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Question 31
Which EIGRP timer determines how long a router waits before declaring a neighbor down?
A) Hello timer
B) Hold timer
C) Active timer
D) Retransmission timer
Answer: B
Explanation:
The hold timer determines how long a router waits before declaring a neighbor down in EIGRP. This timer specifies the maximum time interval that can elapse without receiving a Hello packet from a neighbor before the router considers that neighbor unreachable. When the hold timer expires, the router tears down the neighbor relationship, removes all routes learned from that neighbor, and recalculates its topology. The hold timer provides fault detection capability essential for network convergence during failures.
The default hold timer value is three times the Hello interval. On high-bandwidth interfaces like Ethernet where the Hello interval is 5 seconds, the hold timer defaults to 15 seconds. On low-bandwidth multipoint interfaces with a 60-second Hello interval, the hold timer is 180 seconds. This relationship between Hello and hold timers provides reasonable tolerance for occasional packet loss while still detecting genuine failures relatively quickly.
The hold timer is reset to its maximum value each time a Hello packet is received from the neighbor. This mechanism ensures that as long as Hello packets continue arriving, the neighbor relationship remains active. The timer continuously counts down from its maximum value, and only if it reaches zero without receiving a Hello packet does the router declare the neighbor down. Network administrators can observe the current hold timer countdown using the show ip eigrp neighbors command.
The Hello timer is distinct from the hold timer. The Hello timer controls how frequently the local router sends Hello packets to discover and maintain neighbor relationships. It does not determine when neighbors are declared down. However, the Hello and hold timers must be coordinated appropriately to ensure that under normal conditions, Hello packets arrive well before hold timers expire.
The active timer controls how long a router waits for Reply packets during the DUAL algorithm query process. If this timer expires before all Replies are received, a stuck-in-active condition occurs. The active timer serves a different purpose related to route calculation rather than neighbor maintenance.
The retransmission timer determines how long a router waits for an Acknowledgment before retransmitting a reliable packet. This timer is calculated dynamically based on smooth round-trip time measurements for each neighbor. Understanding these different timers and their roles helps network administrators troubleshoot EIGRP convergence and stability issues effectively.
Question 32
What is the purpose of the EIGRP topology table?
A) To store only the best routes
B) To store all learned routes including successors and feasible successors
C) To store neighbor relationships
D) To store interface configurations
Answer: B
Explanation:
The EIGRP topology table stores all learned routes including successors and feasible successors for each destination network. This comprehensive route database contains every route that EIGRP has learned from neighbors, not just the best routes installed in the routing table. The topology table includes critical information for each route such as feasible distance, advertised distance, metric components, and the next-hop router. This complete view of network topology enables EIGRP to perform rapid convergence when primary paths fail.
Understanding the topology table structure is fundamental to EIGRP operation. For each destination network, the topology table maintains entries for all paths learned from different neighbors. The successor route has the best metric and is installed in the routing table for packet forwarding. Feasible successor routes meet the feasibility condition and serve as pre-computed backup paths ready for immediate use if the successor fails. Other routes that do not meet the feasibility condition remain in the topology table but cannot be used until EIGRP queries neighbors and recalculates the topology.
The topology table enables EIGRP’s fast convergence capability. When a successor route fails, if a feasible successor exists in the topology table, EIGRP can immediately promote it to successor status without running the DUAL algorithm or querying neighbors. This instantaneous failover provides sub-second convergence times significantly faster than protocols that must discover backup paths after failures occur. The topology table essentially maintains a pool of potential routes evaluated for loop-free operation.
The routing table, in contrast to the topology table, contains only the best routes selected from the topology table. These successor routes are actively used for packet forwarding. The routing table represents the subset of topology table entries that have been chosen for actual traffic forwarding based on having the lowest metrics to their respective destinations.
Neighbor relationships are maintained in a separate neighbor table, not the topology table. The neighbor table tracks directly connected EIGRP routers with which the local router has formed adjacencies. It contains information about neighbor IP addresses, local interfaces connecting to neighbors, hold times, and reliability metrics for communication with each neighbor.
The topology table can be viewed using the show ip eigrp topology command, which displays successor and feasible successor routes. The show ip eigrp topology all-links command provides even more comprehensive output including routes that do not qualify as feasible successors, offering complete visibility into all paths EIGRP has learned.
