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Question 181:
What EIGRP command displays routes in the topology table?
A) show ip route
B) show ip eigrp topology
C) show ip eigrp neighbors
D) show ip protocols
Answer: B
Explanation:
The show ip eigrp topology command displays routes in the EIGRP topology table, providing comprehensive visibility into all paths EIGRP has learned to each destination network. This command shows successor and feasible successor routes by default, along with their metrics, next-hop information, and route states. The topology table contains more information than the routing table because it stores all learned routes from all neighbors, not just the best routes installed for forwarding. This complete view enables administrators to understand EIGRP’s knowledge of network topology and available paths.
The command output displays each destination network with its feasible distance shown first, followed by the advertised distance in parentheses, then the next-hop router and outgoing interface. Routes are marked with indicators showing whether they are successors currently used for forwarding or feasible successors available as backup paths. This format provides essential information for understanding EIGRP path selection decisions and verifying that backup routes exist for important destinations requiring high availability.
Additional command options provide different levels of detail about topology table contents. The basic show ip eigrp topology command shows only successor and feasible successor routes that meet the feasibility condition. Adding the all-links keyword displays even routes that do not meet the feasibility condition, showing every path EIGRP has learned regardless of whether it qualifies as a potential backup. The detail keyword expands output to include complete metric component breakdown showing bandwidth, delay, reliability, load, and MTU values used in calculations.
The topology table serves as the foundation for EIGRP’s fast convergence capabilities. By maintaining comprehensive information about all available paths and pre-computing which backup routes are loop-free through the feasibility condition, the topology table enables instant failover when primary paths fail. This pre-computation strategy moves complex processing to stable periods, so backup paths are ready when needed during critical failure moments requiring immediate convergence.
Understanding how to examine the topology table and interpret its contents is fundamental to EIGRP administration and troubleshooting.
Question 182:
Which EIGRP feature allows key rotation without disruption?
A) Multiple passwords
B) Key chains with validity periods
C) Authentication mode switching
D) Router ID changes
Answer: B
Explanation:
EIGRP key chains with validity periods allow key rotation without disruption by supporting multiple keys with overlapping validity periods. Key chains can contain multiple authentication keys, each with configurable send-lifetime and accept-lifetime parameters that specify when each key is valid for sending and receiving authenticated packets. By configuring new keys with future validity periods while maintaining old keys with extended accept lifetimes, administrators can rotate keys gradually without causing neighbor relationship disruptions or routing protocol failures that would impact network operations.
The key rotation process leverages overlapping validity periods to ensure continuous authentication during transitions. Administrators configure new keys with send-lifetime values that begin in the future and accept-lifetime values that start immediately or slightly before the send-lifetime. Old keys remain configured with accept-lifetime values extending past when new keys become active for sending. During the overlap period, routers send packets authenticated with new keys while still accepting packets authenticated with either old or new keys from neighbors.
This graceful transition strategy allows key rotation across multiple routers without requiring simultaneous configuration changes on all devices. As each router is configured with new keys, it begins sending packets with new key authentication while still accepting old key authentication from neighbors not yet updated. After all routers have been configured with new keys and the old key accept-lifetime has expired, the old keys can be removed from key chains without any impact on routing operations or neighbor relationships.
Key chain configuration flexibility enables sophisticated key management strategies appropriate for high-security environments. Organizations can implement regular key rotation schedules to limit the window of compromise if keys are exposed through security breaches. The ability to rotate keys without disrupting routing operations makes regular rotation practical rather than a disruptive event requiring maintenance windows and coordinated downtime across the network infrastructure.
Understanding key chains and their rotation capabilities helps administrators implement strong EIGRP security with regular key updates that maintain routing stability.
Question 183:
What is the EIGRP multicast address for Hello packets?
A) 224.0.0.5
B) 224.0.0.9
C) 224.0.0.10
D) 224.0.0.13
Answer: C
Explanation:
EIGRP uses the multicast address 224.0.0.10 for Hello packets and other EIGRP communications on multicast-capable networks. This well-known multicast address is specifically assigned to EIGRP by IANA for IPv4 routing protocol communications. When EIGRP routers send Hello packets on Ethernet and other multicast-capable interfaces, they address these packets to 224.0.0.10, allowing all EIGRP neighbors on the local segment to receive them simultaneously without requiring individual unicast transmissions to each neighbor device.
