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Question 211:
What EIGRP feature prevents routes from being advertised back to source?
A) Route filtering
B) Split horizon
C) Stub routing
D) Distribute lists
Answer: B
Explanation:
EIGRP split horizon prevents routes from being advertised back to the neighbor from which they were learned implementing a fundamental loop prevention principle. This rule recognizes that sending routing information about a network back to the neighbor that originally provided that information serves no useful purpose and could contribute to routing loops during network changes or failures. Split horizon tracks the incoming interface for each route and automatically suppresses advertisements of that route out the same interface.
Split horizon operates as default behavior in EIGRP without requiring explicit configuration. As EIGRP processes routing updates from neighbors, it records which interface each route was learned from. When preparing routing updates to send to neighbors, split horizon logic checks each route’s incoming interface. If a route was learned from the same interface where an update is being prepared, that specific route is excluded from the update. This suppression occurs transparently improving efficiency.
Split horizon is particularly effective in simple topologies where its basic principle prevents common loop scenarios. On point-to-point links, advertising routes back to the neighbor that provided them serves no purpose and split horizon efficiently eliminates this wasteful traffic. Across large routing tables and many interfaces, split horizon meaningfully reduces protocol overhead while providing loop prevention benefits.
Certain network topologies require split horizon to be disabled for proper operation. Hub-and-spoke Frame Relay and other non-broadcast multi-access topologies where multiple remote sites connect through a single interface on a hub router are classic examples. In these configurations, the hub learns routes from different spoke routers through the same physical interface and split horizon would prevent necessary route propagation between spokes.
Disabling split horizon uses the no ip split-horizon eigrp command on specific interfaces typically on the hub interface in NBMA hub-and-spoke topologies. This configuration allows necessary route propagation while accepting increased risk of routing loops that must be managed through careful network design.
Split horizon works in conjunction with EIGRP’s more advanced loop prevention mechanisms including the feasibility condition and DUAL algorithm.
Question 212:
Which EIGRP metric component represents link errors?
A) Delay
B) Load
C) Reliability
D) MTU
Answer: C
Explanation:
The EIGRP reliability metric component represents link errors and interface dependability based on error rates and keepalive reception. Reliability is expressed as a fraction of 255 where 255 out of 255 indicates perfect reliability with no errors while lower values indicate degraded link quality with higher error rates. The reliability value reflects the dependability of a link based on observed packet loss, bit errors, CRC errors, and keepalive success rates measured over time.
Reliability is one of the five EIGRP metric components that can be weighted by K-values K1 through K5. By default, K4 which weights reliability is set to 0 meaning reliability does not contribute to metric calculations. The default K-value configuration uses only bandwidth weighted by K1 and delay weighted by K3. Reliability remains available for inclusion in metrics if administrators modify K4 to a non-zero value though this is rarely recommended.
Including reliability in metric calculations is generally discouraged because reliability values change dynamically based on current link conditions and error rates. If reliability contributes to metrics, routes could change frequently as link error rates fluctuate due to temporary interference, congestion, or environmental conditions. This route flapping creates routing instability where paths continuously switch based on momentary reliability variations degrading overall network performance.
The reliability value is calculated by the router based on observed link performance including error counters, CRC errors, and keepalive success rates. Interfaces with clean error-free operation maintain reliability values near the maximum of 255 out of 255. Interfaces experiencing errors have lower reliability values proportional to the error rate. These values update continuously as the router monitors interface performance.
Delay represents cumulative latency across path interfaces and does not reflect error rates. Load indicates current traffic levels on interfaces based on utilization. MTU represents maximum transmission unit size. Among these components, reliability specifically indicates link dependability based on errors making it the metric component that represents link error characteristics.
Question 213:
What EIGRP command shows interface Hello intervals?
A) show ip eigrp neighbors
B) show ip eigrp interfaces detail
C) show ip protocols
D) show ip route
Answer: B
Explanation:
The show ip eigrp interfaces detail command displays interface Hello intervals along with other comprehensive EIGRP interface-specific settings. This detailed command output shows the configured Hello interval for each EIGRP-enabled interface allowing administrators to verify timer settings and identify any customized configurations. The detail keyword expands the standard interface display to include timing information, authentication settings, and other advanced parameters.