Question 33
Which EIGRP feature prevents routing loops by checking advertised distance against feasible distance?
A) Split horizon
B) Feasibility condition
C) Route poisoning
D) Holddown timer
Answer: B
Explanation:
The feasibility condition prevents routing loops in EIGRP by checking whether a route’s advertised distance is less than the current feasible distance of the successor route. This mathematical condition is the cornerstone of EIGRP’s loop prevention mechanism and enables the protocol to identify backup routes that are guaranteed to be loop-free. A route can only become a feasible successor if it satisfies this condition, ensuring that using the backup route will not create a routing loop even during topology changes.
The feasibility condition works by comparing two key metrics. The advertised distance is the metric that a neighboring router reports to reach a specific destination. This value represents the distance from the neighbor to the destination. The feasible distance is the best metric from the local router to that destination, which is the metric of the current successor route. By requiring the advertised distance to be less than the feasible distance, EIGRP ensures that the path through the neighbor is not looping back through the local router.
Understanding the mathematical proof behind the feasibility condition helps network administrators appreciate its effectiveness. If a neighbor’s advertised distance to a destination is less than the local router’s feasible distance, the neighbor must be using a path that does not include the local router. This logic guarantees loop-free operation because the neighbor has found a better path independently. The DUAL algorithm leverages this condition to provide instant failover to feasible successors without risk of creating loops.
Split horizon is a different loop prevention mechanism used by distance-vector routing protocols. It prevents a router from advertising routes back out the interface where they were learned. While split horizon helps prevent loops, it operates differently from the feasibility condition and is not specific to EIGRP’s advanced distance-vector operation. EIGRP implements split horizon in addition to the feasibility condition for comprehensive loop prevention.
Route poisoning is a technique where failed routes are advertised with infinite metrics to quickly inform all routers about network failures. Holddown timers prevent routers from accepting new routes to destinations for a specified period after receiving information about route failures. Neither of these mechanisms is the primary loop prevention method in EIGRP. The feasibility condition provides immediate, mathematical certainty about loop-free paths without waiting periods or poison route advertisements.
Question 34
Which command configures the EIGRP Hello interval on an interface?
A) ip hello-interval eigrp
B) eigrp hello-interval
C) hello-interval eigrp
D) ip eigrp hello
Answer: A
Explanation:
The ip hello-interval eigrp command configures the EIGRP Hello interval on an interface. This interface-level command allows administrators to customize how frequently Hello packets are sent on specific interfaces. The complete syntax is ip hello-interval eigrp autonomous-system-number seconds, where the autonomous system number identifies the EIGRP process and seconds specifies the interval value. Customizing Hello intervals may be necessary for unreliable links, high-latency connections, or specific network requirements.
Modifying the Hello interval affects how quickly EIGRP can detect neighbor failures and how much overhead the protocol generates. Shorter Hello intervals enable faster failure detection because neighbors send keepalive messages more frequently. However, shorter intervals also increase bandwidth consumption and CPU utilization for processing Hello packets. Longer intervals reduce overhead but delay failure detection because more time elapses between Hello transmissions.
When changing the Hello interval, administrators must also consider the hold timer. The hold timer determines how long a router waits without receiving Hello packets before declaring a neighbor down. By default, the hold timer is three times the Hello interval. If you modify the Hello interval without adjusting the hold timer proportionally, the relationship between these timers changes, potentially causing unexpected neighbor timeout behavior.
The hold timer is configured separately using the ip hold-time eigrp command on the same interface. Best practice recommends maintaining the default three-to-one ratio between hold time and Hello interval. For example, if you set the Hello interval to 10 seconds, the hold time should be 30 seconds. This ratio provides reasonable tolerance for occasional packet loss while still detecting genuine failures promptly.
EIGRP does not require matching Hello and hold timers between neighbors for adjacency formation. Unlike some routing protocols that enforce timer matching, EIGRP allows neighbors to have different timer values. Each router uses its own locally configured hold timer to determine when its neighbor is down. This flexibility can be useful but also requires careful planning to ensure consistent behavior across the network.