Using multicast for EIGRP communications provides several advantages over broadcast or multiple unicast transmissions. Multicast packets are processed only by routers that have joined the specific multicast group, reducing unnecessary processing on non-EIGRP devices. Switches can forward multicast traffic more efficiently than broadcast traffic using IGMP snooping and multicast forwarding tables. This selective delivery improves overall network efficiency and reduces load on devices not participating in EIGRP routing operations.
On point-to-point interfaces or networks where multicast is not supported, EIGRP automatically reverts to using unicast transmissions. Each neighbor is sent packets directly using unicast addresses instead of multicast. This fallback behavior ensures EIGRP operates correctly across various network types including Frame Relay, ATM, and other non-broadcast multi-access technologies where multicast functionality may be limited or unavailable due to infrastructure constraints.
The 224.0.0.0/24 address range is reserved for local network control traffic and is not forwarded by routers beyond the local broadcast domain. This characteristic ensures that EIGRP multicast packets remain within the local network segment and do not propagate beyond directly connected networks. EIGRP’s use of protocol number 89 and multicast address 224.0.0.10 makes it easily identifiable in packet captures and network monitoring tools during troubleshooting activities.
Other routing protocols use different multicast addresses within the 224.0.0.0/24 range for their communications. OSPF uses 224.0.0.5 for all OSPF routers and 224.0.0.6 for designated routers. RIPv2 uses 224.0.0.9 for routing updates.
Question 184:
Which EIGRP timer prevents indefinite active state?
A) Hello timer
B) Hold timer
C) Active timer
D) Retransmission timer
Answer: C
Explanation:
The EIGRP active timer prevents indefinite active state by limiting how long a route can remain in active state before the router declares a stuck-in-active condition. When a route enters active state because the successor has failed without available feasible successors, the router sends Query packets to neighbors and starts the active timer. The default active timer value is 180 seconds or three minutes. If all Reply packets are not received before this timer expires, the router declares a stuck-in-active condition for that route and resets neighbor relationships with routers that failed to reply.
The active timer serves as a critical safety mechanism preventing routes from remaining indefinitely in active state when query processes encounter problems. Without this timer, routes could remain active forever if query or reply packets are lost, filtered, or if routing loops prevent proper query resolution. By imposing a time limit on active state, the timer ensures that problematic situations eventually trigger corrective action through neighbor relationship resets rather than leaving destinations unreachable indefinitely.
When the active timer expires, significant network disruption occurs as the router resets relationships with non-responding neighbors. All routes learned from those neighbors are removed from the routing and topology tables, forcing recalculation of potentially many routes. This drastic action is necessary because the router cannot determine why neighbors failed to reply and must assume communication failures requiring relationship re-establishment to restore proper routing operations.
Modern EIGRP implementations include SIA-Query and SIA-Reply mechanisms designed to prevent premature active timer expiration. At approximately half the active timer interval, the router sends SIA-Query messages to neighbors that have not yet replied to the original Query. Neighbors respond with SIA-Reply messages indicating they are still processing and need more time. This mechanism allows slow but legitimate convergence to complete without triggering stuck-in-active conditions unnecessarily.
Network administrators can modify the active timer using the timers active-time command in EIGRP router configuration mode, though changes should be made cautiously. Increasing the timer might mask underlying problems rather than solving them, while decreasing it could cause premature stuck-in-active declarations in legitimately slow-converging networks.
Question 185:
What EIGRP configuration enables both IPv4 and IPv6 in one instance?
A) Classic mode
B) Named mode
C) Hybrid mode
D) Dual-stack mode
Answer: B
Explanation:
EIGRP named mode enables both IPv4 and IPv6 in one instance through its address-family configuration structure. This unified approach represents a significant improvement over classic EIGRP configuration which requires completely separate processes for IPv4 and IPv6. Named mode organizes configuration hierarchically with distinct address families for different protocols, enabling administrators to manage multi-protocol routing from a centralized configuration context while maintaining protocol-specific parameters where necessary.
