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Question 61:
What is the default EIGRP metric formula when using default K-values?
A) Bandwidth only
B) Delay only
C) Bandwidth + Delay
D) Bandwidth + Delay + Reliability
Answer: C
Explanation:
When using default K-values, the EIGRP metric formula calculates the composite metric using bandwidth and delay components. The default K-value configuration sets K1 and K3 to 1, while K2, K4, and K5 are set to 0. This configuration means that only the bandwidth (weighted by K1) and delay (weighted by K3) components contribute to the final metric calculation. The formula becomes: metric = ([K1 × bandwidth + K3 × delay] × 256), which simplifies to ([bandwidth + delay] × 256) when using default values.
The bandwidth component is calculated using the formula (10^7 / minimum bandwidth in kbps) × 256. This calculation identifies the slowest link along the entire path and uses that value to represent the bandwidth constraint of the route. Lower bandwidth values result in higher metric components, making paths with higher bandwidth more preferable in EIGRP route selection. For example, a Gigabit Ethernet interface produces a much lower bandwidth metric component than a T1 interface, causing EIGRP to prefer the higher-speed path.
The delay component represents cumulative delay across all interfaces in the path. Unlike bandwidth which uses only the minimum value, delay is summed across every interface that packets traverse from source to destination. Each interface contributes its configured delay value, measured in tens of microseconds, to the total path delay. This cumulative approach provides comprehensive insight into end-to-end latency characteristics of different routes through the network.
Using only bandwidth and delay in the default metric calculation provides stable and predictable routing behavior. These two components are relatively static interface characteristics that do not change frequently during normal network operation. In contrast, reliability and load components change dynamically based on current network conditions such as error rates and traffic levels. Including these dynamic components in metric calculations would cause route flapping, where routes continuously change as conditions fluctuate, creating instability throughout the network.
Network administrators can modify K-values to include other metric components if specific network requirements demand it. However, changing K-values requires careful planning because all routers in the EIGRP autonomous system must use matching K-values to form neighbor adjacencies. Mismatched K-values prevent neighbor relationships from forming, causing routing disruptions. Most production networks maintain the default K-value configuration because it provides excellent routing decisions without the complexity and potential instability of including additional metric components. The bandwidth and delay combination effectively represents path quality while maintaining routing stability.
Question 62:
Which EIGRP packet type is used to acknowledge receipt of reliable packets?
A) Hello
B) Update
C) Acknowledgment
D) Query
Answer: C
Explanation:
EIGRP uses Acknowledgment packets to confirm receipt of reliable packets including Updates, Queries, and Replies. The Acknowledgment packet, often abbreviated as ACK, is a lightweight packet containing only the information necessary to confirm that a specific reliable packet was successfully received. ACK packets reference the sequence number of the packet being acknowledged, allowing the sender to verify that its transmission reached the intended neighbor. This acknowledgment mechanism is fundamental to EIGRP’s Reliable Transport Protocol, ensuring critical routing information is successfully exchanged between neighbors.
When a router sends a reliable packet such as an Update containing routing information, it expects to receive an Acknowledgment from the recipient. If no ACK arrives within the retransmission timeout period, the sender assumes the packet was lost and retransmits it. This process can repeat up to 16 times before the router gives up and resets the neighbor relationship. The ACK mechanism guarantees that important routing updates are not lost during transmission, maintaining synchronized topology information across all EIGRP routers in the autonomous system.
Acknowledgment packets use unreliable delivery, meaning they themselves do not require acknowledgments. This design prevents an infinite acknowledgment loop where ACKs would require their own ACKs. ACK packets are small and efficient, typically containing just the sequence number being acknowledged and minimal header information. They are sent to the unicast address of the neighbor whose packet is being acknowledged, ensuring that acknowledgments reach the correct sender even in multi-access network environments.
The sequence numbering system works in conjunction with acknowledgments to maintain packet ordering and prevent duplicate processing. Each reliable packet carries a unique sequence number that increments with each new transmission. When routers receive reliable packets, they send ACKs referencing those sequence numbers. If a packet is retransmitted due to missing acknowledgment, it carries the same sequence number as the original transmission. Receiving routers recognize duplicate sequence numbers and avoid reprocessing the same routing information multiple times.
Hello packets serve a completely different purpose in EIGRP, functioning as keepalive messages to discover and maintain neighbor relationships. Hello packets use unreliable delivery and do not require acknowledgments because they are sent frequently enough that occasional packet loss can be tolerated. Update and Query packets are reliable packet types that require ACKs but are not themselves acknowledgments. Understanding the distinct role of Acknowledgment packets in EIGRP’s reliable delivery mechanism is essential for troubleshooting convergence issues and neighbor stability problems.
Question 63:
What happens when EIGRP neighbors have mismatched K-values?
