 Bridge B Bridge C Port 2 Port 2 Bridge B ID = 80.00.00.00.0C.BB.BB.BB Bridge C ID = 80.00.00.00.0C.CC.CC.CC LAN Segment 3 100-Mbps Ethernet Cost = 19

  40. Determine Designated Ports Bridge A ID = 80.00.00.00.0C.AA.AA.AA Root Bridge A Designated Port Designated Port Port 1 Port 2 LAN Segment 1 100-Mbps Ethernet Cost = 19 LAN Segment 2 100-Mbps Ethernet Cost = 19 Root Port Root Port Port 1 Port 1 Bridge B Bridge C Port 2 Port 2 Bridge B ID = 80.00.00.00.0C.BB.BB.BB Bridge C ID = 80.00.00.00.0C.CC.CC.CC Lowest Bridge ID Wins! LAN Segment 3 100-Mbps Ethernet Cost = 19 Designated Port

  41. Prune Topology into a Tree! Bridge A ID = 80.00.00.00.0C.AA.AA.AA Root Bridge A Designated Port Designated Port Port 1 Port 2 LAN Segment 1 100-Mbps Ethernet Cost = 19 LAN Segment 2 100-Mbps Ethernet Cost = 19 Root Port Root Port Port 1 Port 1 Bridge B Bridge C Port 2 Port 2 X Bridge B ID = 80.00.00.00.0C.BB.BB.BB Bridge C ID = 80.00.00.00.0C.CC.CC.CC LAN Segment 3 100-Mbps Ethernet Cost = 19 Designated Port Blocked Port

  42. React to Changes Bridge A ID = 80.00.00.00.0C.AA.AA.AA Root Bridge A Designated Port Designated Port Port 1 Port 2 LAN Segment 1 LAN Segment 2 Root Port Root Port Port 1 Port 1 Bridge B Bridge C Port 2 Port 2 Bridge B ID = 80.00.00.00.0C.BB.BB.BB Bridge C ID = 80.00.00.00.0C.CC.CC.CC LAN Segment 3 Designated Port Becomes Disabled Blocked Port Transitions to Forwarding State

  43. 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

  44. 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.

  45. 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

  46. 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.

  47. Rapid Spanning Tree Protocol RSTP 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.

  48. 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.

  49. Rapid Spanning Tree Protocol • Root Bridge High speed Reliable Centered in network topology A switch with the lowest bridge ID Priority field MAC address the lowest MAC address of a switch or bridge

  50. 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

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