The address-family structure within named mode allows protocol-specific configurations while sharing common parameters at the instance level. Each address family contains its own network statements, metrics, and protocol-specific settings for either IPv4 or IPv6. Global parameters configured at the instance level apply to all address families unless overridden within specific address-family contexts. This inheritance model reduces configuration redundancy for parameters that should be consistent across protocols such as authentication settings or timer values.
Named mode configuration begins with the router eigrp command followed by a descriptive instance name rather than a numeric autonomous system number. Within this instance context, administrators enter address-family ipv4 autonomous-system or address-family ipv6 autonomous-system to configure protocol-specific parameters. The autonomous system number is configured within each address family rather than globally, providing flexibility for scenarios requiring different AS numbers for different protocols though typically the same AS number is used.
Classic mode handles each protocol version as a completely independent EIGRP process requiring separate router eigrp and ipv6 router eigrp configurations. Parameters that should be consistent across protocols must be configured multiple times, increasing administrative burden and potential for configuration errors or inconsistencies between protocol implementations.
The transition from classic to named mode supports gradual migration strategies. Both configuration modes can coexist on the same router using different EIGRP instances without interference. 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 maintains backward compatibility.
Question 186:
Which EIGRP metric component uses only the worst link value?
A) Delay
B) Bandwidth
C) Reliability
D) Load
Answer: B
Explanation:
The EIGRP bandwidth metric component uses only the worst link value along the path, specifically the minimum bandwidth encountered across all interfaces in the route. This approach ensures EIGRP identifies and accounts for bottlenecks that limit overall path capacity. When calculating the bandwidth component of a route’s metric, EIGRP examines the bandwidth values of all interfaces in the path and uses the lowest value found. This methodology reflects the reality that a path’s effective throughput capacity is limited by its slowest link regardless of how fast other segments might be.
The bandwidth component calculation uses the formula ten million divided by the minimum bandwidth in kilobits per second, multiplied by 256. This formula produces higher metric values for lower bandwidth paths and lower metric values for higher bandwidth paths, causing EIGRP to prefer routes with greater capacity. For example, if a path includes both Gigabit Ethernet segments with one million kbps bandwidth and a T1 segment with 1544 kbps bandwidth, the bandwidth component uses the T1 value because it represents the bottleneck limiting the entire path’s effective throughput.
This minimum bandwidth approach accurately represents path capacity limitations in real-world network scenarios. Even if most of a path consists of high-speed links, a single low-bandwidth segment constrains the entire path’s throughput to that segment’s capacity. Using the minimum bandwidth for metric calculation ensures EIGRP recognizes these constraints and factors them appropriately into path selection decisions. Routes with bottlenecks receive higher metrics making them less preferable compared to paths with consistently high bandwidth throughout.
The bandwidth value used in calculations is the interface bandwidth configured with the bandwidth command, not necessarily the actual physical interface speed. This configuration-based approach allows administrators to influence EIGRP path selection by adjusting bandwidth values without changing physical infrastructure. By setting bandwidth values higher or lower on specific interfaces, administrators can make paths more or less attractive to EIGRP for traffic engineering purposes.
Delay operates differently by summing values across all interfaces rather than using only the minimum or maximum value.
Question 187:
What EIGRP feature creates summary routes automatically?
A) Auto-summary
B) Manual summarization
C) Route aggregation
D) Network statements
Answer: A
Explanation:
EIGRP auto-summary creates summary routes automatically at classful network boundaries when enabled. This feature was originally enabled by default in EIGRP to reduce routing table size and maintain compatibility with classful routing paradigms. When auto-summary is active, EIGRP automatically creates summary routes at major network boundaries, advertising only classful network addresses rather than specific subnets. While this behavior reduced routing information in simple contiguous networks, it creates fundamental problems in modern networks with discontiguous addressing or VLSM implementations.
Automatic summarization operates without administrator intervention once enabled, summarizing all subnet information to classful boundaries as routes are advertised. For example, if a router has subnets 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24, automatic summarization causes the router to advertise only 172.16.0.0/16 at classful boundaries. This aggregation reduces the amount of routing information propagated through the network but loses the specific subnet detail necessary for correct routing in complex topologies.