A) They form an adjacency but use different metrics
B) They form an adjacency and negotiate common K-values
C) They do not form an adjacency
D) They form an adjacency but cannot exchange routes
Answer: C
Explanation:
EIGRP neighbors with mismatched K-values do not form an adjacency. During the Hello packet exchange that initiates neighbor relationships, routers advertise their configured K-values to potential neighbors. Both routers compare the K-values they receive against their own local configuration. If any of the five K-values differ between the two routers, they refuse to establish a neighbor relationship. This strict requirement ensures that all routers in an EIGRP autonomous system calculate metrics identically, preventing routing inconsistencies that could arise from different metric calculation methods.
The K-values are fundamental to how EIGRP calculates the composite metric for routes. These five values (K1 through K5) weight different metric components including bandwidth, load, delay, reliability, and MTU. If routers used different K-values, they would evaluate the same paths with different metrics, potentially reaching conflicting conclusions about which routes are best. One router might consider a path optimal while another router views it as suboptimal, creating routing loops or persistent convergence issues throughout the autonomous system.
Preventing adjacency formation between routers with mismatched K-values is a protective mechanism built into EIGRP. Rather than allowing inconsistent metric calculations to corrupt routing tables, EIGRP simply refuses to exchange routing information with neighbors that use different calculation methods. This design decision prioritizes routing correctness over connectivity, ensuring that when EIGRP does operate, it operates correctly without hidden inconsistencies that would be difficult to troubleshoot.
Troubleshooting EIGRP neighbor problems should always include verification of K-value consistency. The show ip protocols command displays the configured K-values on each router. If potential neighbors have physical connectivity and properly configured network statements but still fail to form adjacencies, K-value mismatches are a common cause. Comparing the K-values shown in show ip protocols output between the routers quickly identifies this problem.
Modifying K-values requires network-wide coordination because all routers must use matching values. Changing K-values typically involves a maintenance window where all EIGRP routers are reconfigured sequentially or simultaneously to maintain consistent values throughout the autonomous system. The default K-values (K1=1, K2=0, K3=1, K4=0, K5=0) work well for most networks, so changes should only be made when specific requirements justify the complexity. Most production networks never modify K-values from their defaults, avoiding this potential source of neighbor adjacency problems entirely while achieving excellent routing behavior through bandwidth and delay based metric calculations.
Question 64:
Which command displays the EIGRP router ID?
A) show ip eigrp neighbors
B) show ip protocols
C) show ip route
D) show ip eigrp interfaces
Answer: B
Explanation:
The show ip protocols command displays the EIGRP router ID along with other comprehensive protocol configuration information. This command provides a summary of all routing protocols running on the router, and for EIGRP specifically, it shows the router ID, autonomous system number, K-values, administrative distances, network statements, passive interfaces, and various other configuration parameters. The router ID is displayed near the beginning of the EIGRP section in the output, typically in the format “Router ID: 192.168.1.1” or similar depending on the configured or automatically selected value.
The EIGRP router ID serves as a unique identifier for the router within the EIGRP domain. While not used as extensively in EIGRP as in protocols like OSPF, the router ID provides administrative identification that appears in various show commands and log messages. In EIGRP named mode configuration, the router ID becomes more prominent and is used in topology information displays. Having a consistent and predictable router ID simplifies network management, documentation, and troubleshooting activities.
By default, EIGRP selects the router ID from the highest IP address of configured loopback interfaces. Loopback interfaces are preferred because they are virtual interfaces that remain operational as long as the router is running, providing stability regardless of physical interface status. If no loopback interfaces exist, EIGRP selects the highest IP address among all active physical interfaces. This automatic selection ensures every EIGRP router has a router ID without requiring explicit configuration.
Network administrators can manually configure the EIGRP router ID using the eigrp router-id command in EIGRP configuration mode. Manual configuration is recommended in production environments because it provides explicit control over router identification and prevents unexpected changes if interface configurations are modified. The manually configured router ID takes precedence over automatically selected values, ensuring consistent identification across router reboots and configuration changes.
The router ID is selected when the EIGRP process starts and typically remains unchanged unless the process is restarted or the router is rebooted. Adding new interfaces with higher IP addresses after EIGRP has started does not automatically change the router ID. Understanding router ID selection behavior helps prevent confusion during troubleshooting and ensures network documentation accurately reflects router identities.
Other show commands provide different information but do not display the router ID. The show ip eigrp neighbors command shows neighbor relationships but not the local router ID. The show ip route command displays routing table contents without protocol-specific configuration details like router ID. The show ip eigrp interfaces command shows interface participation in EIGRP but not global protocol parameters. For comprehensive EIGRP configuration information including router ID, show ip protocols is the appropriate command to use during verification and troubleshooting activities.
Question 65:
What is the purpose of the EIGRP successors?
