Top-Down Network DesignChapter SevenSelecting Switching and Routing ProtocolsCopyright 2010 Cisco Press & Priscilla Oppenheimer

Switching and Routing ChoicesSwitchingLayer 2 transparent bridging (switching)Multilayer switchingSpanning Tree Protocol enhancementsVLAN technologiesRouting Static or dynamicDistance-vector and link-state protocolsInterior and exteriorEtc.

Selection Criteria for Switching and Routing ProtocolsNetwork traffic characteristicsBandwidth, memory, and CPU usageThe number of peers supportedThe capability to adapt to changes quickly Support for authentication

Making DecisionsGoals must be establishedMany options should be exploredThe consequences of the decision should be investigatedContingency plans should be madeA decision table can be used

Example Decision Table

Transparent Bridging (Switching) TasksForward frames transparentlyLearn which port to use for each MAC addressFlood frames when the destination unicast address hasnt been learned yetFilter frames from going out ports that dont include the destination addressFlood broadcasts and multicasts

Switching Table on a Bridge or Switch

MAC AddressPort12308-00-07-06-41-B900-00-0C-60-7C-0100-80-24-07-8C-02

Cisco Spanning Tree Protocol EnhancementsPortFastUplinkFast and Backbone FastUnidirectional link detectionLoop Guard

Redundant UplinksAccess LayerDistribution LayerCoreLayerSwitch ASwitch BSwitch CPrimary UplinkSecondary UplinkXXX = blocked by STPIf a link fails, how long will STP take to recover?Use UplinkFast to speed convergence

Protocols for Transporting VLAN InformationInter-Switch Link (ISL)Tagging protocolCisco proprietaryIEEE 802.1QTagging protocolIEEE standardVLAN Trunk Protocol (VTP)VLAN management protocol

Selecting Routing ProtocolsThey all have the same general goal:To share network reachability information among routersThey differ in many ways:Interior versus exteriorMetrics supportedDynamic versus static and defaultDistance-vector versus link-sateClassful versus classlessScalability

Interior Versus Exterior Routing ProtocolsInterior routing protocols are used within an autonomous systemExterior routing protocols are used between autonomous systems

Autonomous system (two definitions that are often used): A set of routers that presents a common routing policy to the internetworkA network or set of networks that are under the administrative control of a single entity

Routing Protocol MetricsMetric: the determining factor used by a routing algorithm to decide which route to a network is better than anotherExamples of metrics:Bandwidth - capacityDelay - timeLoad - amount of network traffic Reliability - error rateHop count - number of routers that a packet must travel through before reaching the destination networkCost - arbitrary value defined by the protocol or administrator

Routing AlgorithmsStatic routingCalculated beforehand, offlineDefault routingIf I dont recognize the destination, just send the packet to Router XCiscos On-Demand RoutingRouting for stub networksUses Cisco Discovery Protocol (CDP)Dynamic routing protocolDistance-vector algorithmsLink-state algorithms

Static Routing ExampleRouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.20.2Send packets for subnet 50 to 172.16.20.2 (Router B)e0e0e0s0s1s0s0Router ARouter BRouter CHost AHost CHost B172.16.10.2172.16.30.2172.16.50.2172.16.20.1172.16.40.1172.16.10.1172.16.30.1172.16.50.1172.16.20.2172.16.40.2

Default Routing ExampleRouterA(config)#ip route 0.0.0.0 0.0.0.0 172.16.20.2If its not local, send it to 172.16.20.2 (Router B)e0e0e0s0s1s0s0Router ARouter BRouter CHost AHost CHost B172.16.10.2172.16.30.2172.16.50.2172.16.20.1172.16.40.1172.16.10.1172.16.30.1172.16.50.1172.16.20.2172.16.40.2