Timer modifications should be implemented cautiously in production networks. Inappropriate timer values can cause neighbor flapping where adjacencies repeatedly form and tear down, creating routing instability. Testing timer changes in lab environments before deployment helps identify potential issues. Documentation of customized timer values is essential for troubleshooting and future network maintenance activities.
Question 35
What is the default administrative distance for EIGRP external routes?
A) 90
B) 110
C) 120
D) 170
Answer: D
Explanation:
The default administrative distance for EIGRP external routes is 170, which makes them less preferred than most other routing information sources. External routes are those redistributed into EIGRP from other routing protocols or sources such as OSPF, BGP, RIP, or static routes. The higher administrative distance reflects the fact that external routes originate outside the EIGRP autonomous system and may be less trustworthy than routes learned through native EIGRP operation.
Administrative distance provides a mechanism for routers to select the best path when multiple routing protocols advertise the same destination network. Lower administrative distance values indicate higher trustworthiness and preference. With EIGRP external routes having an administrative distance of 170, they are less preferred than EIGRP internal routes (90), OSPF routes (110), and RIP routes (120), but more preferred than unknown or unreliable sources.
The distinction between internal and external routes in EIGRP is important for proper route selection. Internal routes are learned from other EIGRP routers within the same autonomous system through normal EIGRP operation. These routes maintain the administrative distance of 90 because they represent trusted information from within the routing domain. External routes, having been redistributed from other sources, receive the higher administrative distance of 170 to reflect their external origin.
When routes are redistributed into EIGRP, they are tagged as external and carry this designation throughout the EIGRP domain. All EIGRP routers that learn these redistributed routes recognize them as external and apply the appropriate administrative distance. This consistent treatment ensures predictable routing behavior when multiple protocols coexist in the network and prevents routing loops that could occur from mutual redistribution scenarios.
Network administrators can modify administrative distance values using the distance command in EIGRP router configuration mode. This command allows separate specification of administrative distances for internal routes, external routes, and specific route sources. Customization might be necessary during network migrations, when implementing complex routing policies, or when specific paths must be preferred despite protocol defaults.
Understanding administrative distance differences helps troubleshoot unexpected routing behavior. If both EIGRP and OSPF advertise the same network, administrators must know which route will be installed based on administrative distance. The show ip protocols command displays configured administrative distances for all routing protocols, while show ip route shows the administrative distance of installed routes. Proper documentation of any customized administrative distance values prevents confusion during troubleshooting activities.
Question 36
Which EIGRP configuration mode supports IPv4 and IPv6 in a single instance?
A) Classic mode
B) Named mode
C) Wide metric mode
D) Multi-protocol mode
Answer: B
Explanation:
EIGRP named mode supports IPv4 and IPv6 in a single instance through its address-family configuration structure. This unified approach represents a significant improvement over classic EIGRP configuration, which requires separate processes for IPv4 and IPv6. Named mode organizes configuration hierarchically with address families for different protocols, allowing administrators to manage multi-protocol routing from a centralized configuration context. This design simplifies configuration and improves management efficiency in dual-stack networks.
The named mode configuration structure includes multiple sections for organizing EIGRP parameters. The address-family section contains protocol-specific configurations for IPv4 or IPv6, including network statements, autonomous system numbers, and address-family-specific parameters. The af-interface section allows interface-specific configurations that apply to particular address families. The topology section manages topology-specific settings. This hierarchical organization provides clear separation of concerns while maintaining all configurations within a single EIGRP instance.
Classic mode EIGRP requires completely separate processes for IPv4 and IPv6 routing. Administrators must configure router eigrp for IPv4 and ipv6 router eigrp for IPv6, with each process using separate commands and configuration contexts. This separation leads to duplicated effort and potential inconsistencies between IPv4 and IPv6 configurations. Named mode eliminates this redundancy by unifying both protocols under a single router eigrp statement identified by a descriptive name.
Wide metrics are a feature available in named mode that uses 64-bit values instead of the 32-bit values used in classic mode. While wide metrics provide better granularity for high-speed interfaces, they represent a feature of named mode rather than a separate configuration mode. Wide metrics help prevent metric calculations from reaching maximum values in large networks with very high-speed links where traditional 32-bit metrics might overflow.
The transition from classic to named mode does not require network downtime. Both configuration modes can coexist on the same router using different EIGRP instances. Additionally, routers running classic mode can form neighbor relationships with routers running named mode for the same autonomous system because the on-wire protocol format remains compatible. This compatibility allows gradual migration strategies without disrupting existing routing operations.
