Q: OSPF adjacencies without DR/BDR elections With which OSPF network type do routers form adjacencies but do not run a DR/BDR election process?

Introduction / Context: OSPF behavior varies with the underlying network type. DR/BDR elections occur on multiaccess networks to reduce adjacency overhead. On links that inherently have only two endpoints, there is no need for a DR/BDR, yet full adjacencies still form. Recognizing these differences aids design and troubleshooting. Given Data / Assumptions: Network types considered: point-to-point, broadcast multi-access, NBMA. Adjacency formation is required in all OSPF network types. The question asks specifically where DR/BDR is not elected. Concept / Approach: DR/BDR election is only needed when more than two routers share a segment, as on Ethernet (broadcast) or Frame Relay (NBMA). On point-to-point links (for example, serial PPP links or point-to-point subinterfaces), only two routers exist; therefore, OSPF forms a full adjacency without any DR/BDR roles. Step-by-Step Solution: Classify network type: point-to-point means two routers only.Apply OSPF rule: DR/BDR elections occur only on multiaccess segments.Conclude: point-to-point links have no DR/BDR election but do form full adjacencies. Verification / Alternative check: Command show ip ospf interface on a serial PPP link reports “Network Type POINT_TO_POINT, No designated router on this network,” confirming the behavior. Why Other Options Are Wrong: B: Area 0 is a logical construct; DR/BDR depends on network type, not area number. C/D: Both are multiaccess and do run DR/BDR elections. E: Virtual links traverse areas but do not change the underlying network-type rules. Common Pitfalls: Assuming all Ethernet links are point-to-point; Ethernet is broadcast multi-access in OSPF, and elections occur. Final Answer: Point-to-point

Question 1

Where OSPF elects a DR/BDR: Which OSPF network types perform Designated Router (DR) and Backup Designated Router (BDR) elections?

Accepted Answer

Introduction / Context: OSPF uses DR/BDR elections to reduce adjacencies on multiaccess media. Not all network types perform elections; understanding where they occur is key for design and troubleshooting adjacencies. Given Data / Assumptions: (1) Broadcast (2) Point-to-point (3) NBMA (Non-Broadcast Multi-Access) (4) NBMA point-to-point (not an election scenario) Concept / Approach: DR/BDR is used on multiaccess segments: Broadcast Ethernet and NBMA networks (e.g., Frame Relay multipoint). Point-to-point links have only two neighbors, so elections are unnecessary and not performed. Step-by-Step Solution: Identify multiaccess types: Broadcast (1), NBMA (3).Eliminate point-to-point (2) and NBMA point-to-point (4) because there is no multiaccess adjacency explosion to optimize.Select '1 and 3'. Verification / Alternative check: On Ethernet (broadcast), show ip ospf interface shows DR/BDR. On serial point-to-point links, no such election occurs; neighbors form full adjacency directly. Why Other Options Are Wrong: Including point-to-point introduces a type where DR/BDR is not used. 'All of the above' incorrectly assumes elections everywhere. Common Pitfalls: Confusing Frame Relay point-to-point subinterfaces (no election) with multipoint (election); assuming passive interfaces affect DR/BDR—passive controls hellos, not election rules. Final Answer: 1 and 3

Question 2

Routing protocols that support VLSM, summarization, and discontiguous networks: From the list provided, which protocols meet all three capabilities?

Accepted Answer

Introduction / Context: Modern network designs rely on Variable Length Subnet Masking (VLSM), route summarization, and support for discontiguous networks. Protocol choice affects scalability and address efficiency. Given Data / Assumptions: Protocols listed: (1) RIPv1, (2) IGRP, (3) EIGRP, (4) OSPF, (5) BGP, (6) RIPv2. We seek those that support VLSM, summarization, and discontiguous networking. Concept / Approach: Classless protocols that carry subnet masks support VLSM and discontiguous topologies (no automatic classful summarization). EIGRP, OSPF, and RIPv2 are classless and support manual summarization. RIPv1 and IGRP are classful (no VLSM), disqualifying them. Although BGP is also classless, it is not included in the correct option grouping for this question set. Step-by-Step Solution: Eliminate classful: RIPv1 (1), IGRP (2).Retain classless IGPs: EIGRP (3), OSPF (4), RIPv2 (6).Choose combination '3, 4 and 6'. Verification / Alternative check: Configuration examples: EIGRP 'no auto-summary' and manual 'ip summary-address'; OSPF area and ABR summarization; RIPv2 includes subnet masks and supports 'no auto-summary'. These confirm all three capabilities. Why Other Options Are Wrong: Options containing RIPv1/IGRP include protocols that lack VLSM and break on discontiguous networks. 'All of the above' is invalid because not all listed protocols support the required features. Common Pitfalls: Assuming RIPv2 cannot handle discontiguous networks (it can, with auto-summary disabled); forgetting that IGRP is classful and deprecated. Final Answer: 3, 4 and 6

Question 3

Administrative distance (AD) of OSPF: What is the default administrative distance value used by OSPF routes in Cisco IOS?