Interface Hello intervals determine how frequently EIGRP sends Hello packets on each specific interface for neighbor discovery and maintenance. Default intervals are 5 seconds on high-bandwidth interfaces and 60 seconds on low-bandwidth NBMA interfaces. The show ip eigrp interfaces detail output displays these values allowing verification that appropriate intervals are configured for each interface type.
The detailed output includes Hello interval information along with hold time values showing the complete timing configuration for neighbor relationship maintenance. Additionally, the output shows next Hello packet transmission timing indicating when the next Hello will be sent. This comprehensive timing information helps administrators understand EIGRP behavior and troubleshoot timing-related issues.
The show ip eigrp neighbors command displays neighbor relationships and shows hold time remaining before neighbors are declared down but does not display the Hello interval configuration. While the hold time value provides indirect indication of timing because it’s typically three times the Hello interval, the neighbors command does not explicitly show Hello interval settings.
The show ip protocols command provides summary information about EIGRP configuration including network statements, administrative distances, and K-values but does not display interface-specific Hello interval settings. Hello intervals are configured per-interface and require interface-level display commands to verify.
The show ip route command displays routing table contents without any protocol timing information. Hello intervals relate to protocol operation and neighbor maintenance rather than routing table entries making the route command inappropriate for viewing timer configurations.
Question 214:
Which EIGRP route state allows immediate packet forwarding?
A) Active
B) Passive
C) Query
D) Transitional
Answer: B
Explanation:
The EIGRP passive route state allows immediate packet forwarding because routes in passive state have valid successor routes installed in the routing table ready for use. When a route is in passive state, the router has successfully calculated the best path, has a successor route available, and is not performing any route recalculation activities. Passive state represents normal stable routing operation where routing information is settled and the network is fully converged for that destination.
Routes in passive state may also have feasible successors pre-computed and stored in the topology table providing immediate backup paths if the successor fails. These pre-qualified backup routes enable sub-second convergence without leaving passive state because feasible successors can be instantly promoted to successor status when primary paths fail. The combination of an active forwarding path and pre-computed backups represents ideal EIGRP routing conditions.
Active state contrasts sharply with passive state indicating ongoing convergence activity where routes cannot be used for forwarding. Routes enter active state when their successors fail and no feasible successors exist requiring execution of the DUAL algorithm’s query process. During active state, the router cannot forward packets to the destination using EIGRP until convergence completes and a new successor is selected. Active state represents impaired connectivity and instability.
The goal of proper EIGRP network design is maintaining routes in passive state with transitions to active state being rare events requiring genuine topology recalculation. Networks with adequate redundancy and properly configured metrics typically have feasible successors for most destinations allowing routes to remain passive even during failures. Only when backup paths are unavailable or fail to meet the feasibility condition do routes enter active state.
Query and transitional are not actual EIGRP route states. Query is a packet type used during active state convergence but not a route state itself. Routes exist in either passive or active states with passive being the desired condition allowing immediate packet forwarding. Understanding this distinction helps administrators interpret EIGRP behavior and maintain stable routing operations.
The goal of proper EIGRP network design is maintaining routes in passive state with transitions to active state being rare events requiring genuine topology recalculation. Networks with adequate redundancy and properly configured metrics typically have feasible successors for most destinations allowing routes to remain passive even during failures. Only when backup paths are unavailable or fail to meet the feasibility condition do routes enter active state.
Query and transitional are not actual EIGRP route states. Query is a packet type used during active state convergence but not a route state itself. Routes exist in either passive or active states with passive being the desired condition allowing immediate packet forwarding. Understanding this distinction helps administrators interpret EIGRP behavior and maintain stable routing operations.
Question 215:
What EIGRP configuration prevents EIGRP on specific interfaces?
A) Stub routing
B) Passive interface
C) Route filtering
D) Network removal
Answer: B
Explanation:
EIGRP passive interface configuration prevents EIGRP operation on specific interfaces by stopping all EIGRP packet transmission and reception including Hello packets. When an interface is configured as passive using the passive-interface command in EIGRP router configuration mode, that interface stops sending Hello packets preventing neighbor adjacencies from forming. The interface also stops processing any EIGRP packets received effectively isolating it from EIGRP protocol operations completely.
Despite preventing EIGRP protocol communication, passive interfaces still advertise their connected networks in EIGRP updates sent out other active interfaces. This behavior 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 these interfaces as passive, administrators prevent unnecessary EIGRP traffic and potential security risks while ensuring the networks are still advertised.
