EIGRP

More scalable than RIPSophisticated metricMultiple-path supportIntroducing IGRP

BandwidthDelayReliabilityLoadingMTUIGRP Composite Metric

Maximum 6 paths (default = 4)Within metric varianceNext-hop router closer to destinationIGRP Unequal Multiple Paths

Configuring IGRPRouter(config-router)#network network-numberSelects participating attached networksRouter(config)#router igrp autonomous-systemDefines IGRP as the IP routing protocol

Configuring IGRP (cont.)Router(config-router)#traffic-share {balanced | min}Controls how load-balanced traffic is distributedRouter(config-router)#variance multiplierControls IGRP load balancing

IGRP Configuration Example

Verifying the IGRP Configuration

Displaying the IP Routing Table

debug ip igrp transaction Command

debug ip igrp events CommandRouterA#debug ip igrp eventsIGRP event debugging is onRouterA#00:23:44: IGRP: sending update to 255.255.255.255 via Ethernet0 (172.16.1.1)00:23:44: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes.00:23:44: IGRP: Total routes in update: 200:23:44: IGRP: sending update to 255.255.255.255 via Serial2 (10.1.1.1)00:23:45: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.00:23:45: IGRP: Total routes in update: 100:23:48: IGRP: received update from 10.1.1.2 on Serial200:23:48: IGRP: Update contains 1 interior, 1 system, and 0 exterior routes.00:23:48: IGRP: Total routes in update: 2

Updating Routing Information Example

Updating Routing Information Example (Cont.)

Updating Routing Information Example (Cont.)

Updating Routing Information Example (Cont.)

SummaryIGRP has several key features such as increased scalability, a sophisticated metric, and multiple paths. IGRP uses a composite routing metric that can include bandwidth, delay, reliability, loading, and MTU value. The IGRP composite routing metric supports multiple paths between source and destination. Use the router igrp and network commands to create an IGRP routing process. Use the variance and traffic-share commands to configure IGRP load balancing. Use the show ip protocols and show ip route commands to display information about your IGRP configuration. Use the debug ip igrp transaction command to display transaction information on IGRP routing transactions and the debug ip igrp events command to display a summary of the IGRP routing information.

An Overview of IGRPDis-advantages of RIPHop count metric and the 15-hop network sizeAdvantages of IGRP over RIPUnequal-cost load sharingAn update period three times longer than RIP'sA more efficient update packet formatDisadvantage of both IGRP and EIGRP is that they are proprietary to Cisco and therefore limited to Cisco productsIGRP might be slower to converge than RIP due to update timer is 90 secAdvantages of EIGRPCan route not only IP but also IPX and AppleTalkIGRP summarizes addresses at network boundariesSplit horizon with poisoned reverse, triggered updates, and holddown timers are used for stabilityClassful distance vector protocol that periodically broadcasts its entire routing table with the exception of routes suppressed by split horizon to all its neighbors

IGRP Timers and Stability FeaturesThe IGRP update period is 90 secondsIGRP uses less bandwidth for periodic updates when compared to RIPWhen a route is first learned, the invalid timer for that route is set for 270 secondsFlush timer is set for 630 secondCisco CommandTimers basic update invalid holddown flush [sleeptime]Holddown timer is 280 seconds If a destination becomes unreachable or if the next-hop router increases metric of destination enough to cause triggered updateIGRP MetricsMetric are bandwidth, delay, load, and reliabilityBandwidth is a static number used for metric calculation only and does not necessarily reflect the actual bandwidth of the linkBWIGRP = 107/1544 = 6476Delay is a three-octet number expressed in the same 10-microsecond units as specified by the delay commandDLYIGRP = DLY/10 = 50/10 = 5, or 0x000005

EIGRP

Process DomainsRouting Domains - a set of routers running one or more IGPs under a common administration and communicate via an EGP like BGP Process domain - is a group of routers sharing routing information by means of a single routing processIGRP classifies route entries into one of three categories: interior routes, system routes, and exterior routes

EIGRPAn interior route is a path to a subnet of the network address of the data link on which the update is being broadcast A system route is a path to a network address, which has been summarized by a network boundary router An exterior route is a path to a network that has been flagged as a default network LeHand advertise subnet 192.168.2.192/26 to Tully as internal route (Subnet of NetworkLeHand advertises Network 192.168.3.0 to Tully as a system route (Summary)LeHand advertises Network 192.168.1.0 as an external route (Network in another AS)