A) To serve as backup routes only
B) To provide the best route currently used for forwarding
C) To advertise routes to neighbors
D) To store routing updates
Answer: B
Explanation:
EIGRP successors provide the best route currently used for forwarding packets to destination networks. The successor is the route with the lowest feasible distance among all paths to a particular destination. When EIGRP calculates metrics for all available paths, the route with the best metric becomes the successor and is installed in the routing table for active packet forwarding. Only successor routes are used to forward traffic under normal circumstances, ensuring that packets follow the optimal path based on EIGRP’s metric calculations.
The successor route represents the result of EIGRP’s path selection process. After receiving routing updates from multiple neighbors, the router calculates the feasible distance for each path by adding the local link cost to the neighbor’s advertised distance. The path with the lowest total metric becomes the successor. This calculation occurs continuously as routing updates arrive, ensuring the router always uses the best available path based on current network conditions and topology information.
Understanding the relationship between successors and feasible successors is crucial for comprehending EIGRP’s fast convergence capabilities. While the successor is the best route actively used for forwarding, feasible successors are pre-computed backup routes that meet the feasibility condition. These backup routes remain in the topology table ready for immediate use if the successor fails. When a successor route fails and feasible successors exist, EIGRP can instantly promote a feasible successor to successor status without running the DUAL algorithm or querying neighbors, providing sub-second convergence.
The successor route’s metric becomes the reference point for evaluating potential feasible successors. For a backup route to qualify as a feasible successor, its advertised distance must be less than the successor’s feasible distance. This mathematical relationship ensures that feasible successors are loop-free and can be used immediately without risk of creating routing loops. The feasibility condition is the foundation of EIGRP’s ability to provide rapid convergence while maintaining loop-free routing throughout topology changes.
Multiple equal-cost paths can simultaneously serve as successor routes, enabling equal-cost load balancing. When multiple paths have identical metrics to the same destination, EIGRP installs all of them in the routing table up to the maximum-paths limit. Traffic is then distributed across these equal-cost successors, maximizing bandwidth utilization and providing redundancy. This load balancing capability enhances network performance and reliability.
The show ip route command displays successor routes installed in the routing table, while the show ip eigrp topology command shows both successors and feasible successors in the topology table. Understanding successor route selection and maintenance is fundamental to EIGRP operation and essential for troubleshooting routing behavior, optimizing path selection, and ensuring network traffic follows intended paths through the infrastructure.
Question 66:
Which EIGRP configuration prevents the router from sending Query packets to neighbors?
A) Passive interface
B) Stub router
C) Route filtering
D) Summarization
Answer: B
Explanation:
EIGRP stub router configuration prevents the router from sending Query packets to neighbors during route recalculation. When a router is configured as a stub, it informs its neighbors of this status during adjacency formation. Neighbors then understand that the stub router should never be queried for alternative paths and will not propagate Query packets to it during convergence operations. This behavior is fundamental to stub router functionality and applies regardless of which specific stub options are configured, making stub routers ideal for spoke locations in hub-and-spoke topologies.
The stub router feature significantly improves network stability and convergence time by limiting query scope. In traditional EIGRP networks without stub configuration, Query packets can propagate throughout the entire autonomous system when routers lose successor routes and have no feasible successors. This widespread query propagation can lead to extended convergence times and potentially cause stuck-in-active conditions. By designating appropriate routers as stubs, queries are contained at higher hierarchical levels, dramatically reducing the number of routers involved in convergence calculations.
Stub routers are configured using the eigrp stub command with various options controlling which routes the stub advertises. Available options include connected (advertise connected networks), summary (advertise summary routes), static (advertise redistributed static routes), redistributed (advertise redistributed routes from other protocols), and receive-only (advertise no routes at all). These options provide flexibility in controlling route advertisements while maintaining the core stub behavior of never sending Query packets to neighbors.
The query limitation applies only to the stub router’s outbound queries. Stub routers can still receive Query packets from their neighbors and must respond with Reply packets. When a stub router receives a Query, it responds based on its local routing information. If the stub has a route to the queried destination, it includes that information in the Reply. If not, it responds indicating the destination is unreachable from its perspective. This response obligation ensures proper DUAL algorithm operation while still achieving the convergence benefits of query containment.
Passive interfaces operate differently from stub configuration. Passive interfaces completely prevent EIGRP protocol communication on specified interfaces, stopping both sending and receiving of all EIGRP packets including Hellos, Updates, Queries, and Replies. In contrast, stub routers participate fully in EIGRP protocol operations including forming neighbor relationships and exchanging routing information. The key distinction is that stub routers specifically prevent sending Query packets while allowing all other normal EIGRP operations.
Network designers should carefully plan stub router deployment to ensure proper network functionality. Stub routers cannot serve as transit paths between different EIGRP neighbors because they do not propagate queries and have limited route advertisement capabilities. Understanding these limitations prevents connectivity problems in complex topologies and ensures stub configuration aligns with intended network design and traffic flow patterns.