Accepted Answer

Introduction / Context: Administrative distance (AD) is a Cisco metric that ranks the trustworthiness of a route source. When multiple protocols know a path to the same destination, the route with the lowest AD is installed in the routing table. Given Data / Assumptions: Default Cisco AD values are assumed. Protocol in question: OSPF (an open standard link-state IGP). Concept / Approach: Common defaults: Connected 0, Static 1, eBGP 20, EIGRP internal 90, IGRP 100, OSPF 110, IS-IS 115, RIP 120, EIGRP external 170, iBGP 200. Knowing these helps predict which protocol’s route will win in a tie for prefix length and metric. Step-by-Step Solution: Recall OSPF's default AD = 110.Compare to others to understand selection order if multiple routes exist. Verification / Alternative check: show ip route marks each route source with a code; the installed route aligns with the lowest AD among candidates of equal prefix specificity. Why Other Options Are Wrong: 90 is EIGRP internal; 100 is IGRP; 120 is RIP. These are not the OSPF default. Common Pitfalls: Confusing metric values (cost, hop count) with AD; assuming AD varies per vendor—defaults can differ outside Cisco. Final Answer: 110

Question 4

EIGRP control-plane tables maintained via Hellos/updates: Which EIGRP data structures are kept in RAM and maintained by periodic Hello and triggered update packets?

Accepted Answer

Introduction / Context: Enhanced Interior Gateway Routing Protocol (EIGRP) maintains several tables in memory to track neighbors and routes. Understanding which tables exist clarifies how EIGRP forms adjacencies and computes loop-free paths. Given Data / Assumptions: (1) Neighbor table (2) STP table (3) Topology table (4) DUAL table Concept / Approach: EIGRP uses Hello packets to discover/maintain neighbors (Neighbor table) and updates/queries to exchange route information (Topology table). The Diffusing Update Algorithm (DUAL) is the algorithm run against the topology table; it is not itself a distinct 'table'. STP is a Layer 2 protocol unrelated to EIGRP. Step-by-Step Solution: Select Neighbor table (1) – built/maintained with Hellos.Select Topology table (3) – updated by routing updates/queries/replies.Exclude STP (2) – unrelated.Exclude 'DUAL table' (4) – DUAL is the computation, not a stored table. Verification / Alternative check: Commands: show ip eigrp neighbors and show ip eigrp topology display these structures. No 'DUAL table' command exists; DUAL results are the successor/feasible successor selections within the topology table. Why Other Options Are Wrong: Including STP or a 'DUAL table' introduces items that do not belong to EIGRP’s routing database. Common Pitfalls: Confusing DUAL (algorithm) with a data store; mixing Layer 2 STP with Layer 3 routing protocols. Final Answer: 1 and 3

Question 5

Migrating from IGRP to EIGRP without redistribution: Your network runs IGRP in autonomous system (AS) 10. You want to introduce EIGRP gradually without configuring redistribution. Which single command enables an easy migration path?

Accepted Answer

Introduction / Context: EIGRP is backward compatible with IGRP when the same autonomous system (AS) number is used. This allows smooth coexistence and automatic exchange of routes during migration, avoiding immediate manual redistribution. Given Data / Assumptions: Existing IGRP AS: 10. Goal: begin running EIGRP while minimizing configuration complexity. Desire: avoid explicit redistribution at first. Concept / Approach: By enabling EIGRP with the identical AS number (10), the router engages in automatic route exchange/compatibility features between EIGRP and IGRP. This eases phased migration: neighbors still using IGRP can interoperate while EIGRP is introduced and later fully replaces IGRP. Step-by-Step Solution: Enter routing process: router eigrp 10Activate networks with the same statements used under IGRP.Gradually convert other routers to EIGRP using AS 10. Verification / Alternative check: Use show ip protocols and show ip route to verify learned routes and their codes; watch for EIGRP codes (D) appearing alongside or replacing IGRP codes (I). Why Other Options Are Wrong: Using a different AS (A) breaks the compatibility shortcut; you would need redistribution. (C) explicitly configures redistribution, which the question aims to avoid. (D) is not a valid IOS command. Common Pitfalls: Forgetting to disable IGRP after full migration; neglecting no auto-summary if discontiguous networks exist; mixing AS numbers unintentionally. Final Answer: router eigrp 10

Question 6

Placing network 10.2.3.0/24 into OSPF area 0: Which two commands together achieve this (process ID 10, area 0)?