EIGRP Metrics - Bandwidth and DelayMetric=[k1*BWIGRP(min)+(k2*BWIGRP(min))/(256-LOAD)+k3*DLYIGRP(sum)] x[k5/(RELIABILITY+k4)],Metric = BWIGRP(min) + DLYIGRP(sum)metric weights tos k1 k2 k3 k4 k5 (Default Value k1=k3=1 and k2=k4=k5=0)

MediaBandwidthBWIGRPDelayDLYIGRP100M ATM100000K100100mS10Fast Ethernet100000K100100mS10FDDI100000K100100mS10HSSI45045K22220000mS200016M Token Ring16000K625630mS63Ethernet10000K10001000mS100T11544K647620000mS2000DS064K15625020000mS200056K56K17857120000mS2000Tunnel9K1111111500000mS50000

EIGRP MetricsMinimum bandwidth on the route from Casablanca to subnet 172.20.40.0/24 is 512K, at Quebec. BWIGRP(min) = 107/512 = 19531Delay of the route is (1000 + 20000 + 20000 + 5000) = 46000/10 Micro Sec

EIGRP MetricsBWIGRP(min) = 107/512 = 19531DLYIGRP(sum) = 46000/10 = 4600Metric = BWIGRP(min) + DLYIGRP(sum) = 19531 + 4600 = 24131

Metric for the route from Yalta to subnet 172.20.4.0/24 is different from the metric for the route from Casablanca to subnet 172.20.40.0/24.This is due to the differences in the configured bandwidth on the link between Yalta and Quebec and To the differences in the delay on the outgoing interfaces to the two destination subnets

EIGRPEIGRP is occasionally described as a distance vector protocol that acts like a link-state protocol, also called as Hybrid Routing ProtocolEIGRP updates are still vectorsthey are nonperiodic, partial, and boundedNonperiodic means that updates are not sent at regular intervalsonly when a metric or topology change occursPartial means that the updates will include only routes that have changed, not every entry in the route tableBounded means that the updates are sent only to affected routersEIGRP ClasslessEIGRP can route IP, IPX and AppleTalkEIGRP scales the metric components by 256Multiply the bandwidth and delay components by 256 for a metric of 256 x 24131 = 6177536Support for IPv4 and IPv6Communication via Reliable Transport Protocol (RTP)Best path selection via Diffusing Update Algorithm (DUAL)Support for summaries and discontiguous networksEfficient neighbor discoverySupport for VLSM/CIDRpackets can be authenticated using an MD5 cryptographic checksum

EIGRPReliable Transport Delivery:Reliable delivery means that delivery is guaranteed and that packets will be delivered in order.Guaranteed delivery is accomplished by means of a Cisco-proprietary algorithm known as reliable multicast, using the reserved class D address 224.0.0.10. Each neighbor receiving a reliably multicast packet unicasts an acknowledgment

EIGRP - RTPEIGRP Packet TypesHellos are used by the neighbor discovery and recovery process. Hello packets are multicast and use unreliable delivery. ip hello-interval eigrpAcknowledgments (ACKs) are Hello packets with no data in them. ACKs are always unicast and use unreliable delivery Updates convey route informationQueries and Replies are used by the DUAL finite state machine to manage its diffusing computations. Queries can be multicast or unicast, and replies are always unicast. Both queries and replies use reliable delivery.If any packet is reliably multicast and an ACK is not received from a neighbor, the packet will be retransmitted as a unicast to that unresponding neighbor. If an ACK is not received after 16 of these unicast retransmissions, the neighbor will be declared dead.The time to wait for an ACK before switching from multicast to unicast is specified by the multicast flow timer. The time between the subsequent unicasts is specified by retransmission timeout (RTO). Both the multicast flow timer and the RTO are calculated for each neighbor from the smooth round-trip time (SRTT). The SRTT is the average elapsed time, measured in milliseconds, between the transmission of a packet to the neighbor and the receipt of an acknowledgment

EIGPR - Neighbor Discovery/RecoveryHellos are unicast every 60 seconds or 5 SecOn most networks, Hellos are multicast every five secondsOn multipoint X.25, Frame Relay, and ATM interfaces, with access link speeds of T1 or slower, Hellos are unicast every 60 secThe hold time (3 Times) tells the router the maximum time it should wait to receive subsequent Hellos. If the hold timer expires before a Hello is received, the neighbor is declared unreachable. ip hold-time eigrpBefore EIGRP routers are willing to exchange routes with each other, they must become neighbors. Three Conditions must be met to form adjancencyHello or ACK receivedAS numbers matchIdentical metrics (K values)EIGRP routers that belong to different autonomous systems (ASes) dont automatically share routing information and dont become neighbors.When does EIGRP advertises its entire routing table??when it discovers a new neighbor and forms an adjacency with it through the exchange of Hello packets. Both neighbors advertise their entire routing tables to one another.