Question 67:
What is the default EIGRP administrative distance for summary routes?
A) 5
B) 90
C) 110
D) 170
Answer: A
Explanation:
The default EIGRP administrative distance for summary routes is 5, which is lower than most other route types. When a router advertises a summary route using the ip summary-address eigrp command, it automatically installs a local route to the Null0 interface for that summary address with an administrative distance of 5. This low administrative distance ensures that the Null0 summary route is preferred over any more specific routes learned from other sources, creating a critical loop prevention mechanism for route summarization.
The Null0 route created during summarization serves as a discard route for packets destined to addresses within the summary range that do not actually exist in the network. Without this protective mechanism, packets for non-existent destinations within the summary could be forwarded according to less specific routes such as default routes, potentially creating routing loops. The Null0 interface is a virtual interface that drops all traffic sent to it, similar to /dev/null in Unix systems. Packets forwarded to Null0 simply disappear, preventing them from circulating through the network.
The administrative distance of 5 for summary Null0 routes is strategically chosen within the hierarchy of administrative distances. It is lower than the administrative distance of EIGRP internal routes (90) and EIGRP external routes (170), ensuring that the summary Null0 route takes precedence over any EIGRP-learned routes. However, it is higher than connected routes (0) and most static routes (default 1), allowing more specific routes to override the summary when they exist. This ordering creates the desired behavior where specific routes are used when available, but the Null0 route catches traffic for non-existent destinations within the summary range.
Network administrators can modify the administrative distance of summary routes using an optional parameter in the ip summary-address eigrp command. The complete syntax includes the autonomous system number, summary address, subnet mask, and optionally the administrative distance. Changing the administrative distance might be necessary when multiple routers advertise overlapping summaries and specific preference control is needed. However, the default value of 5 works correctly for most implementations.
Verification of summary route configuration includes examining the Null0 route in the routing table. The show ip route command displays the summary route pointing to Null0 with its administrative distance of 5. This route appears as a static-like entry even though it was automatically created by EIGRP. Understanding this automatic route creation is important for troubleshooting connectivity issues that might arise from overly broad summarization where legitimate traffic is being dropped by the Null0 route.
The low administrative distance of summary routes ensures proper loop prevention while maintaining flexibility for more specific routes to override the summary when appropriate. This design allows EIGRP summarization to operate safely and predictably, providing the scalability benefits of route aggregation without introducing routing loops or forwarding problems for legitimate traffic destinations.
Question 68:
Which EIGRP feature prevents routing updates from being sent out the interface where they were learned?
A) Route filtering
B) Split horizon
C) Passive interface
D) Stub configuration
Answer: B
Explanation:
Split horizon prevents routing updates from being sent out the interface where they were learned, serving as a fundamental loop prevention mechanism in EIGRP. This rule recognizes that advertising a route back to the neighbor that originally provided it would be counterproductive and potentially contribute to routing loops during convergence or failure scenarios. Split horizon operates by tracking which interface each route was learned from and automatically suppressing advertisements of that specific route out the same interface.
The split horizon mechanism improves routing stability by preventing obviously circular routing information. When a router learns about a destination network from a neighbor on a particular interface, there is no benefit to advertising that same network back out the interface to that neighbor. The neighbor already knows about the network, so receiving the advertisement again provides no useful information and unnecessarily consumes bandwidth. More importantly, such circular advertisements could cause routing loops if the original neighbor had lost its path and was relying on other routers for alternative routing information.
In most network topologies, split horizon operates transparently and beneficially without requiring administrator intervention. Standard point-to-point links, Ethernet networks, and most modern network designs work correctly with split horizon enabled. However, certain topologies require split horizon to be disabled for proper route propagation. Hub-and-spoke Frame Relay networks and other non-broadcast multi-access environments where multiple remote sites connect through a single interface on a hub router are classic examples where split horizon can prevent necessary route advertisements.
In hub-and-spoke NBMA topologies, the hub router learns routes from spoke routers through a single physical interface. With split horizon enabled, the hub cannot advertise routes learned from one spoke to other spokes because they all share the same physical interface. This limitation prevents spokes from learning about networks at other spokes, causing connectivity problems. Disabling split horizon on the hub interface using the no ip split-horizon eigrp command allows the hub to advertise routes between spokes while accepting the increased risk of routing loops that must be managed through careful network design.
Split horizon works in conjunction with other EIGRP loop prevention mechanisms including the feasibility condition and DUAL algorithm. While split horizon provides basic loop prevention for simple scenarios, the feasibility condition offers mathematical certainty of loop-free backup paths. The DUAL algorithm ensures overall loop-free routing during topology changes. These multiple layers of loop prevention create a robust framework that maintains routing stability under various network conditions.