Named mode configuration follows a specific hierarchy starting with router eigrp followed by the instance name. Within this context, administrators enter address-family ipv4 autonomous-system or address-family ipv6 autonomous-system to configure protocol-specific settings. Interface configurations use af-interface statements. Understanding this structure is essential for implementing named mode in production networks and leveraging its organizational benefits.
Question 37
What does EIGRP use to ensure reliable delivery of Update packets?
A) TCP
B) UDP
C) Reliable Transport Protocol
D) SCTP
Answer: C
Explanation:
EIGRP uses Reliable Transport Protocol (RTP) to ensure reliable delivery of Update packets and other critical routing information. RTP is a proprietary transport mechanism developed specifically for EIGRP that operates at the network layer using protocol number 89. Unlike routing protocols that rely on TCP or UDP for transport, EIGRP implements its own reliability mechanisms through RTP, providing precise control over packet delivery and acknowledgment requirements based on packet type.
RTP provides both reliable and unreliable delivery modes depending on packet type requirements. Update, Query, and Reply packets require reliable delivery because they contain critical routing information that must be guaranteed to reach neighbors. When a router sends these packet types, it expects an Acknowledgment packet in return. If no acknowledgment arrives within the retransmission timeout period, the packet is retransmitted up to 16 times before the neighbor relationship is reset.
Hello and Acknowledgment packets use unreliable delivery through RTP. These packets do not require acknowledgments because they are transmitted frequently and their occasional loss can be tolerated. Hello packets serve as keepalive messages sent every few seconds, making reliable delivery unnecessary and wasteful of resources. Acknowledgment packets themselves cannot require acknowledgments as this would create an infinite acknowledgment loop.
RTP implements sequence numbers to track packets and prevent duplicate processing. Each reliable packet carries a unique sequence number that increments with each transmission. Receiving routers use these sequence numbers to identify which packets need acknowledgment and to detect retransmissions. This sequencing mechanism ensures that routing information is processed in the correct order and that retransmitted packets are not processed multiple times.
TCP would provide reliable delivery but adds overhead that EIGRP avoids through RTP’s targeted approach. TCP establishes connections, maintains windows, and implements complex congestion control mechanisms that are unnecessary for EIGRP’s specific communication patterns. RTP provides exactly the reliability features EIGRP needs without the overhead of full TCP sessions between every pair of neighbors.
UDP provides unreliable delivery and would require EIGRP to implement its own reliability mechanisms on top. While this approach is used by some routing protocols, EIGRP’s designers chose to implement RTP with integrated reliability features tailored specifically for routing protocol requirements. This custom implementation optimizes performance for EIGRP’s communication patterns and provides precise control over reliability behavior. Understanding RTP operation helps troubleshoot EIGRP convergence issues and neighbor stability problems.
Question 38
Which EIGRP packet contains the K-values used for metric calculation?
A) Update
B) Query
C) Hello
D) Reply
Answer: C
Explanation:
EIGRP Hello packets contain the K-values used for metric calculation, allowing routers to verify metric compatibility before forming neighbor relationships. The K-values are metric weights that determine which components contribute to the composite EIGRP metric calculation. There are five K-values (K1 through K5) that weight bandwidth, load, delay, reliability, and MTU components respectively. By including K-values in Hello packets, EIGRP ensures that neighboring routers use consistent metric calculation methods, preventing routing inconsistencies.
When two EIGRP routers exchange Hello packets, they compare their K-values as part of the neighbor adjacency process. If the K-values do not match between potential neighbors, the routers will not form an adjacency. This strict requirement ensures that all routers in an EIGRP autonomous system calculate metrics identically. Mismatched K-values would cause different routers to evaluate paths differently, potentially leading to routing loops or suboptimal path selection across the network.
The default K-values are K1=1, K2=0, K3=1, K4=0, and K5=0, which means only bandwidth (K1) and delay (K3) are used in the default metric calculation. These defaults provide stable and predictable routing behavior because bandwidth and delay are relatively static interface characteristics. Network administrators can modify K-values using the metric weights command, though this is rarely necessary or recommended in production environments.