EIGRP TablesNeighbor table - Each router keeps state information about adjacent nei.IP address of the neighbor and the interface on which the neighbor's Hellos are receivedTopology Table Routes received from the neighbors are stored hereRouting Table contains best path to the destination network

EIGRP TermsFeasible distance (FD)This is the best metric along all paths to a remote network,Reported/advertised distanceThis is the metric of a remote network, as reported by a neighborFeasible successorA Backup route to the destination networkSuccessorbest route to a remote network.A successor route is used by EIGRP to forward traffic to a destination and is stored in the routing tableDiffusing Update Algorithm (DUAL) EIGRP uses DUAL for selecting and maintaining the best path to each remote network.Local ComputationDiffusion Computation

EIGRP - DUALUpon startup, a router uses Hellos to discover neighbors and to identify itself to neighbors. When a neighbor is discovered, EIGRP will attempt to form an adjacency with that neighbor. An adjacency is a logical association between two neighbors over which route information is exchanged. When adjacencies have been established, the router will receive updates from its neighbors. The updates will contain all routes known by the sending routers and the metrics of those routes. For each route, the router will calculate a distance based on the distance advertised by the neighbor and the cost of the link to that neighbor.The lowest calculated metric to each destination will become the feasible distance (FD) of that destinationWhen an EIGRP router is performing no diffusing computations, each route is in the passive state

EIGRP - DUALA router will reassess its list of feasible successors for a routeA change in the cost of a directly connected linkA change in the state (up or down) of a directly connected linkThe reception of an update packetThe reception of a query packetThe reception of a reply packetThe first step in its reassessment is a local computationIf the feasible successor with the lowest distance is different from the existing successor, the feasible successor will become the successor.If the new distance is lower than the FD, the FD will be updated.If the new distance is different from the existing distance, updates will be sent to all neighborsIf a feasible successor cannot be found in the topology table, the router will begin a diffusing computation and route will change to active stateA router begins a diffusing computation by sending queries to all of its neighborsFor each neighbor to which a query is sent, the router will set a reply status flag (r) to keep track of all outstanding queries. The diffusing computation is complete when the router has received a reply to every query sent to every neighborIf all expected replies are not received before the Active time expires, the route is declared stuck-in-active (SIA).

Case Study 1 - EIGRP ConfigurationStep 1: Enable EIGRP with the command router eigrp process-idStep 2: Specify each major network on which to run EIGRP with the network commandEarhartrouter eigrp 15 network 172.20.0.0Cochranrouter eigrp 15 network 172.20.0.0 network 192.167.17.0Lindberghrouter eigrp 15 network 172.20.0.0 network 192.167.16.0

Case Study 1 - EIGRP Configuration

Case Study 2: Unequal-Cost Load BalancingEIGRP will do equal-cost load balancing under the same CEF/fast/process switching16 parallel routes of equal costRoute from Earhart's S0.1 interface to network 192.168.17.0, the minimum bandwidth is 1544KDLYEIGRP(sum) (serial and Fast Ethernet):256 x ( 2000 + 10 ) = 514560BWEIGRP(min) is 256 x (107/1544) = 1657856Composite metric of the route is 514560 + 1657856 = 2172416Minimum bandwidth on route via Earhart's S0.3 to 192.168.17.0 is 256KDLYEIGRP(sum) is the same as on the first routeComposite metric for this route is 256 x (107/256) + 514560 = 10514432EIGRP will simply select the path with the lowest metric costEarhart is using Low Cost link via Serial 0.1, with a metric of 2172416 to 192.168.17.0