Verification of split horizon configuration is done using the show ip interface command, which indicates whether split horizon is enabled for EIGRP on each interface. Understanding split horizon behavior is essential for troubleshooting routing problems in complex topologies, particularly when implementing hub-and-spoke designs or dealing with NBMA technologies. Proper split horizon configuration balances loop prevention benefits against the need for complete route propagation in specialized network architectures.
Question 69:
What is the EIGRP smooth round-trip time used for?
A) Calculating hold timers
B) Calculating retransmission timeout
C) Determining metric values
D) Selecting successor routes
Answer: B
Explanation:
The EIGRP smooth round-trip time (SRTT) is used for calculating the retransmission timeout (RTO) value. SRTT measures the average time between sending a reliable packet to a neighbor and receiving the acknowledgment for that packet. This measurement provides insight into the communication quality and latency between EIGRP neighbors. By tracking SRTT, EIGRP can adapt its retransmission behavior to network conditions, waiting longer for acknowledgments on high-latency links and retransmitting sooner on low-latency links where delays likely indicate packet loss rather than normal propagation time.
The SRTT value is calculated using a weighted average that smooths out temporary variations in round-trip time. When a router sends a reliable packet and receives an acknowledgment, it measures the actual round-trip time for that exchange. This measured value is incorporated into the SRTT using a smoothing algorithm that prevents temporary spikes or dips from causing dramatic changes. The smoothing mechanism ensures that SRTT represents consistent communication characteristics rather than reacting too strongly to individual packet exchanges that might be anomalous.
The retransmission timeout is directly derived from the SRTT value. EIGRP multiplies the SRTT by a factor to determine how long to wait for an acknowledgment before assuming a packet was lost and needs retransmission. If the RTO is too short, the router will unnecessarily retransmit packets that are simply delayed but not lost, creating additional network overhead. If the RTO is too long, the router will wait excessive time before retransmitting actually lost packets, slowing convergence. The SRTT-based RTO calculation balances these concerns by adapting to actual observed network behavior.
Network administrators can monitor SRTT and RTO values using the show ip eigrp neighbors command, which displays these statistics for each neighbor relationship. High SRTT values indicate network latency or congestion between neighbors, while rapidly changing SRTT values suggest unstable or congested network conditions. Consistently high RTO values may indicate that the network path between neighbors has significant latency requiring longer retransmission timeouts to avoid unnecessary packet retransmissions.
SRTT and RTO measurements are maintained independently for each neighbor relationship. Different neighbors may experience different network conditions based on their physical paths, link types, and intermediate network equipment. By maintaining separate statistics per neighbor, EIGRP optimizes its reliable delivery mechanism for the specific characteristics of each neighbor relationship. This per-neighbor adaptation ensures efficient protocol operation across diverse network environments with varying latency and reliability characteristics.
Understanding SRTT and RTO helps troubleshoot EIGRP convergence and stability issues. If neighbors frequently reset or stuck-in-active conditions occur, examining SRTT and RTO values can reveal underlying network quality problems. Very high values suggest latency issues that may require network infrastructure improvements, while erratic values indicate unstable conditions causing packet loss and retransmissions. Monitoring these metrics provides valuable insight into EIGRP communication health and helps identify network segments requiring attention to improve routing protocol performance and overall network stability.
Question 70:
Which EIGRP command configures authentication on an interface?
A) ip authentication mode eigrp
B) authentication mode eigrp
C) eigrp authentication
D) ip eigrp authentication
Answer: A
Explanation:
The ip authentication mode eigrp command configures the authentication mode on an EIGRP interface. This interface configuration command specifies whether MD5 or SHA-256 authentication should be used for EIGRP packets sent and received on that specific interface. The complete syntax is ip authentication mode eigrp autonomous-system-number [md5 | sha-256], where the autonomous system number identifies the EIGRP process and the authentication type is specified as either md5 or sha-256. This command must be accompanied by the ip authentication key-chain eigrp command to specify which key chain contains the authentication keys.
EIGRP authentication provides security by preventing unauthorized routers from forming neighbor relationships and injecting false routing information into the network. Without authentication, any device capable of sending EIGRP packets could potentially become a neighbor and either learn routing information or corrupt routing tables with incorrect data. Authentication ensures that only routers configured with correct authentication credentials can participate in EIGRP operations, protecting the routing infrastructure from malicious attacks or accidental misconfigurations.
Two authentication modes are supported in modern Cisco IOS versions: MD5 and SHA-256. MD5 authentication has been available in EIGRP for many years and provides adequate security for most implementations through message digest algorithms. SHA-256 authentication was introduced in later IOS versions as a more secure alternative with stronger cryptographic protection against various attack vectors. Organizations with high security requirements should prefer SHA-256 when available on their platforms. Both methods use key chains to define authentication keys, allowing flexible key management with validity periods and key rotation capabilities.
