Lectures Top-Down Network Design - Chapter 7: Selecting Switching and Routing Protocols

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1. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Top-DNtkDiDown Network Design Chapter Seven Selecting Switching and Routing Protocols Copyright 2010 Cisco Press & Priscilla Oppenheimer Switching and Routing Choices • Switching – Layer 2 transparent bridging (switching) – Multilayer switching – Spanning Tree Protocol enhancements – VLAN technologies • Routing – Static or dynamic – Distance-vector and link-state protocols – Interior and exterior –Etc. Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 1
2. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Selection Criteria for Switching and Routing Protocols • Network traffic characteristics • Bandwidth, memory, and CPU usage • The number of peers supported • The capability to adapt to changes quickly • Support for authentication Making Decisions • Goals must be established • Many options should be explored • The consequences of the decision should be investigated • Contingency plans should be made • A decision table can be used Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 2
3. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Example Decision Table Transparent Bridging (Switching) Tasks • Forward frames transppyarently • Learn which port to use for each MAC address • Flood frames when the destination unicast address hasn’t been learned yet • Filter frames from going out ports that don’ t include the destination address • Flood broadcasts and multicasts Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 3
4. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Switching Table on a Bridge or Switch MAC Address Port 08-00-07-06-41-B9 1 00-00-0C-60-7C-01 2 00-80-24-07-8C-02 3 Cisco Spanning Tree Protocol Enhancements • PortFast • UplinkFast and Backbone Fast • Unidirectional link detection • Loop Guard Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 4
5. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Redundant Uplinks Core Layer X Distribution Layer Switch B Switch C Primary Secondary Uplink X Uplink Access Layer Switch A X = blocked by STP • If a link fails, how long will STP take to recover? • Use UplinkFast to speed convergence Protocols for Transporting VLAN Information • Inter-Switch Link (ISL) – Tagging protocol – Cisco proprietary • IEEE 802.1Q – Tagging protocol – IEEE standard • VLAN Trunk Protocol (VTP) – VLAN management protocol Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 5
6. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Selecting Routing Protocols • They all have the same general goal: – To share network reachability information among routers • They differ in many ways: – Interior versus exterior – Metrics supported – Dynamic versus static and default – Distance-vector versus link-sate – Classful versus classless – Scalability Interior Versus Exterior Routing Protocols • Interior routing protocols are used within an autonomous system • Exterior routing protocols are used between autonomous systems Autonomous syy(stem (two definitions that are often used) : “A set of routers that presents a common routing policy to the internetwork” “A network or set of networks that are under the administrative control of a single entity” Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 6
7. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Routing Protocol Metrics • Metric: the determining factor used by a routinggg algorithm to decide which route to a network is better than another • Examples of metrics: – Bandwidth - capacity – Delay - time – Load - amount of network traffic – Reliability - error rate – HtHop count - numbftthtkttber of routers that a packet must travel through before reaching the destination network – Cost - arbitrary value defined by the protocol or administrator Routing Algorithms • Static routing – Calculated beforehand, offline • Default routing – “If I don’t recognize the destination, just send the packet to Router X” • Cisco’s On-Demand Routing – Routing for stub networks – Uses Cisco Discovery Protocol (CDP) • Dynamic routing protocol – Distance-vector algorithms – Link-state algorithms Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 7
8. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Static Routing Example 172.16.20.1172.16.20.2 172.16.40.1 172.16.40.2 Router A Router B Router C s0 s0 s0 s1 e0 e0 e0 172.16.10.1 172.16.30.1 172.16.50.1 Host AHost B Host C 172.16.10.2 172.16.30.2 172.16.50.2 RouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.20.2 Send packets for subnet 50 to 172.16.20.2 (Router B) Default Routing Example 172.16.20.1172.16.20.2 172.16.40.1 172.16.40.2 Router A Router B Router C s0 s0 s0 s1 e0 e0 e0 172.16.10.1 172.16.30.1 172.16.50.1 Host AHost B Host C 172.16.10.2 172.16.30.2 172.16.50.2 RouterA(config)#ip route 0.0.0.0 0.0.0.0 172.16.20.2 If it’s not local, send it to 172.16.20.2 (Router B) Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 8
9. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Distance-Vector Routing • Router maintains a routing table that lists known networks, direction (vector) to each network, and the distance to each network • Router periodically (every 30 seconds, for example) transmits the routing table via a broadcast packet that reaches all other routers on the local segments • Router updates the routing table, if necessary, based on received broadcasts Distance-Vector Routing Tables Router A Router B 172.16.0.0 192.168.2.0 Router A’s Routing Table Router B’s Routing Table Network Distance Send To Network Distance Send To 172.16.0.0 0 Port 1 192.168.2.0 0 Port 1 192.168.2.0 1 Router B 172.16.0.0 1 Router A Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 9
10. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Link-State Routing • Routers send updates only when there’s a change • Router that detects change creates a link- state advertisement (LSA) and sends it to neighbors • Neighbors propagate the change to their neighbors • Routers update their topological database if necessary Distance-Vector Vs. Link-State • Distance-vector algorithms keep a list of networks, with next hop and distance (metric) information • Link-state algorithms keep a database of routers and links between them – Link-state algorithms think of the internetwork as a graph instead of a list – When changes occur, link-state algorithms apply Dijkstra’s shortest-path algorithm to find the shortest path between any two nodes Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 10
11. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Choosing Between Distance- Vector and Link-State Choose Distance-Vector Choose Link-State • Simple, flat topology • Hierarchical topology • Hub-and-spoke topology • More senior network • Junior network administrators administrators • Fast convergence is critical • Convergence time not a big concern Dynamic IP Routing Protocols Distance-Vector Link-State • Routing Information • Open Shortest Path First Protocol (RIP) Version 1 (OSPF) and 2 • Intermediate System-to- • Interior Gateway Routing Intermediate System (IS-IS) Protocol (IGRP) • Enhanced IGRP • Border Gateway Protocol (BGP) Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 11
12. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Routing Information Protocol (RIP) • First standard routing protocol developed for TCP/IP environments – RIP Version 1 is documented in RFC 1058 (1988) – RIP Version 2 is documented in RFC 2453 (1998) • Easy to configure and troubleshoot • Broadcasts its routing table every 30 seconds; 25 routes per packet • Uses a single routing metric (hop count) to measure the distance to a destination network; max hop count is 15 RIP V2 Features • Includes the subnet mask with route updates – Supports prefix routing (classless routing, supernetting) – Supports variable-length subnet masking (VLSM) • Includes simple authentication to foil crackers sending routing updates Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 12
13. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols IGRP Solved Problems with RIP • 15-hop limitation in RIP – IGRP supports 255 hops • Reliance on just one metric (hop count) – IGRP uses bandwidth, delay, reliability, load – (By default just uses bandwidth and delay) • RIP's30 s 30-second update timer – IGRP uses 90 seconds EIGRP • Adjusts to changes in internetwork very quickly • Incremental updates contain only changes, not full routing table • Updates are delivered reliably • Router keeps track of neighbors’ routing tables and uses them as feasible successor • Same metric as IGRP, but more granularity (32 bits instead of 24 bits) Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 13
14. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Open Shortest Path First (OSPF) • Open standard, defined in RFC 2328 • Adjusts to changes quickly • Supports very large internetworks • Does not use a lot of bandwidth • Authenticates protocol exchanges to meet security goals OSPF Metric • A single dimensionless value called cost. A network administrator assigns an OSPF cost to each router interface on the path to a network. The lower the cost, the more likely the interface is to be used to forward data traffic. • On a Cisco router, the cost of an interface defaults to 100,000,000 divided by the bandwidth for the interface. For example, a 100-Mbps Ethernet interface has a cost of 1. Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 14
15. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols OSPF Areas Connected via Area Border Routers (ABRs) Area 0 (Backbone) ABRABR ABR Area 1Area 2 Area 3 IS-IS • Intermediate System-to-Intermediate StSystem • Link-state routing protocol • Designed by the ISO for the OSI protocols • Integrated IS-IS handles IP also Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 15
16. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Border Gateway Protocol (BGP) • Allows routers in different autonomous systems to exchange routing information – Exterior routing protocol – Used on the Internet among large ISPs and major companies • Supports route aggregation • Main metric is the length of the list of autonomous system numbers, but BGP also supports routing based on policies Summary • The selection of switching and routing protocols should be based on an analysis of – Goals – Scalability and performance characteristics of the protocols • Transparent bridging is used on modern switches – But other choices involve enhancements to STP and protocols for transporting VLAN information • There are many types of routing protocols and many choices within each type Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 16
17. Top-Down Network Design, Ch. 7: Selecting Switching and Routing Protocols Review Questions • What are some options for enhancing the Spanning Tree Protocol? • What factors will help you decide whether distance-vector or link-state routing is best for your design customer? • What factors will help you select a specific routing protocol? • Why do static and default routing still play a role in many modern network designs? Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 17