The passive-interface command is configured under EIGRP router configuration mode specifying which interface should be made passive. Alternatively, the passive-interface default command configures all interfaces as passive by default requiring administrators to use no passive-interface commands to selectively enable EIGRP on specific interfaces. This default-deny approach is often preferred for security because it requires explicit configuration to enable EIGRP preventing accidental protocol operation on unintended interfaces.
Passive interfaces provide security benefits by preventing unauthorized routers from forming EIGRP neighbor relationships. Without passive interface configuration on untrusted segments, any device sending EIGRP Hello packets could potentially become a neighbor and either learn routing information or inject false routes. Making untrusted interfaces passive eliminates this risk completely.
Stub routing limits query propagation and route advertisement but still forms neighbor relationships and exchanges routing information. Passive interfaces completely prevent all EIGRP communication including neighbor formation. Route filtering controls which routes are learned or advertised but allows normal protocol operation. Network removal stops advertising specific networks but does not prevent protocol operation on interfaces. Only passive interface configuration completely prevents EIGRP on specified interfaces.
Question 216:
Which EIGRP command shows feasible successors?
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 feasible successors along with successor routes providing comprehensive visibility into all paths EIGRP has evaluated for each destination. This command shows routes that meet the feasibility condition and qualify as pre-computed backup paths ready for immediate use if successor routes fail. The topology table contains both currently active routes and backup routes making it essential for verifying network resilience.
The command output displays each destination network with its feasible distance followed by advertised distance in parentheses, then 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 backups. For each destination, administrators can see all pre-qualified backup paths and verify redundancy.
Feasible successors appear in the topology table with notation indicating they meet the feasibility condition and are available for instant promotion to successor status if needed. The presence of feasible successors for critical destinations indicates the network can achieve rapid sub-second convergence during failures. Routes lacking feasible successors represent potential convergence risks where failures will cause longer outages during active state recalculation.
The show ip route command displays only the routing table which contains successor routes currently installed for packet forwarding. While the routing table shows what paths are actively used, it does not reveal backup routes or feasible successors that exist in the topology table. The routing table represents a subset of topology table information.
The show ip eigrp neighbors command displays neighbor relationships including IP addresses, interfaces, and timing information but does not show route information or feasible successors. Neighbor relationships provide the foundation for EIGRP operation but route details require topology table examination.
The show ip protocols command shows protocol configuration parameters without specific route information or feasible successor details. Understanding that feasible successors are visible only in the topology table helps administrators use appropriate commands during troubleshooting.
Question 217:
What EIGRP metric component is configured not measured?
A) Delay
B) Load
C) Reliability
D) None, all are measured
Answer: A
Explanation:
The EIGRP delay metric component is configured using the delay command rather than being measured from actual packet transmission times. Administrators explicitly set delay values on interfaces using the delay command followed by the value in tens of microseconds. Default delay values are assigned automatically based on interface type but these are configured defaults not measurements of actual latency. This configuration-based approach allows network designers to influence EIGRP path selection through delay manipulation.
Delay configuration provides a powerful mechanism for EIGRP traffic engineering. Since delay values are set through configuration rather than measured from real traffic, administrators can adjust them to influence path selection without affecting actual packet forwarding performance. By increasing delay values on paths that should be less preferred, those routes receive higher EIGRP metrics making them less attractive. This metric manipulation enables sophisticated routing control.
The configured nature of delay differs fundamentally from load and reliability which are measured dynamically based on actual interface conditions. Load represents current interface utilization measured as a fraction of configured bandwidth. Reliability indicates link dependability based on observed error rates, CRC errors, and keepalive success. Both components continuously update based on real-time measurements of interface performance.
Default delay values assigned to different interface types are configured values not measurements. Fast Ethernet interfaces receive a default delay of 100 representing 1000 microseconds, serial interfaces typically have higher defaults, and various other interface types have appropriate configured defaults. These defaults can be modified using the delay command whenever traffic engineering requirements dictate different values.
Understanding that delay is configured rather than measured helps administrators recognize it as a tool for path manipulation. Unlike load and reliability that reflect actual current conditions and change dynamically, delay remains static at configured values until administratively changed. This stability makes delay suitable for traffic engineering while the dynamic nature of load and reliability makes them problematic for metric calculations due to potential route flapping.