Update packets contain routing information including destination networks, metrics, and next-hop addresses, but they do not include K-values. The K-values are established during the Hello exchange and apply to all subsequent metric calculations. Once neighbors agree on K-values during adjacency formation, these values do not need to be included in every routing update.
Query and Reply packets also do not contain K-values as these are used during route recalculation when successors fail. These packets contain metric information for specific destinations being queried, calculated using the K-values that were agreed upon during the initial Hello exchange. The K-values remain constant throughout the neighbor relationship unless the EIGRP process is reconfigured and restarted.
Modifying K-values affects the entire EIGRP domain because all routers must use matching values. Changing K-values requires careful planning and typically involves a maintenance window where all routers are reconfigured sequentially. Verification using show ip protocols displays the configured K-values, allowing administrators to confirm consistency across the network before and after changes.
Question 39
What is the significance of the EIGRP reported distance?
A) It is the metric from the local router to the destination
B) It is the metric advertised by a neighbor to reach a destination
C) It is the administrative distance of the route
D) It is the hop count to the destination
Answer: B
Explanation:
The EIGRP reported distance, also called advertised distance, is the metric advertised by a neighbor to reach a destination network. This value represents the distance from the neighboring router to the destination, not including the link between the local router and that neighbor. The reported distance is crucial for the feasibility condition calculation that determines whether a route can serve as a feasible successor. Understanding reported distance helps network administrators analyze EIGRP path selection and troubleshoot convergence issues.
The reported distance differs from the feasible distance, which is the total metric from the local router to the destination through a specific neighbor. The feasible distance equals the reported distance plus the cost of the link between the local router and the advertising neighbor. This relationship means that the feasible distance is always greater than or equal to the reported distance. The router uses feasible distance to select the best path (successor), while reported distance is used in the feasibility condition.
The feasibility condition compares reported distance against feasible distance to identify loop-free backup paths. A route qualifies as a feasible successor only if its reported distance is less than the feasible distance of the current successor route. This comparison ensures that the backup path does not loop back through the local router. By examining reported distances, EIGRP mathematically guarantees loop-free routing without requiring global network topology knowledge.
Administrative distance is completely different from reported distance. Administrative distance is a value between 0 and 255 that represents the trustworthiness of routing information sources. EIGRP internal routes have an administrative distance of 90, while external routes have 170. Administrative distance determines which routing protocol’s information is installed in the routing table when multiple protocols advertise the same destination.
Hop count represents the number of routers a packet traverses to reach a destination. While EIGRP tracks hop count internally up to a maximum of 255 hops, it does not use hop count in its metric calculations. EIGRP is an advanced distance-vector protocol that uses a composite metric based on bandwidth and delay by default, rather than simple hop count like traditional distance-vector protocols such as RIP.
The show ip eigrp topology command displays both reported distance and feasible distance for routes in the topology table. The output format shows feasible distance first, followed by reported distance in parentheses. Analyzing these values helps administrators understand why certain routes become feasible successors while others do not, which is essential for optimizing network design and troubleshooting convergence problems.
Question 40
Which command displays EIGRP-enabled interfaces and their configuration?
A) show ip interface brief
B) show ip eigrp interfaces
C) show interfaces
D) show ip protocols
Answer: B
Explanation:
The show ip eigrp interfaces command displays EIGRP-enabled interfaces and their configuration, providing essential information for verifying and troubleshooting EIGRP operation. This command shows which interfaces are actively participating in EIGRP, the number of peers (neighbors) on each interface, pending routes waiting to be sent, and other interface-specific EIGRP parameters. The output helps administrators quickly identify which interfaces are running EIGRP and whether they are functioning correctly.
The command output includes the interface name, number of EIGRP peers on each interface, transmit queue information showing pending updates, and mean smooth round-trip time for reliable packet delivery. This information is valuable for identifying interfaces with communication problems, verifying that EIGRP is enabled on expected interfaces, and troubleshooting neighbor formation issues. If an interface does not appear in the output, EIGRP is not running on that interface.
Adding the detail keyword to the command (show ip eigrp interfaces detail) provides extensive additional information including authentication configuration, Hello and hold timer values, split-horizon settings, next Hello packet transmission time, and other interface-specific parameters. The detailed output is particularly useful for troubleshooting authentication problems, timer misconfigurations, and other interface-level issues that might prevent proper EIGRP operation.