Case Study 2: Unequal-Cost Load BalancingAdditional configuration is needed to enable unequal-cost load balancingThe variance command is used to determine which routes are feasible for unequal-cost load sharingVariance defines a multiplier by which a metric may differ, from the metric of the lowest-cost routeAny route whose metric exceeds the metric of the lowest-cost route, multiplied by the variance, will not be considered a feasible routeThe default variance is one, meaning that metrics of multiple routes must be equal, to load balance. Variance must be specified in whole numbersmetric of Earhart's route through S0.3 is 10514432/2172416 = 4.8 times larger than the metric of the S0.1 route

Case Study 2: Unequal-Cost Load BalancingTo do unequal-cost load balancing over both links, the variance at Earhart should be fiveFor every five packets sent over the 1544K link (to next hop 172.20.15.6), one packet is sent over the 256K link (to next hop 172.20.15.10)Use PING to verify variance command

Load Sharing and Cisco Express ForwardingPer destination load sharing distributes load according to dest. AddressOn most platforms, CEF is default switching mode for IPv4, but not IPv6What is CEF?? Very efficient switching processCEF builds a forwarding information base (FIB) with information obtained from the route tableAll the destination networks entered in the route table are entered into the CEF FIB. If the route table is stable and not changing, the FIB will not changeCEF uses a separate table, the adjacency table, to maintain Layer 2 forwarding information for each entry in the FIBAdjacency table is created as Layer 2 information is learned, through IP ARP Both the FIB and adjacency table are created before packets need to be forwardedCEF performs per-destination load sharing by default (per source-destination pair)Per packet load sharing is another method available to CEF switched IPv4 packetsCEF is enabled globally on a router, use the commands show ip cefRouter(config)#ip cef Per packet load sharing is enabled using interface command ip load-sharing per-packetPer destination load sharing is enabled using ip load-sharing per-destinationCEF and fast switching can be turned off with no ip cef and no ip route-cache, and the router is performing unequal-cost, per packet load balancing

Per Destination Load Sharing and Fast SwitchingLoad sharing is per destination if the packet is fast switched or CEF switched using the default CEF configurationFast switching works as follows:When a router switches the first packet to a particular destination, a route table lookup is performed and an exit interface is selected.The necessary data-link information to frame the packet for the selected interface is then retrieved (from the ARP cache, for instance), and the packet is encapsulated and transmitted.The retrieved route and data-link information is then entered into a fast switching cache.As subsequent packets to the same destination enter the router, the information in the fast cache allows the router to immediately switch the packet without performing another route table and ARP cache lookup.While switching time and processor utilization are decreased, fast switching means that all packets to a specific destination, not source-destination pair, are routed out the same interface

Per Packet Load Sharing and Process SwitchingLoad sharing is per packet if process switching is used or if the CEF configuration was modifiedProcess switching simply means that for every packet, the router performs a route table lookup, selects an interface, and then looks up the data link information. Because each routing decision is independent for each packet, all packets to the same destination are not forced to use the same interface. To enable process switching on an interface, use the command no ip route-cache for IPv4The debug ip packet command displays only process switched packets

Case Study 3: Setting Maximum PathsThe maximum number of routes over which EIGRP can load balance is set with the maximum-paths paths commandpaths may be any number from 1 to 16Diagram below shows three parallel paths of varying costs from Earhart to address 172.18.0.0. The network administrator wants to load balance over a maximum of only two of these routes while ensuring that if either of these paths should fail, the third route will replace it

Case Study 3: Setting Maximum PathsThe metrics from Earhart to network 172.18.0.0 areVia S0.4: 256 x (9765 + (2000 + 10)) = 3014400Via S0.5: 256 x (19531 + (2000 + 10)) = 5514496Via S0.6: 256 x (78125 + (2000 + 10)) = 20514560The metric of the S0.6 route is 6.8 times as large as the lowest-cost metric, so the variance is sevenEarhart's configuration uses the variance command and the maximum-paths command to provide unequal-cost load sharing over a specified maximum number of pathsrouter eigrp 15 variance 7 network 172.20.0.0 maximum-paths 2The variance command ensures that any of the three routes to 172.18.0.0 is feasible (Interview Question)The maximum-paths command limits the load-sharing group to only the two best routes (Interview Question)

Case Study 3: Setting Maximum PathsRoute table for Earhart, before and after failure of one of three links, shows the results of using the variance and maximum-paths commands to configure load sharing to 172.18.0.0