Modern Cisco IOS versions disable automatic summarization by default, recognizing that contemporary networks almost universally use VLSM and CIDR addressing requiring preservation of specific subnet mask information. However, understanding auto-summary behavior remains important because it may be encountered in networks with older configurations or when working with legacy equipment. The command no auto-summary explicitly disables this feature in EIGRP router configuration mode.
Automatic summarization causes severe problems in discontiguous network topologies where the same major network number appears in multiple locations with different subnets. With auto-summary enabled, both locations advertise identical classful summaries, creating ambiguity where routers cannot determine which direction leads to specific subnets within the summarized range. This ambiguity causes packet loss and connectivity failures that can be difficult to diagnose.
Manual summarization using the ip summary-address eigrp command provides controlled aggregation at strategic network locations chosen by administrators. Unlike automatic summarization’s rigid classful boundaries, manual summarization offers flexibility to create summaries at any prefix length aligned with network hierarchy. Manual summarization is the recommended approach for modern EIGRP deployments requiring route aggregation benefits.
Question 188:
Which EIGRP command modifies the administrative distance?
A) distance eigrp
B) administrative-distance
C) eigrp distance
D) metric distance
Answer: A
Explanation:
The distance eigrp command modifies the administrative distance for EIGRP routes, allowing customization of route preference when multiple routing protocols advertise the same destination. This command is configured in EIGRP router configuration mode and allows separate specification of administrative distances for internal routes and external routes. The syntax is distance eigrp internal-distance external-distance where the first value sets the administrative distance for internal EIGRP routes and the second value sets it for external routes redistributed into EIGRP.
Administrative distance represents the trustworthiness of routing information sources, with lower values indicating higher preference. By default, EIGRP internal routes have an administrative distance of 90 making them more preferred than OSPF routes with 110 and RIP routes with 120. EIGRP external routes have a default administrative distance of 170 making them less preferred than most other routing protocols. These default values work well for most networks but can be customized when specific routing policy requirements dictate different preferences.
Modifying administrative distance is sometimes necessary during network migrations when implementing routing policies or when certain paths should be preferred despite protocol defaults. For example, during a migration from RIP to EIGRP, an administrator might temporarily increase EIGRP’s administrative distance to allow RIP routes to be preferred while validating EIGRP operation before fully transitioning. Once validation is complete, the administrative distance can be restored to default values.
The distance eigrp command affects only EIGRP route preferences globally for the entire EIGRP process. For more granular control over specific routes or sources, the distance command can be used with access lists to modify administrative distance for particular destinations or route sources. This finer control enables sophisticated routing policies where different routes receive different preference treatments based on their characteristics or origins.
Administrative distance has only local significance on each router and is not exchanged between routers during routing protocol operation. Each router independently evaluates administrative distance when selecting routes from available options. This local nature means different routers could potentially make different routing decisions if their administrative distance configurations differ, requiring careful planning to ensure consistent behavior.
Question 189:
What is the EIGRP feasibility condition purpose?
A) Calculate metrics
B) Prevent routing loops
C) Determine hold timers
D) Configure authentication
Answer: B
Explanation:
The EIGRP feasibility condition prevents routing loops by mathematically determining which backup routes are guaranteed to be loop-free without requiring complete network topology knowledge. The feasibility condition states that 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 mathematical relationship guarantees that the backup path does not loop through the local router because the neighbor advertising the route has found a better path than routing through the local router would provide.
The mathematical proof behind the feasibility condition is straightforward and elegant. If a neighbor’s advertised distance to a destination is less than the local router’s best metric to that destination, the neighbor must be using a path that does not include the local router. This logical relationship provides certainty about loop freedom without requiring the local router to know the complete network topology or trace the exact path used by the neighbor. The feasibility condition’s simplicity and mathematical certainty make it a powerful loop prevention mechanism.