The show ip interface brief command displays a summary of all interfaces including their IP addresses and status, but it does not show EIGRP-specific information. This command is useful for verifying basic IP connectivity and interface states but does not indicate which interfaces are participating in EIGRP or provide any routing protocol details.
The show interfaces command displays detailed Layer 1 and Layer 2 information about physical and logical interfaces including hardware addresses, bandwidth settings, encapsulation types, and traffic statistics. While useful for troubleshooting physical connectivity issues, this command does not show routing protocol configuration or EIGRP-specific parameters.
The show ip protocols command provides a summary of all routing protocols running on the router including EIGRP, but it does not list individual interfaces. This command shows global EIGRP parameters such as autonomous system number, K-values, administrative distances, and network statements, but for interface-specific information, show ip eigrp interfaces is the appropriate command. Understanding which command provides specific information types improves troubleshooting efficiency and reduces diagnostic time.
Question 41
What is the purpose of EIGRP passive interfaces?
A) To disable EIGRP entirely on the router
B) To prevent sending and receiving EIGRP packets while still advertising the network
C) To accept but not send routing updates
D) To enable EIGRP authentication
Answer: B
Explanation:
EIGRP passive interfaces prevent sending and receiving EIGRP packets on specified interfaces while still advertising the connected networks of those interfaces in EIGRP. This configuration is useful for interfaces connected to networks where no EIGRP neighbors should exist, such as user access networks, DMZ segments, or connections to non-EIGRP devices. By configuring interfaces as passive, administrators prevent unnecessary EIGRP traffic and potential security risks while ensuring the networks are still advertised throughout the EIGRP domain.
The passive-interface command is configured under the EIGRP router configuration mode. When applied to an interface, that interface stops sending Hello packets, which prevents neighbor adjacencies from forming. The interface also stops processing any EIGRP packets received, effectively isolating it from EIGRP protocol operations. However, the network connected to the passive interface is still included in EIGRP advertisements sent out other active interfaces.
Using passive interfaces provides security benefits by preventing unauthorized routers from forming EIGRP neighbor relationships. Without passive interface configuration, any device that sends EIGRP Hello packets could potentially become a neighbor and either learn routing information or inject false routes into the network. Making untrusted interfaces passive eliminates this risk by preventing EIGRP protocol communication entirely.
The passive-interface default command configures all interfaces as passive by default, then administrators use no passive-interface commands to selectively enable EIGRP on specific interfaces. This approach is often preferred because it is more secure, requiring explicit configuration to enable EIGRP protocol communication. This default-deny approach prevents accidentally enabling EIGRP on new interfaces without proper configuration review.
Passive interfaces differ from stub routers, which limit query propagation and route advertisement but still form neighbor relationships and exchange routing information. Passive interfaces completely prevent EIGRP protocol communication on specified interfaces while still advertising connected networks. Stub routers participate fully in EIGRP but advertise only specific route types and never propagate queries.
Verification of passive interface configuration can be done using the show ip protocols command, which lists all interfaces configured as passive. The show ip eigrp interfaces command will not display passive interfaces because they are not actively participating in EIGRP protocol communication. Understanding passive interface behavior helps administrators design secure EIGRP deployments that minimize attack surfaces while maintaining complete route advertisement throughout the autonomous system.
Question 42
Which EIGRP metric component is calculated using (10^7 / minimum bandwidth)?
A) Delay
B) Bandwidth
C) Reliability
D) Load
Answer: B
Explanation:
The EIGRP bandwidth metric component is calculated using the formula (10^7 / minimum bandwidth in kbps) × 256. This calculation uses the minimum bandwidth value encountered along the entire path to the destination, meaning the slowest link in the path determines the bandwidth component of the metric. The formula produces higher metric values for lower bandwidth paths and lower metric values for higher bandwidth paths, causing EIGRP to prefer routes with higher bandwidth capacity.
Understanding the bandwidth calculation formula explains why EIGRP prefers certain paths over others. A Fast Ethernet interface with 100,000 kbps bandwidth produces a metric component of (10,000,000 / 100,000) × 256 = 25,600. A T1 interface with 1,544 kbps bandwidth produces (10,000,000 / 1,544) × 256 = 1,657,856, which is much higher. The lower metric value makes the Fast Ethernet path more attractive to EIGRP for route selection.