Accepted Answer

Introduction / Context: To advertise a subnet into OSPF, you must enter the OSPF process and add a matching network statement with an appropriate wildcard mask and area assignment. Precision matters: typos such as 'area0' will not work. Given Data / Assumptions: OSPF process ID: 10. Prefix: 10.2.3.0/24. Goal: place it in area 0. Concept / Approach: The two necessary commands are: (2) router ospf 10 to enter the process, and (5) network 10.2.3.0 0.0.0.255 area 0 to match the /24 and bind it to area 0. Using the classful 'network 10.0.0.0' overmatches and is not specific; 'area0' without a space is invalid syntax. Step-by-Step Solution: Start OSPF: router ospf 10Advertise subnet: network 10.2.3.0 0.0.0.255 area 0 Verification / Alternative check: Run show ip ospf interface and show ip route ospf to confirm that interfaces matching 10.2.3.0/24 join area 0 and routes appear with OSPF codes (O). Why Other Options Are Wrong: (3) network 10.0.0.0 classful match is not the precise /24 specification. (4) 'area0' is syntactically wrong; IOS requires a space: area 0. Common Pitfalls: Using classful network statements unintentionally; forgetting wildcard masks; misplacing the OSPF process ID and area number formatting. Final Answer: 2 and 5

Question 7

Routing protocol selection — classless support with distance-vector behavior A network administrator must configure a router with a distance-vector (or advanced distance-vector) routing protocol that fully supports classless routing with VLSM and CI…

Accepted Answer

Introduction / Context: Choosing the correct interior gateway protocol (IGP) depends on capabilities such as classless routing, route summarization, and operational model (distance-vector vs. link-state). Many legacy protocols are classful and cannot carry variable-length subnet masks (VLSM). This question tests recognition of which distance-vector family protocol supports classless routing in modern IPv4 designs. Given Data / Assumptions: We need a distance-vector or advanced distance-vector protocol. The protocol must support classless routing, VLSM, and CIDR. Candidate protocols listed include IGRP, OSPF, RIPv1, EIGRP, and IS-IS. Concept / Approach: Classless routing protocols carry subnet mask information in updates and therefore support VLSM and CIDR. RIPv1 and IGRP are classful (no mask in updates). EIGRP is an advanced distance-vector (sometimes called hybrid) that includes mask information and supports VLSM and CIDR. OSPF and IS-IS are link-state, not distance-vector. Therefore, among distance-vector options, EIGRP is the correct choice. Step-by-Step Solution: Identify classful protocols: RIPv1 and IGRP → no VLSM/CIDR support.Identify classless protocols: EIGRP, OSPF, IS-IS → include mask in advertisements.Filter by distance-vector family: EIGRP qualifies; OSPF/IS-IS are link-state.Conclude that EIGRP uniquely satisfies both conditions. Verification / Alternative check: Configuration guides show EIGRP supports route summarization, VLSM, unequal-cost load balancing, and includes the subnet mask in its updates. In contrast, IGRP and RIPv1 lack mask carriage and are deprecated for classless designs. Why Other Options Are Wrong: IGRP: Classful; cannot carry masks, no VLSM/CIDR. OSPF: Classless but link-state, not distance-vector. RIPv1: Classful; no subnet mask in updates. IS-IS: Classless link-state; not distance-vector. Common Pitfalls: Confusing protocol families (calling EIGRP link-state) or assuming all modern protocols are link-state. Also, mixing up RIPv2 (classless) with RIPv1 (classful). Final Answer: EIGRP

Question 8

OSPF Router ID selection without a loopback interface If multiple routers in one area share the same OSPF priority and a router has no loopback interface configured, which address is chosen as the OSPF Router ID on that device?

Accepted Answer

Introduction / Context: OSPF requires a 32-bit Router ID (RID) to uniquely identify each router within the OSPF domain. Understanding OSPF RID selection rules helps predict neighbor relationships, LSA origination, and troubleshooting outcomes when loopbacks are absent or misconfigured. Given Data / Assumptions: No loopback interface is configured on the router. Multiple routers use the same OSPF priority (not directly relevant to RID selection but common in exams). OSPF will select a RID automatically if not manually set. Concept / Approach: OSPF selects the Router ID using a deterministic order: (1) the manually configured RID if present; else (2) the highest IP address on any loopback interface; else (3) the highest IP address on any active physical interface. Because the problem specifies no loopback, rule (3) applies. Priority values affect DR/BDR elections on multiaccess segments, not the RID selection process itself. Step-by-Step Solution: Confirm there is no configured RID and no loopback interfaces.Enumerate active physical interfaces and note their IPs.Choose the numerically highest IP address among those physical interfaces.That address becomes the OSPF Router ID until changed or the process restarts with different interface IPs. Verification / Alternative check: Use the command to view RID: show ip ospf; it will display the chosen RID. Adding a loopback with a higher IP and restarting OSPF will change the RID unless statically set. Why Other Options Are Wrong: A/C/D: OSPF selects the highest IP, and loopback takes precedence over physical, but loopbacks are absent in this scenario. E: The startup-config order does not influence RID selection. Common Pitfalls: Assuming the lowest IP is chosen, or confusing DR/BDR priority with RID rules. RID is independent and persists until the OSPF process is cleared or the device reloads. Final Answer: The highest IP address of any physical interface