Routes meeting the feasibility condition are identified as feasible successors and stored in the topology table as pre-qualified backup paths. These routes can be used immediately when primary paths fail without any risk of creating routing loops during the failover process. The pre-qualification through the feasibility condition enables EIGRP’s sub-second convergence capability while maintaining absolute loop freedom throughout topology changes. This combination of rapid convergence and guaranteed loop prevention represents one of EIGRP’s most significant innovations over simpler routing protocols.
The feasibility condition’s strictness requires that advertised distance be strictly less than feasible distance, not less than or equal. This strict inequality ensures conservative loop prevention where even routes with advertised distances equal to the feasible distance are excluded from feasible successor status. While such routes might actually be loop-free in many specific scenarios, the strict condition provides mathematical certainty without requiring complex analysis of particular network topologies.
Understanding the feasibility condition and its mathematical basis helps administrators appreciate EIGRP’s sophisticated loop prevention mechanisms and design networks that maximize feasible successor availability.
Question 190:
Which EIGRP packet is sent in response to a Query?
A) Update
B) Reply
C) Acknowledgment
D) Hello
Answer: B
Explanation:
The Reply packet is sent in response to a Query packet in EIGRP’s DUAL algorithm operation. When a router loses its successor route and has no feasible successor available, it sends Query packets to all neighbors asking if they have viable paths to the unreachable destination. Each neighbor receiving a Query must respond with a Reply packet either providing alternative route information if available or indicating that no path exists from its perspective. This Query-Reply exchange is fundamental to DUAL algorithm operation during route recalculation ensuring loop-free path discovery.
Reply packets contain routing information in response to queries. If a neighbor has a route to the queried destination, the Reply includes metric information for that route allowing the querying router to evaluate the path quality. If the neighbor does not have a route to the destination, it sends a Reply indicating the destination is unreachable from its perspective. The router that sent the original Query waits for Replies from all neighbors before completing its route calculation and transitioning the route from active to passive state.
The Query-Reply mechanism can propagate through multiple router hops in the network creating diffusing computations. When a router receives a Query for a destination and does not have a feasible successor, it may propagate the Query to its own neighbors. Those neighbors must then send Replies, and the process continues until all queries are resolved with replies flowing back through the network. This cascading effect can lead to extended convergence times in large networks without proper design considerations like route summarization and stub routing.
The router must receive Reply packets from all queried neighbors before completing active state processing. Only after receiving all Replies can the router evaluate available alternatives and select a new successor route if one exists. The route then transitions from active back to passive state with normal forwarding resuming through the newly selected path. If the active timer expires before all Replies arrive, a stuck-in-active condition occurs requiring neighbor relationship resets.
Understanding the Query-Reply mechanism helps administrators troubleshoot convergence issues and stuck-in-active conditions that may arise in EIGRP networks.
Question 191:
What EIGRP configuration limits route advertisements from spokes?
A) Passive interface
B) Route filtering
C) Stub routing
D) Distribute lists
Answer: C
Explanation:
EIGRP stub routing limits route advertisements from spoke routers in hub-and-spoke topologies by restricting which route types can be advertised to neighbors. Stub routers are configured using the eigrp stub command with options specifying which route categories should be advertised such as connected networks, summary routes, static routes, or redistributed routes. This controlled advertisement prevents spoke routers from inadvertently becoming transit paths between different network segments while still allowing them to advertise their local networks appropriately.
The stub designation is communicated to neighbors during adjacency formation through special flags in Hello packets. When a router knows its neighbor is configured as stub, it understands two important behaviors: the stub will advertise only specific route types based on its stub configuration, and the stub should never receive Query packets during route recalculation. This information allows neighbors to optimize their behavior avoiding attempts to use the stub as a transit path and preventing query propagation that would be unproductive.
Common stub configurations include different combinations of route types appropriate for various spoke router roles. The eigrp stub connected command allows advertising only directly connected networks, suitable for simple spoke sites with local networks but no additional routing information to contribute. Adding summary to create eigrp stub connected summary allows advertising both connected networks and configured summary routes. The redistributed option permits advertising routes imported from other sources while receive-only creates the most restrictive configuration where the stub advertises no routes.