Case Study 4: Multiple EIGRP ProcessesBleriot's configuration - Each EIGRP process will run only on the interfaces of the networks specifiedrouter eigrp 15network 172.20.0.0 router eigrp 10network 10.0.0.0

Case Study 4 Passive InterfacesUsing the passive-interface command prevents EIGRP Hellos from being sent on data links where they don't belong. Note that because Cochran's interfaces are in network 172.20.0.0, the passive-interface command is used to restrict unnecessary routing protocol trafficFor EIGRP, this command blocks unnecessary Hellos. No adjacencies will be formed; therefore, no other EIGRP traffic will be sent.Cochran's EIGRP 15 and EIGRP 10 configuration requires passive interfaces because the same major network number is used on both of Cochran's connected interfacesrouter eigrp 15 passive-interface Fastethernet0/1 network 172.20.0.0 router eigrp 10 passive-interface Serial0/0.1 passive-interface Serial 0/0.2 network 172.20.0.0

Case Study 5: Disabling Automatic SummarizationBy default, EIGRP summarizes at network boundaries as do the protocols covered in previous chapters.Unlike those protocols, however, EIGRP's automatic summarization can be disableddisable automatic summarization on Post using the command no auto-summaryDisable automatic summarization prevents ambiguous routing to network by avoiding discontiguous subnets

Case Study 6: Stub RoutingUnderstand how DUAL works If Earhart's link to Yeager, goes down, Earhart sends queries to all its neighborsSuppose If a problem develops on the link to Lindbergh before Earhart has received a response to the query it sent about Yeager's addresses, Yeager's addresses will remain Active on Earhart, even if the link between Earhart and Yeager comes back up.A router that has EIGRP Stub neighbors will not send queries to the stubsThereby eliminating the chance that a stub-configured remote site will cause stuck in active conditions, and routing instabilities in other parts of the network

Case Study 6: Stub RoutingHub and Spoke Designeigrp stub {connected | redistributed | static | summary | receive-only}Johnson's EIGRP stub router configurationrouter eigrp 15 eigrp stubThe command eigrp stub causes Johnson to send updates containing its connected and summary routes only (not any remote routes)No configuration changes are required on Earhart, the hub routerTo verify a neighbor is configured as a stub router, use the command show ip eigrp neighbor detail on the hub routerConfiguring stub routing with EIGRP greatly increases the scalability of an EIGRP network, by minimizing queries, and thus the amount of time that network outages require addresses to be in an active state

Case Study 6: Address SummarizationRouter Yeager, has a single link to Earhart. Six addresses that Earhart must advertise to Yeager can be summarized with two aggregate addressesip summary-address eigrp command will automatically suppress the advertisement of the more specific networksinterface Ethernet2 ip address 10.15.15.254 255.255.255.252 ip summary-address eigrp 15 172.0.0.0 255.0.0.0 ip summary-address eigrp 15 192.168.16.0 255.255.240.0

Case Study 7 - AuthenticationMD5 cryptographic checksums are only authentication supported in EIGRP and where as RIPv2 and OSPF, support both MD5 and clear-text passwordsThe steps for configuring EIGRP authentication areStep1. Define a key chain with a name.Step2. Define the key or keys on the key chain.Step3. Enable authentication on an interface and specify the key chain to be used.Step4. Optionally configure key management. (accept-lifetime and send-lifetime)key chain Edwards key 1 key-string PanchoBarnes interface Serial0/0.1 ip address 172.20.15.6 255.255.255.252 ip authentication key-chain eigrp 15 Edwards ip authentication mode eigrp 15 md5 interface Serial0/0.2 ip address 172.20.15.10 255.255.255.252 ip authentication key-chain eigrp 15 Edwards ip authentication mode eigrp 15 md5

Purpose: The figure introduces the IGRP routing protocol. IGRP is a sophisticated distance vector routing protocol.Emphasize: The Interior Gateway Routing Protocol (IGRP) is a dynamic distance-vector routing protocol designed by Cisco in the mid-1980s for routing in an autonomous system that contains large, arbitrarily complex networks with diverse bandwidth and delay characteristics. Historically, IGRP became one of the success factors for the early Cisco IOS software capabilities because of its superiority to RIP version 1.The important IGRP characteristics are as follows:More scalability than RIP Fast response to network changesSophisticated metricMultiple-path support