Distance-Vector RoutingRouter maintains a routing table that lists known networks, direction (vector) to each network, and the distance to each networkRouter periodically (every 30 seconds, for example) transmits the routing table via a broadcast packet that reaches all other routers on the local segmentsRouter updates the routing table, if necessary, based on received broadcasts

Distance-Vector Routing TablesRouter ARouter B172.16.0.0192.168.2.0NetworkDistanceSend To

172.16.0.0 0Port 1192.168.2.0 1Router B

NetworkDistanceSend To

192.168.2.0 0Port 1 172.16.0.0 1Router A

Router As Routing TableRouter Bs Routing Table

Link-State RoutingRouters send updates only when theres a changeRouter that detects change creates a link-state advertisement (LSA) and sends it to neighborsNeighbors propagate the change to their neighborsRouters update their topological database if necessary

Distance-Vector Vs. Link-StateDistance-vector algorithms keep a list of networks, with next hop and distance (metric) informationLink-state algorithms keep a database of routers and links between themLink-state algorithms think of the internetwork as a graph instead of a listWhen changes occur, link-state algorithms apply Dijkstras shortest-path algorithm to find the shortest path between any two nodes

Choosing Between Distance-Vector and Link-StateChoose Distance-VectorSimple, flat topologyHub-and-spoke topologyJunior network administratorsConvergence time not a big concern

Choose Link-StateHierarchical topologyMore senior network administratorsFast convergence is critical

Dynamic IP Routing ProtocolsDistance-VectorRouting Information Protocol (RIP) Version 1 and 2Interior Gateway Routing Protocol (IGRP)Enhanced IGRPBorder Gateway Protocol (BGP)

Link-StateOpen Shortest Path First (OSPF)Intermediate System-to-Intermediate System (IS-IS)

Routing Information Protocol (RIP)First standard routing protocol developed for TCP/IP environmentsRIP Version 1 is documented in RFC 1058 (1988)RIP Version 2 is documented in RFC 2453 (1998)Easy to configure and troubleshootBroadcasts its routing table every 30 seconds; 25 routes per packetUses a single routing metric (hop count) to measure the distance to a destination network; max hop count is 15

RIP V2 FeaturesIncludes the subnet mask with route updatesSupports prefix routing (classless routing, supernetting)Supports variable-length subnet masking (VLSM)Includes simple authentication to foil crackers sending routing updates

IGRP Solved Problems with RIP15-hop limitation in RIPIGRP supports 255 hopsReliance on just one metric (hop count) IGRP uses bandwidth, delay, reliability, load(By default just uses bandwidth and delay)RIP's 30-second update timer IGRP uses 90 seconds

EIGRPAdjusts to changes in internetwork very quicklyIncremental updates contain only changes, not full routing table Updates are delivered reliablyRouter keeps track of neighbors routing tables and uses them as feasible successorSame metric as IGRP, but more granularity (32 bits instead of 24 bits)

Open Shortest Path First (OSPF)Open standard, defined in RFC 2328Adjusts to changes quicklySupports very large internetworksDoes not use a lot of bandwidthAuthenticates protocol exchanges to meet security goals

OSPF MetricA 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.

OSPF Areas Connected via Area Border Routers (ABRs)Area 1Area 3Area 2Area 0 (Backbone)ABRABRABR

IS-ISIntermediate System-to-Intermediate SystemLink-state routing protocolDesigned by the ISO for the OSI protocolsIntegrated IS-IS handles IP also

Border Gateway Protocol (BGP)Allows routers in different autonomous systems to exchange routing informationExterior routing protocolUsed on the Internet among large ISPs and major companiesSupports route aggregationMain metric is the length of the list of autonomous system numbers, but BGP also supports routing based on policies

SummaryThe selection of switching and routing protocols should be based on an analysis ofGoalsScalability and performance characteristics of the protocolsTransparent bridging is used on modern switchesBut other choices involve enhancements to STP and protocols for transporting VLAN informationThere are many types of routing protocols and many choices within each type

Review QuestionsWhat 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?