Stub configuration differs from passive interfaces which completely prevent EIGRP protocol communication on specified interfaces stopping all EIGRP packets including Hellos and preventing neighbor formation entirely. Stub routers in contrast form complete neighbor adjacencies and participate in normal EIGRP operations aside from their restricted route advertisement and query behavior. This distinction makes stub configuration more appropriate for spoke routers that need full routing information.
Network designers should carefully plan stub router deployment ensuring the configuration aligns with topology requirements. Routers configured as stubs cannot serve as transit paths between other EIGRP neighbors because of their limited route advertisements.
Question 192:
Which EIGRP feature provides proportional traffic distribution?
A) Equal-cost load balancing
B) Variance
C) Traffic-share count
D) Maximum-paths
Answer: C
Explanation:
The EIGRP traffic-share count feature provides proportional traffic distribution across multiple paths with different metrics when unequal-cost load balancing is enabled. Traffic share counts are automatically calculated by EIGRP based on the relative metrics of paths being used simultaneously for load balancing. These counts specify how traffic should be distributed proportionally ensuring that routes with better metrics receive more traffic than routes with worse metrics. This intelligent distribution optimizes bandwidth utilization while preferring higher-quality paths.
Traffic share count calculation is based on the inverse relationship between metric and traffic proportion. Routes with lower metrics meaning better paths receive higher traffic share counts indicating more traffic should be directed to these preferred routes. For example, if one route has a metric of 1000 and another has a metric of 2000, the first route receives twice as much traffic as the second route. This proportional distribution ensures efficient use of all available bandwidth while still preferring better-quality paths for the majority of traffic.
The traffic share count is visible in the output of the show ip route command when multiple EIGRP routes to the same destination are installed in the routing table for load balancing. Each route displays its traffic share count indicating the relative proportion of traffic it will carry. Network administrators can use this information to verify that load balancing is operating as expected and that traffic distribution aligns with path quality as determined by EIGRP metric calculations.
EIGRP’s automatic calculation of traffic share counts eliminates the need for manual traffic distribution configuration. The protocol intelligently determines appropriate traffic proportions based on metric calculations, adapting automatically to changes in network conditions. If path metrics change due to bandwidth modifications or topology changes, traffic share counts are recalculated to maintain optimal traffic distribution reflecting current path characteristics.
The traffic share count mechanism works only when variance is configured to allow unequal-cost load balancing and when multiple qualifying routes exist. With variance set to the default value of 1, only equal-cost load balancing occurs and traffic is distributed evenly regardless of metric differences.
Question 193:
What EIGRP timer determines query response timeout?
A) Hello timer
B) Hold timer
C) Active timer
D) Query timer
Answer: C
Explanation:
The EIGRP active timer determines query response timeout by limiting how long a router waits for Reply packets in response to Query packets before declaring a stuck-in-active condition. When a route enters active state because the successor has failed without available feasible successors, the router sends Query packets to all neighbors and starts the active timer. The default active timer value is 180 seconds or three minutes. If all Reply packets are not received from queried neighbors before this timer expires, the router declares the route stuck-in-active and resets neighbor relationships with routers that failed to reply.
The active timer serves as a critical safety mechanism preventing routes from remaining indefinitely in active state when convergence encounters problems. Without this timer limitation, routes could remain active forever if query or reply packets are lost, filtered by access lists, or if network conditions prevent proper query resolution. By imposing a time limit on active state duration, the timer ensures that problematic situations eventually trigger corrective action through neighbor relationship resets rather than leaving destinations unreachable indefinitely.
When the active timer expires before all replies are received, significant network disruption occurs as the router resets relationships with non-responding neighbors. All routes learned from those neighbors are removed from the routing and topology tables forcing recalculation of potentially many routes throughout the network. This drastic action is necessary because the router cannot determine why neighbors failed to reply and must assume communication failures requiring relationship re-establishment to restore proper routing operations.
Modern EIGRP implementations include SIA-Query and SIA-Reply mechanisms designed to prevent premature active timer expiration during legitimate slow convergence. At approximately half the active timer interval, the router sends SIA-Query messages to neighbors that have not yet replied to the original Query. Neighbors respond with SIA-Reply messages indicating they are still processing the query and need additional time. This mechanism allows slow but legitimate convergence to complete without triggering stuck-in-active conditions unnecessarily.