Purpose: This figure presents the IGRP metric with its five possible components. Emphasize : Bandwidth and delay are the two metrics that are most commonly used. They also comprise the default metric.Note: Changing IGRP metrics can have great impact on network performance.Describe the IGRP 24-bit metric field, as follows:BandwidthMinimum bandwidth on the route, in kilobits per second.DelayRoute delay, in tens of microseconds.ReliabilityLikelihood of successful packet transmission, expressed as an integer from 0 to 255.LoadingEffective bandwidth of path.MTUMinimum MTU in path, expressed in bytes.The following equation calculates the metric. It is presented for instructors and is not required to be taught:metric = [k1 x bandwidth + (k2 x bandwidth) / (256 - load) + k3 x delay]If k5 does not equal 0, an additional operation is done:metric = metric x (k5/(reliability + k4))The default constant values are k1 = k3 = 1 and k2 = k4 = k5 = 0. Again, if default values are set, metric = bandwidth + delay.The constants (k1, k2, k3) can be changed using the metric weights command. Changes to the IGRP constant values should be made with great care.Purpose: The figure presents how IGRP load sharing improves throughput and increases reliability.Emphasize: Only feasible paths can be used for IGRP load sharing.Load-balancing methods vary according to the switching mode because the data structures for process switching, fast switching, and autonomous switching are all different. When process switching, the processor load-balances packet by packet. When fast, autonomous, or silicon switching, load balancing is done destination by destination.By default, the amount of variance is set to one, which results in equal-cost load balancing.You can use the default-metric command to change the default metric.Transition: The following pages describe how to configure the IGRP routing protocol.

Slide 1 of 2Purpose: This figure explains how to use the router igrp and network commands to configure an IGRP process.Emphasize: Note that the AS keyword is required for IGRP.You can use multiple network commands to specify all networks that are to participate in the IGRP process. Only those networks specified will be published to other routers.

Slide 2 of 2Purpose: This figure displays the commands to allow load sharing and load balancing in an IGRP environment.Emphasize: Note the router configuration mode to the students.

Purpose: The figure shows how the IGRP commands operate on the example network.Emphasize: An administrator only specifies directly connected networks that should be published to other routers.Without the network command, nothing is advertised. With a network command, the router will advertise every subnet within the Class A, B, or C network specified in the configuration.Purpose: This figure shows how the show ip protocol command is used to monitor IGRP operation.Emphasize: The command displays the routing protocols that are active on the router for IP. It also gives network and timer information. In this example, IGRP is displayed.Point out the timing information.Point out the list of networks for which the router is injecting routes.Point out the administrative distance metric.

Purpose: This figure displays the show ip route command, which displays the contents of the routers IP routing table. Emphasize: This is the same command presented earlier in the chapter. Discuss the IP routing table in detail. The [100/90956] represent the administrative distance and metric, respectively.Discuss the following fields:IRefers to routes learned from IGRP.viaRefers to the router that informed us about this route.00:00:23 timer valueIGRP updates are every 90 seconds. Ask, How long until the next update?The interfaces used for the best path.Purpose: This figure presents the debug ip igrp transaction command.Emphasize: The debug ip igrp transaction command displays all updates that are sent from and received by your router.Purpose: This figure presents the debug ip igrp events command.Emphasize: When there are many networks in your routing table, displaying every update for every route can flood the console and make the router unusable.The debug ip igrp events command displays a summary of IGRP routing messages. This command indicates the source and destination of each update, as well as the number of routes in each update. Messages are not generated for each route.Slide 1 of 4Purpose: The next few pages illustrate IGRP update operation when network information changes. Emphasize: Network 172.16.0.0 fails. Router A sends a triggered update to router B. Slide 2 of 4Purpose: This figure continues to illustrate IGRP update operation when network information changes. Emphasize: Router B receives the triggered update from router A, sends a poison reverse to router A, and sends a triggered update to router C.Slide 3 of 4Purpose: This figure continues to illustrate IGRP update operation when network information changes. Emphasize: Router B places route to network 172.16.0.0 in hold-down state. The route is marked as possibly down while in holddown. Router B will still attempt to forward packets destined to 172.16.0.0.Slide 4 of 4Purpose: This figure continues to illustrate IGRP update operation when network information changes. Emphasize: If the link comes back up, router A will send another triggered update to router B. Router B will not update its routing table until the hold-down timer expires. Purpose: This slide discuss the initial configurations on the routers and switches. Note: There is no setup mode on the Catalyst 1900 switch.