The minimum bandwidth approach means that a path is only as good as its slowest link. If a route includes both Gigabit and Fast Ethernet segments, the bandwidth metric component uses the Fast Ethernet value (100 Mbps) because it is lower. This behavior ensures EIGRP accounts for bottlenecks in paths and does not overestimate path capacity based on high-speed segments while ignoring slower links that limit actual throughput.
Network administrators can manipulate EIGRP path selection by adjusting interface bandwidth values using the bandwidth command. This command does not change actual interface speed but modifies the value EIGRP uses in metric calculations. By lowering bandwidth values on undesired paths or raising them on preferred paths, administrators influence EIGRP route selection without changing physical infrastructure.
The factor of 10^7 in the formula represents a reference bandwidth of 10 Mbps (10,000,000 bps). This value was chosen when EIGRP was designed and Fast Ethernet was common. The multiplication by 256 scales the result to fit EIGRP’s metric calculation requirements. While these specific values may seem arbitrary, they create appropriate metric ranges that allow EIGRP to distinguish between paths of different bandwidths effectively.
Delay is calculated differently from bandwidth, using cumulative delay values summed across all interfaces rather than minimum values. The delay formula is (sum of delays in tens of microseconds) × 256. Both bandwidth and delay components are combined in the default EIGRP metric formula, with each component weighted by K-values. Understanding these calculations helps administrators predict EIGRP path selection behavior and design networks that automatically select optimal paths.
Question 43
What happens when an EIGRP router receives a route with a higher sequence number than expected?
A) The packet is accepted and processed normally
B) The packet is dropped as corrupted
C) The router requests retransmission
D) The neighbor relationship is reset
Answer: A
Explanation:
When an EIGRP router receives a route with a higher sequence number than expected, the packet is accepted and processed normally. EIGRP uses sequence numbers in the Reliable Transport Protocol to track packets and ensure reliable delivery of critical routing information. Each reliable packet (Update, Query, Reply) carries a sequence number that increments with each transmission. Receiving routers use these sequence numbers to identify packets, send acknowledgments, and maintain proper packet ordering.
Sequence numbers allow EIGRP to handle out-of-order packet arrival gracefully. In networks with multiple paths or varying latency, packets may arrive in different orders than they were sent. Higher-than-expected sequence numbers indicate that packets have arrived out of order or that some packets were lost. EIGRP processes the higher-numbered packet and updates its expected sequence number accordingly. The sender’s retransmission mechanism ensures that any missing lower-numbered packets are retransmitted.
When a router sends a reliable packet, it increments its local sequence number and includes this value in the packet header. The receiving router acknowledges the packet using an ACK packet that references the received sequence number. If the sender does not receive an acknowledgment within the retransmission timeout period, it retransmits the packet with the same sequence number. The receiver recognizes retransmissions by matching sequence numbers and avoids processing duplicate packets.
The sequence number mechanism prevents duplicate processing of retransmitted packets. If a packet is received twice due to delayed acknowledgment and retransmission, the receiver compares the sequence number against previously processed packets. Duplicate sequence numbers indicate retransmissions that should be acknowledged but not reprocessed. This duplicate detection prevents routing table inconsistencies that could arise from processing the same update multiple times.
Sequence numbers do not cause neighbor resets under normal circumstances. EIGRP’s sequence tracking is designed to handle lost packets, out-of-order delivery, and retransmissions without disrupting neighbor relationships. Only severe communication failures such as consistent packet loss, acknowledgment timeouts exceeding retry limits, or hold timer expiration cause neighbor resets. Sequence number handling is part of normal protocol operation that maintains reliability without triggering adjacency failures.
Monitoring sequence numbers helps troubleshoot EIGRP communication problems. The show ip eigrp neighbors command displays the last sequence number received from each neighbor. Stagnant sequence numbers suggest that updates are not being received, while rapidly changing values indicate active route exchanges. Understanding sequence number operation provides insight into EIGRP’s reliable delivery mechanism and helps diagnose convergence issues or communication failures between neighbors.
Question 44
Which EIGRP command shows the amount of time a route has been in active state?