Authentication must be configured consistently on both sides of an EIGRP neighbor relationship. Both routers must use the same authentication mode (MD5 or SHA-256) and share identical key values in their key chains. Mismatched authentication configurations prevent adjacency formation because packets fail authentication validation. The key-string value in key chains is case-sensitive and must match exactly between neighbors. Even minor differences in configuration prevent successful authentication.
Key chains provide sophisticated authentication management through multiple keys with different validity periods. This feature enables key rotation without disrupting EIGRP operations. Administrators can configure overlapping validity periods for old and new keys, allowing a transition period where both keys are accepted. After all routers have been configured with the new key, the old key can be removed. This capability enhances security by allowing regular key changes without network downtime.
Configuration verification involves checking that authentication settings match on both neighbors. The show ip eigrp interfaces detail command displays authentication configuration including the mode and key chain name. If neighbors fail to form adjacencies despite apparent connectivity, authentication mismatches are a common cause. Debug commands can show authentication failures if troubleshooting requires deeper analysis. Proper authentication configuration significantly enhances EIGRP security without substantially impacting protocol performance or introducing operational complexity when properly implemented.
Question 71:
What is the purpose of the EIGRP advertised distance?
A) It is the metric from the local router to the destination
B) It is the metric reported by a neighbor to reach a destination
C) It determines the administrative distance
D) It sets the hop count limit
Answer: B
Explanation:
The EIGRP advertised distance, also known as reported distance, is the metric reported by a neighbor to reach a destination network. This value represents the distance from the neighboring router to the destination, excluding the cost of the link between the local router and that neighbor. The advertised distance is critical for the feasibility condition calculation that determines whether a route can serve as a feasible successor. Understanding advertised distance is fundamental to comprehending EIGRP’s loop prevention mechanism and rapid convergence capabilities.
The advertised distance differs fundamentally from the feasible distance. While advertised distance represents only the neighbor’s portion of the path, feasible distance is the total metric from the local router to the destination through that specific neighbor. The feasible distance equals the advertised distance plus the cost of the link between the local router and the advertising neighbor. This relationship means feasible distance is always greater than or equal to advertised distance. The router compares feasible distances across all neighbors to select the best path (successor), while advertised distances are used in the feasibility condition to identify loop-free backup paths.
The feasibility condition is the mathematical foundation of EIGRP’s loop prevention mechanism. A route can only qualify as a feasible successor if its advertised 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 advertised distances, EIGRP mathematically guarantees loop-free routing without requiring complete network topology knowledge. If a neighbor’s advertised distance is less than the local router’s best metric, that neighbor must be using a path that does not include the local router.
When a router receives routing updates from neighbors, each update contains the advertised distance for the included networks. The local router uses these advertised distances both for calculating feasible distances and for evaluating the feasibility condition. Routes that meet the feasibility condition are stored in the topology table as feasible successors, ready for immediate use if the successor fails. This pre-computation of backup paths enables EIGRP’s sub-second convergence capability.
The show ip eigrp topology command displays both feasible distance and advertised distance for routes in the topology table. The output format typically shows feasible distance first, followed by advertised distance in parentheses, such as “FD is 28160 via 192.168.1.2 (28416)” where 28160 is the feasible distance and 28416 is the advertised distance. Analyzing these values helps administrators understand why certain routes qualify as feasible successors while others do not.
Network administrators troubleshooting EIGRP path selection should examine advertised distances to understand EIGRP’s routing decisions. If expected backup routes are not becoming feasible successors, comparing their advertised distances against the successor’s feasible distance reveals whether they meet the feasibility condition. Understanding this relationship between advertised distance, feasible distance, and the feasibility condition is essential for optimizing EIGRP network designs and ensuring desired backup path availability.
Question 72:
Which EIGRP feature allows different routing information for IPv4 and IPv6 in a single instance?
A) Classic mode
B) Named mode
C) Stub mode
D) Wide metric mode
Answer: B
Explanation:
EIGRP named mode allows different routing information for IPv4 and IPv6 in a single instance through its address-family configuration structure. This unified approach represents a significant improvement over classic EIGRP, which requires completely separate processes for IPv4 and IPv6 routing. Named mode organizes configuration hierarchically with distinct address families for different protocols, enabling administrators to manage multi-protocol routing from a centralized configuration context. This design simplifies configuration management and reduces complexity in dual-stack networks where both IPv4 and IPv6 must be supported simultaneously.
The named mode configuration structure includes multiple organizational sections. The address-family section contains protocol-specific configurations for IPv4 or IPv6, including network statements, autonomous system numbers, and address-family-specific parameters. Within each address family, administrators can configure all the routing parameters specific to that protocol while sharing common settings at the instance level. The af-interface section provides interface-specific configurations that apply to particular address families, and the topology section manages topology-specific settings. This hierarchical organization provides clear separation between protocol-specific and shared configurations.