Question 218:
Which EIGRP feature requires matching between neighbors?
A) Router ID
B) Autonomous system number
C) Hold timer
D) Maximum-paths
Answer: B
Explanation:
The EIGRP autonomous system number requires matching between neighbors for adjacency formation. The AS number serves as the fundamental identifier for an EIGRP routing domain and must be identical on all routers that should participate in the same EIGRP instance. When routers exchange Hello packets during neighbor discovery, they include their autonomous system numbers. If the AS numbers do not match between potential neighbors, routers refuse to establish adjacencies because they recognize they belong to different routing domains.
The autonomous system number is configured using the router eigrp command followed by the AS number which can range from 1 to 65535. This number must be consistent across all routers in the EIGRP domain. A single router can run multiple EIGRP processes simultaneously by configuring different AS numbers with each process maintaining completely separate neighbor relationships, topology tables, and routing information.
In addition to matching AS numbers, K-values must also be identical between neighbors because they determine how metrics are calculated. If K-values differ, routers cannot consistently evaluate paths and must not exchange routing information. Authentication configuration must match if configured requiring both routers to use the same authentication mode and shared keys. However, among all these requirements, the AS number match is the most fundamental.
Router IDs do not need to match between EIGRP neighbors and should be unique within the EIGRP domain to provide distinct identification for each router. Duplicate router IDs cause administrative confusion but do not prevent adjacency formation. The router ID serves identification purposes rather than being a matching criterion.
Hold timers and maximum-paths are local configuration parameters that affect routing decisions independently on each router without requiring coordination with neighbors. Hold timers determine when each router declares its neighbors down based on local timer expiration. Maximum-paths limits route installation locally. Neither parameter is exchanged or validated for matching during neighbor formation. Only the AS number requires strict matching for EIGRP adjacencies to form successfully.
Question 219:
What is the EIGRP query scope reduction benefit?
A) Larger routing tables
B) Faster convergence
C) More authentication
D) Higher bandwidth usage
Answer: B
Explanation:
EIGRP query scope reduction provides faster convergence as its primary benefit by limiting how many routers must participate in route recalculation when routes enter active state. Query scope reduction is achieved through features like stub routing and route summarization that prevent queries from propagating to network segments where they would not yield useful alternative path information. By containing queries to smaller regions, convergence completes more quickly because fewer routers are involved in the DUAL algorithm’s diffusing computation.
When queries propagate widely throughout a network, each router receiving a query must process it and send a reply. If routers lack feasible successors, they may further propagate queries to their own neighbors creating cascading query waves. The time required for all routers to process queries and all replies to flow back accumulates potentially extending convergence to several seconds or longer in large networks. Query scope reduction dramatically decreases this convergence time.
Stub routing prevents queries from being sent to stub-configured routers typically deployed at spoke sites. Since spoke routers in hub-and-spoke topologies generally cannot provide alternative paths to destinations beyond the hub, preventing query propagation to them eliminates wasted processing without losing any viable path discovery. In networks with dozens or hundreds of spokes, this containment provides substantial convergence time reduction.
Route summarization creates query boundaries at summarization points. When a router advertises a summary route, it does not propagate queries beyond that boundary for specific networks within the summary range. This containment limits query scope to the region where detailed topology knowledge exists preventing queries from propagating throughout the entire EIGRP domain for every route failure.
Query scope reduction does not increase routing table size, in fact summarization often reduces table size. It does not provide authentication which is a separate security feature. It does not increase bandwidth usage, rather it reduces it by preventing unnecessary query packet transmission to routers that cannot contribute useful information. The convergence speed improvement represents the primary and most significant benefit of query scope reduction making it essential for scalable EIGRP deployments.
Question 220:
Which EIGRP packet contains K-values?
A) Update
B) Query
C) Hello
D) Reply
Answer: C
Explanation:
EIGRP Hello packets contain K-values 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. By including K-values in Hello packets, EIGRP ensures that neighboring routers use consistent metric calculation methods preventing routing inconsistencies that could arise from different routers evaluating paths differently.
When two EIGRP routers exchange Hello packets during the neighbor discovery process, they compare their K-values as part of adjacency formation. 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 make conflicting routing decisions potentially leading to routing loops or suboptimal path selection.