A) show ip route
B) show ip eigrp topology active
C) show ip eigrp neighbors
D) show ip protocols
Answer: B
Explanation:
The show ip eigrp topology active command shows routes that are currently in active state and displays how long each route has been active. This command is essential for troubleshooting stuck-in-active (SIA) conditions and understanding convergence behavior during network topology changes. The output includes destination networks in active state, the time elapsed since they entered active state, and which neighbors have replied to queries. This information helps identify convergence problems and diagnose why routes remain in active state longer than expected.
Routes enter active state when the successor fails and no feasible successor exists in the topology table. During active state, the router sends Query packets to neighbors requesting alternative path information. The router must receive Reply packets from all queried neighbors before it can complete route recalculation and return the route to passive state. The time spent in active state directly impacts convergence speed and can indicate network problems if excessively long.
The show ip eigrp topology active command displays which neighbors have sent replies and which have not. This information is critical for identifying the source of convergence delays. If one neighbor consistently fails to reply quickly, it may indicate problems with that neighbor’s connectivity, processing capacity, or route filtering configuration. Administrators can use this information to focus troubleshooting efforts on specific network areas or devices.
When routes remain in active state approaching the SIA timer limit (default 180 seconds), administrators should investigate immediately. The active command output helps determine whether the problem is isolated to specific destinations or affecting many routes simultaneously. Widespread active state issues often indicate network-wide problems such as routing loops, configuration errors, or infrastructure failures requiring immediate attention.
The show ip route command displays the routing table contents but does not show EIGRP state information or distinguish between passive and active routes. It shows only successor routes currently installed for packet forwarding, providing no visibility into routes undergoing recalculation or queries in progress.
The show ip eigrp neighbors command displays neighbor relationship information including adjacency status, hold times, and communication statistics, but it does not show route states or active convergence processes. The show ip protocols command provides protocol configuration summaries without specific route state information. Understanding which command provides which information improves troubleshooting efficiency when diagnosing EIGRP convergence problems or stuck-in-active conditions.
Question 45
What is the effect of configuring EIGRP stub connected on a router?
A) The router advertises only connected routes
B) The router advertises all routes
C) The router stops forming neighbor relationships
D) The router enters passive mode
Answer: A
Explanation:
Configuring EIGRP stub connected on a router causes that router to advertise only its directly connected routes to EIGRP neighbors. The stub configuration prevents the router from advertising any other route types including static routes, summary routes, or routes learned from EIGRP neighbors. This restrictive advertisement policy combined with stub router behavior creates an ideal configuration for spoke routers in hub-and-spoke topologies where spokes should advertise only their local networks without participating in transit routing.
Stub routers inform neighbors of their stub status during adjacency formation using a special flag in EIGRP packets. When a router knows its neighbor is a stub, it never sends Query packets to that stub during route recalculation. This query containment significantly improves convergence time by limiting the scope of DUAL algorithm queries. In large networks with many stub spokes, this behavior prevents query propagation to hundreds of routers that would not provide useful alternative path information anyway.
The connected option is one of several stub router options that can be configured individually or in combination. Other options include summary (advertise summary routes), static (advertise redistributed static routes), and redistributed (advertise routes redistributed from other protocols). Multiple options can be combined, such as eigrp stub connected summary, allowing the router to advertise both connected networks and configured summary routes while maintaining stub behavior.
The receive-only stub option is the most restrictive, preventing the router from advertising any routes at all. A receive-only stub only receives routing information from neighbors without contributing any route advertisements. This configuration might be used for routers that need EIGRP routing information but should not advertise any networks themselves, such as monitoring devices or management stations.
Stub configuration does not prevent neighbor relationship formation. Stub routers form complete EIGRP adjacencies with neighbors and exchange routing information according to their stub configuration. The stub designation affects only query propagation and route advertisement behavior, not the fundamental neighbor relationship. Verification using show ip eigrp neighbors displays stub routers with stub status indicated in the output.
Network designers should carefully plan stub router deployment to ensure proper connectivity. Stub routers cannot serve as transit paths between EIGRP neighbors because they do not advertise routes learned from other routers. If network topology requires a router to forward traffic between different EIGRP neighbors, that router cannot be configured as stub. Understanding these limitations prevents connectivity problems and ensures stub configuration aligns with network topology requirements.