Network administrators can modify the active timer using the timers active-time command in EIGRP router configuration mode with values ranging from 1 to 65535 minutes.
Question 194:
Which EIGRP command displays packet retransmission counts?
A) show ip eigrp neighbors
B) show ip eigrp topology
C) show ip eigrp traffic
D) show ip protocols
Answer: C
Explanation:
The show ip eigrp traffic command displays packet retransmission counts along with comprehensive EIGRP packet statistics for all packet types. This command shows detailed counters for Hello, Update, Query, Reply, and Acknowledgment packets including both sent and received counts. The retransmission statistics are particularly valuable for troubleshooting because they reveal communication problems between EIGRP neighbors where packets are being lost and requiring repeated transmission attempts before successful delivery.
Retransmission counts indicate how many reliable packets required multiple transmission attempts because acknowledgments were not received within expected timeframes. When retransmission counts are high, it suggests that packets are being lost between routers due to network congestion, faulty hardware, buffer overflows, or link quality degradation. Each retransmission represents a packet that had to be sent multiple times before being acknowledged, indicating underlying network problems that should be investigated to prevent routing instability or neighbor relationship failures.
The traffic statistics also show the relative frequency of different packet types providing insight into EIGRP behavior and network stability. In healthy stable networks, Hello packets should predominate because they are sent continuously for neighbor maintenance every 5 seconds on high-bandwidth links while other packet types appear only during topology changes. Networks with continuously high Update, Query, or Reply packet counts may be experiencing routing instability, frequent topology changes, or configuration problems causing repeated convergence cycles.
Monitoring EIGRP traffic over time helps establish baseline behavior for comparison during troubleshooting. Administrators can periodically capture traffic statistics to understand normal patterns in their specific network environments. When problems occur, comparing current statistics to established baselines quickly reveals abnormal behavior patterns. For example, sudden increases in Query packets might indicate that routes are losing feasible successors and entering active state requiring investigation.
The command output can be cleared using clear ip eigrp traffic resetting all counters to zero. This capability allows administrators to measure traffic patterns over specific time periods rather than since the last router restart.
Question 195:
What is the EIGRP default variance value?
A) 0
B) 1
C) 2
D) 4
Answer: B
Explanation:
The EIGRP default variance value is 1, which allows only equal-cost load balancing where routes must have identical metrics to be used simultaneously for traffic distribution. When variance is set to 1, EIGRP installs multiple routes in the routing table only if they have exactly the same metric as the successor route. This default behavior is similar to how most routing protocols handle multiple paths to destinations, distributing traffic evenly across all paths with identical costs up to the maximum-paths limit.
The variance value of 1 effectively disables unequal-cost load balancing in EIGRP. While EIGRP has the capability to perform unequal-cost load balancing through variance configuration, this advanced feature is not active by default. Network administrators must explicitly configure variance to a value greater than 1 to enable unequal-cost load balancing allowing routes with different metrics to be used simultaneously for traffic distribution based on their relative path quality.
When variance is increased above 1, it acts as a multiplier determining how much worse a backup route’s metric can be compared to the successor route’s metric while still qualifying for load balancing. For example, setting variance to 2 allows EIGRP to use routes with metrics up to twice the successor’s metric for load balancing. Setting variance to 3 allows routes with metrics up to three times the successor’s metric. This flexibility enables utilization of multiple paths with varying qualities.
Important restrictions apply regardless of variance setting. Only routes that satisfy the feasibility condition can be used for load balancing, ensuring loop-free operation. A route with a metric within the variance range but failing the feasibility condition will not be used because it cannot be guaranteed loop-free. This requirement maintains EIGRP’s loop prevention guarantees while enabling multipath utilization.
The default variance of 1 provides conservative behavior where only known equal-quality paths are used for load balancing. This default is appropriate for most networks where administrators prefer explicit control over when and how unequal-cost load balancing is enabled. Organizations wanting to leverage unequal-cost load balancing must consciously configure variance based on their specific network topology and traffic engineering requirements.