Classic mode EIGRP handles IPv4 and IPv6 as completely separate processes. Administrators must configure router eigrp for IPv4 and ipv6 router eigrp for IPv6, with each process using separate commands and existing in separate configuration contexts. This separation leads to duplicated configuration effort and potential inconsistencies between IPv4 and IPv6 implementations. Parameters that should be consistent across both protocols must be configured twice, increasing administrative burden and the likelihood of configuration errors.
Named mode unifies both protocols under a single router eigrp statement identified by a descriptive name rather than a numeric autonomous system number. Within this single instance, separate address families maintain protocol-specific configurations while sharing common parameters. For example, authentication settings, timer values, and other parameters that should be consistent across protocols can be configured once at the instance level and inherited by all address families. This approach reduces configuration redundancy and ensures consistency.
The transition from classic to named mode can occur without network disruption. 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 number because the on-wire protocol format remains compatible between modes. This compatibility allows phased migration strategies where different routers transition to named mode at different times without affecting overall routing operations.
Understanding named mode’s multi-protocol capabilities helps network administrators plan dual-stack deployments efficiently. By leveraging named mode’s unified configuration approach, organizations can simplify their routing configurations while maintaining complete IPv4 and IPv6 routing functionality. The centralized management, reduced configuration redundancy, and improved organization make named mode the recommended approach for new EIGRP deployments, especially in networks requiring both IPv4 and IPv6 support.
Question 73:
What is the default behavior of EIGRP when multiple equal-cost paths exist?
A) Uses only the first path discovered
B) Installs all paths up to the maximum-paths limit
C) Alternates between paths for each packet
D) Uses the path with the lowest IP address
Answer: B
Explanation:
When multiple equal-cost paths exist to the same destination, EIGRP installs all paths up to the maximum-paths limit in the routing table. This equal-cost load balancing capability allows EIGRP to utilize multiple paths with identical metrics simultaneously, distributing traffic across all available routes. The default maximum-paths value is 4, meaning EIGRP will install up to four equal-cost routes for any given destination. This load balancing improves bandwidth utilization, provides redundancy, and enhances overall network performance by spreading traffic across multiple links instead of limiting forwarding to a single path.
Equal-cost load balancing occurs automatically when EIGRP calculates identical metrics for multiple paths to the same destination. The metric calculation uses bandwidth and delay by default, so paths with the same minimum bandwidth and cumulative delay will have equal costs. When the router identifies multiple equal-cost paths during metric calculation, it designates all of them as successor routes. All successors are then installed in the routing table up to the maximum-paths limit, making them available for forwarding traffic.
Traffic distribution across equal-cost paths depends on the switching method configured on the router. Process switching distributes traffic on a per-packet basis, alternating between available paths for successive packets to the same destination. Fast switching and CEF switching distribute traffic on a per-destination basis, using hash algorithms based on source and destination information to select paths. The per-destination approach maintains packet ordering for individual flows while still achieving load distribution across multiple flows. This behavior is generally preferable for most applications because it prevents packet reordering within flows while still utilizing all available paths.
The maximum-paths value can be modified using the maximum-paths command in EIGRP router configuration mode. The command syntax is maximum-paths number, where number ranges from 1 to 32 on modern Cisco platforms. Setting maximum-paths to 1 effectively disables load balancing, causing only a single best route to be installed even if multiple equal-cost paths exist. Increasing maximum-paths beyond 4 allows more parallel routes when network topology provides many equal-cost paths to destinations.
Equal-cost load balancing differs from unequal-cost load balancing enabled through the variance feature. Equal-cost balancing requires no special configuration beyond ensuring maximum-paths is set appropriately and multiple equal-metric paths exist. Variance allows load balancing across paths with different metrics, requiring explicit variance configuration and adherence to the feasibility condition. Understanding both types of load balancing helps administrators design networks that efficiently utilize available bandwidth.
Network administrators should design topologies to take advantage of equal-cost load balancing when multiple paths are available. Using consistent link speeds and careful subnet design can create scenarios where multiple paths naturally have equal metrics. Verifying load balancing operation involves examining the routing table with show ip route to confirm multiple paths are installed, and monitoring interface statistics to observe traffic distribution across the parallel links.
Question 74:
Which EIGRP packet type initiates route recalculation when no feasible successor exists?
A) Hello
B) Update
C) Query
D) Acknowledgment
Answer: C
Explanation:
The Query packet type initiates route recalculation in EIGRP when no feasible successor exists after a successor route fails. When a router loses its best path to a destination and has no pre-computed backup route available in the topology table, it must actively search for alternative paths. The router enters active state for that destination and sends Query packets to all its EIGRP neighbors, asking whether they have viable routes to the unreachable network. This query process is a fundamental component of the DUAL algorithm that ensures loop-free routing even during complex topology changes.