The default K-values are K1 equals 1, K2 equals 0, K3 equals 1, K4 equals 0, and K5 equals 0 meaning only bandwidth and delay contribute to default metric calculations. 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.
Update packets contain routing information including destination networks, metrics, and next-hop addresses but 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 are used during route recalculation and contain metric information for specific destinations being queried but not K-values. These packets contain metrics 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.
Question 221:
What EIGRP configuration enables authentication?
A) Key chains only
B) Authentication mode only
C) Both authentication mode and key chain
D) Password command
Answer: C
Explanation:
EIGRP authentication requires both authentication mode configuration and key chain configuration to function properly. The authentication mode specifies whether MD5 or SHA-256 hashing should be used via the ip authentication mode eigrp command on interfaces. The key chain provides the actual authentication keys through the ip authentication key-chain eigrp command linking interfaces to configured key chains. Both components must be properly configured for authentication to protect EIGRP communications.
Authentication mode configuration alone is insufficient because it only specifies the cryptographic algorithm without providing the actual keys used for authentication. The ip authentication mode eigrp command with md5 or sha-256 parameter tells the router which hashing algorithm to use but without keys from a key chain, the router cannot generate or verify authentication hashes. The mode selection determines cryptographic strength but keys provide the actual shared secrets.
Key chains alone are also insufficient because without authentication mode configuration, EIGRP does not know to authenticate packets or which algorithm to use. Key chains can be defined in global configuration containing multiple keys with validity periods, but until authentication mode is configured on interfaces and those interfaces are linked to key chains, the keys are not used. The key chain provides authentication material but the mode activates authentication.
The two-step configuration approach separates the cryptographic method selection from key management. Authentication mode is configured per interface specifying which algorithm protects that interface’s EIGRP traffic. Key chains are defined globally and can be referenced by multiple interfaces allowing centralized key management. The interface then links to the appropriate key chain creating the complete authentication configuration.
Password commands exist in various Cisco configurations but EIGRP specifically uses the key chain mechanism rather than simple password commands. Key chains provide sophisticated features like multiple keys with validity periods enabling graceful key rotation. This flexibility surpasses simple password-based authentication making key chains the appropriate mechanism for EIGRP security.
Both authentication mode and key chain configuration are mandatory for EIGRP authentication making this a two-component requirement that administrators must understand and implement completely for security to function.
Question 222:
Which EIGRP timer prevents neighbor flapping?
A) Hello timer
B) Hold timer
C) Active timer
D) Retransmission timer
Answer: B
Explanation:
The EIGRP hold timer prevents neighbor flapping by providing tolerance for occasional Hello packet loss before declaring neighbors down. The hold timer determines how long a router waits without receiving Hello packets before declaring a neighbor unreachable. By default, the hold timer is three times the Hello interval providing a buffer that allows one or two consecutive Hello packets to be lost due to temporary network conditions without causing neighbor relationship resets.
Neighbor flapping occurs when neighbor relationships repeatedly form and tear down causing routing instability as routes are continuously added and removed from routing tables. This disruptive behavior often results from hold timers that are too short relative to network conditions causing neighbors to be declared down during brief communication interruptions. The default hold timer value provides appropriate tolerance preventing flapping in most network environments.
On high-bandwidth interfaces with 5-second Hello intervals, the default 15-second hold timer allows up to two Hello packets to be lost without triggering neighbor failure detection. This tolerance accommodates temporary packet loss due to congestion, buffer overflows, or brief link quality degradation without causing routing disruption. Networks with occasional transient issues benefit from this built-in tolerance maintaining stable neighbor relationships.
If hold timers are configured too short, minor network issues cause repeated neighbor failures and recoveries creating constant routing churn. Each neighbor reset causes removal of all routes learned from that neighbor followed by route recalculation and re-learning when the relationship re-establishes. This continuous convergence activity degrades network performance and stability. Appropriate hold timer values prevent this problematic behavior.
The Hello timer controls how frequently Hello packets are sent for neighbor discovery and maintenance but does not directly prevent flapping. Shorter Hello intervals enable faster failure detection but without corresponding hold timer adjustments could increase flapping sensitivity. The active timer limits how long routes can remain in active state but does not affect neighbor relationship stability. The retransmission timer controls reliable packet retransmission timing without affecting neighbor relationships. Only the hold timer provides the tolerance buffer that prevents neighbor flapping from transient issues.