Query packets contain information about the destination network that has become unreachable. Neighbors receiving Query packets must respond with Reply packets indicating whether they have paths to the queried destination. If a neighbor has a route, the Reply includes metric information for that path. If the neighbor has no route, it indicates the destination is unreachable from its perspective. The router that sent the Query must wait for Replies from all queried neighbors before it can complete its route recalculation and select a new successor route.
The query process can propagate through multiple router hops across the network. When a router receives a Query for a destination and does not have a feasible successor for that destination, it may need to propagate the Query to its own neighbors. This cascading creates a diffusing computation where queries spread through the network until they reach routers that have alternative paths or reach the boundaries of the EIGRP domain. All queries must eventually be resolved with Replies flowing back through the network to the original querying router.
The active state during query processing represents a period of route computation where the destination is temporarily unreachable through EIGRP. The route remains in active state until all queries are resolved and a new successor is selected, at which point it transitions back to passive state. The duration of active state directly impacts convergence time and represents the period during which traffic to that destination cannot be forwarded using EIGRP. Minimizing active state duration is critical for maintaining network stability.
Query scoping techniques such as route summarization and stub router configuration limit how far queries propagate through the network. When a router advertises a summary route, it does not propagate queries for specific networks within that summary beyond the summarization point. Stub routers never receive queries from their neighbors, containing queries at the hub level in hub-and-spoke topologies. These techniques significantly reduce convergence time by limiting the number of routers involved in query processing.
Understanding the query mechanism helps administrators troubleshoot convergence issues and stuck-in-active conditions. If routes frequently enter active state or remain active for extended periods, examining query propagation patterns reveals where convergence delays originate. Proper network design with adequate backup paths meeting the feasibility condition prevents most query scenarios, while query scoping techniques minimize convergence impact when queries become necessary. The query process, while more complex than simple feasible successor promotion, ensures EIGRP can find alternative paths even in failure scenarios where pre-computed backups are unavailable.
Question 75:
What is the purpose of EIGRP wide metrics?
A) To support higher bandwidth interfaces
B) To reduce routing table size
C) To improve convergence time
D) To enable authentication
Answer: A
Explanation:
EIGRP wide metrics support higher bandwidth interfaces by using 64-bit values instead of the 32-bit values used in classic EIGRP metrics. As network speeds continue to increase beyond 10 Gigabit Ethernet to 40 Gigabit, 100 Gigabit, and higher, the 32-bit metric calculation used in classic EIGRP approaches its maximum representable values. Wide metrics provide the necessary range and granularity to accurately represent metrics for very high-speed interfaces without hitting calculation limits or losing the ability to distinguish between different path qualities. This enhancement ensures EIGRP remains viable as network infrastructure continues advancing to higher speeds.
The 32-bit metric limitation becomes problematic in networks with very high-speed links. The bandwidth component of EIGRP’s metric calculation is (10^7 / bandwidth in kbps) × 256. For extremely high bandwidth interfaces, this calculation produces very small values that eventually lack sufficient granularity to meaningfully distinguish between paths of different speeds. As the formula approaches its limits, rounding errors and insufficient precision make it difficult for EIGRP to select optimal paths based on bandwidth differences. Wide metrics solve this problem by expanding the value range available for metric representation.
Wide metrics are available in EIGRP named mode configuration. When configuring named mode EIGRP, administrators can enable wide metrics to take advantage of the expanded metric range. Classic mode EIGRP continues using 32-bit metrics for backward compatibility. The on-wire protocol remains compatible between routers using different metric widths because EIGRP scales metrics appropriately when forming adjacencies between routers with different capabilities. This compatibility allows gradual migration to wide metrics without requiring simultaneous network-wide upgrades.
In addition to supporting higher bandwidth interfaces, wide metrics provide better granularity across all interface speeds. The increased precision allows EIGRP to make finer distinctions between paths, potentially improving path selection decisions. The expanded range also provides headroom for future network growth, ensuring that metric calculations remain accurate as organizations continue upgrading their infrastructure to higher-speed technologies. Wide metrics essentially future-proof EIGRP metric calculations against continuing bandwidth increases.
Network administrators deploying or upgrading to very high-speed infrastructure should consider implementing named mode with wide metrics. This configuration ensures that EIGRP can accurately calculate and compare metrics for 40 Gigabit, 100 Gigabit, and future higher-speed interfaces. Without wide metrics, EIGRP’s ability to distinguish between different high-speed paths diminishes, potentially leading to suboptimal routing decisions. The transition to wide metrics is straightforward in named mode and provides long-term benefits for network scalability.
Understanding wide metrics helps administrators plan EIGRP deployments in modern high-speed networks. As organizations invest in faster infrastructure to support increasing bandwidth demands, the routing protocol must accurately represent these capabilities in its metric calculations. Wide metrics ensure that EIGRP continues providing optimal path selection regardless of how fast network speeds become, maintaining the protocol’s effectiveness and relevance in contemporary and future network environments where 100 Gigabit and higher speeds become commonplace.