Question 223:
What EIGRP feature allows IPv4 and IPv6 in one process?
A) Classic mode
B) Named mode
C) Hybrid mode
D) Dual-protocol mode
Answer: B
Explanation:
EIGRP named mode allows both IPv4 and IPv6 routing in one process through its address-family configuration structure. This unified approach represents a significant improvement over classic EIGRP which requires completely separate processes for IPv4 using router eigrp and IPv6 using ipv6 router eigrp. Named mode organizes configuration hierarchically with distinct address families for different protocols enabling centralized management 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 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 reduces configuration redundancy for parameters that should be consistent across protocols.
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 providing flexibility while maintaining logical grouping.
Classic mode requires separate EIGRP processes for IPv4 and IPv6 with completely independent configurations. Parameters that should be consistent across protocols must be configured twice increasing administrative burden and potential for errors. Named mode eliminates this redundancy by providing a single unified process managing both protocol versions from one configuration context.
The transition from classic to named mode supports gradual migration. 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 maintains backward compatibility. This compatibility allows phased migration without disrupting routing operations. Named mode’s multi-protocol capability makes it the recommended approach for dual-stack networks.
Question 224:
Which EIGRP command displays wide metric support?
A) show ip eigrp topology
B) show ip protocols
C) show ip eigrp neighbors
D) show version
Answer: B
Explanation:
The show ip protocols command displays wide metric support status for EIGRP along with other protocol configuration parameters. This command shows whether the EIGRP instance is using traditional 32-bit metrics or enhanced 64-bit wide metrics providing visibility into the metric calculation framework being employed. Wide metrics are available in EIGRP named mode and provide expanded range necessary for accurately representing metrics on very high-speed interfaces.
Wide metrics use 64-bit values instead of the 32-bit values used in classic EIGRP providing the range and granularity needed to handle increasingly fast network interfaces. As network speeds advance beyond 10 Gigabit to 40 Gigabit, 100 Gigabit, and higher, traditional 32-bit metrics approach their representable limits. Wide metrics solve this problem by providing expanded numerical range preventing metric calculations from reaching maximum values.
The show ip protocols output includes information about EIGRP operational mode indicating whether named mode with wide metrics is active or whether classic mode with traditional metrics is being used. This visibility helps administrators verify that appropriate metric calculations are configured for their network infrastructure particularly when deploying very high-speed interfaces requiring wide metric support.
In addition to metric width information, show ip protocols displays comprehensive EIGRP configuration including autonomous system number, K-values, administrative distances, network statements, and various other parameters. This consolidated view makes it the primary command for verifying overall EIGRP configuration and operational characteristics including advanced features like wide metrics.
The show ip eigrp topology command displays routing information in the topology table with metric values but does not explicitly show whether wide metrics are enabled or what metric framework is being used. The show ip eigrp neighbors command displays neighbor relationships without configuration details. The show version command shows router hardware and software information without protocol-specific configuration details. Only show ip protocols provides explicit visibility into wide metric support and EIGRP operational mode.
Question 225:
What is the EIGRP administrative distance for summary routes?
A) 1
B) 5
C) 90
D) 170
Answer: B
Explanation:
The EIGRP administrative distance for summary routes is 5, which applies specifically to the Null0 routes automatically created when summary routes are configured. When a router advertises a summary route using the ip summary-address eigrp command, it simultaneously creates a local route to the Null0 interface for the summary address range with an administrative distance of 5. This Null0 route serves as a critical loop prevention mechanism by providing a discard path for packets destined to addresses within the summary range that do not correspond to actual existing subnets.
The administrative distance of 5 is strategically chosen to create appropriate routing preferences within the routing table hierarchy. It is lower than EIGRP internal routes which have an administrative distance of 90 and EIGRP external routes with administrative distance of 170. This lower value ensures the Null0 summary route takes precedence over any EIGRP-learned routes preventing more specific EIGRP routes from overriding the summary’s loop prevention function.
However, the administrative distance of 5 is higher than connected routes with administrative distance of 0 and most static routes with default administrative distance of 1. This ordering allows more specific routes to override the summary when they exist which is precisely the desired behavior. If a specific subnet within the summary range exists as a connected route or static route, that more specific entry is used for forwarding rather than the Null0 discard route.