Question 9

OSPF areas and backbone design — choose the correct statements Consider these statements about OSPF areas:  You must configure separate loopback interfaces in each area.  Area numbers can be assigned up to 65,535.  The backbone area is called area 0…

Accepted Answer

Introduction / Context: OSPF divides large networks into areas to improve scalability, reduce LSDB size, and speed convergence. Understanding the special role of the backbone (area 0) and how areas interconnect is essential for designing or troubleshooting multi-area deployments. Given Data / Assumptions: We evaluate five statements regarding OSPF areas and backbone rules. We assume standard OSPF behavior without vendor-specific deviations. Virtual links may be used to logically connect an area to area 0 when no direct connection exists. Concept / Approach: The backbone is always area 0 and acts as the transit area for inter-area traffic. All other areas must connect to area 0, either directly or via a virtual link. Loopback interfaces are not required per area; they are commonly used for stable Router IDs. Area numbers are 32-bit; 65,535 is a valid decimal value but not the upper bound. Hierarchical designs typically use multiple areas; saying you do not need multiple areas contradicts the point of hierarchy. Step-by-Step Solution: Assess (1): False. Loopbacks are optional and not required per area.Assess (2): Misleading. Area IDs are 32-bit; limiting to 65,535 is not the absolute maximum.Assess (3): True. Area 0 is the backbone.Assess (4): False. Hierarchical OSPF motivates multiple areas.Assess (5): True. All areas must connect to area 0, possibly with a virtual link.Therefore, the correct combination is 3 and 5 only. Verification / Alternative check: Design guides consistently state area 0 as the backbone and require all other areas to be attached to it, directly or logically (virtual link), ensuring proper inter-area route distribution. Why Other Options Are Wrong: B: Includes statement (2), which is not strictly correct as an upper bound claim. C/D/E: Include false statements (1 or 4) or omit requirement (5). Common Pitfalls: Assuming loopbacks are per-area requirements, or misreading the limits on area numbering. Also forgetting virtual links can satisfy the connection-to-backbone rule. Final Answer: 3 and 5 only

Question 10

OSPF process identifier (process ID) — scope and purpose Which statements correctly describe the OSPF process identifier used when enabling OSPF on a router?

Accepted Answer

Introduction / Context: When you start OSPF on a router, you specify a process ID. Engineers often confuse this numeric value with AS numbers or area numbers. Understanding what the process ID does and does not control prevents configuration errors and failed adjacency formation. Given Data / Assumptions: The OSPF process ID is a router-local parameter. Neighbors do not exchange or compare process IDs. Multiple OSPF processes can run on one router for segmentation or migration. Concept / Approach: OSPF’s process ID identifies a local OSPF instance on the router. It is locally significant; different routers can use different process IDs and still form adjacencies, as long as other parameters (area, timers, network statements, authentication) match. The process ID is not optional—you must provide one even if only a single OSPF instance runs—but its numeric value is arbitrary and chosen by the administrator for local organization. Step-by-Step Solution: Recognize that OSPF neighbors never validate one another’s process ID.Confirm that the process ID names the local database instance on the router.Eliminate claims of global significance or matching requirements across neighbors.Conclude that the correct statements are “locally significant” and “needed to identify the local instance.” Verification / Alternative check: Change the process ID on one router and keep all other parameters identical; adjacencies still form. Conversely, mismatched area IDs or authentication prevent adjacency regardless of identical process IDs. Why Other Options Are Wrong: B/D: Incorrect; process ID does not need to match across routers or areas. C: Not optional; you must specify one to enable OSPF. E: LSDB checksums are not derived from process ID. Common Pitfalls: Believing the process ID functions like an AS number in EIGRP/OSPFv3 contexts; it does not. It is a local label only. Final Answer: It is locally significant and is needed to identify a unique local OSPF database instance.

Question 11

OSPF network types — where is a Backup Designated Router (BDR) elected? Identify the OSPF network types that perform DR/BDR elections on multiaccess segments.

Accepted Answer

Introduction / Context: On multiaccess networks, OSPF elects a Designated Router (DR) and Backup DR (BDR) to reduce adjacency count and LSA flooding. Knowing which network types trigger these elections is crucial to predicting neighbor behavior and LSA flow. Given Data / Assumptions: OSPF recognizes several network types: broadcast multi-access (for example, Ethernet), non-broadcast multi-access (NBMA, for example, Frame Relay), and point-to-point. DR/BDR election occurs only on multiaccess segments. Concept / Approach: Broadcast multi-access and NBMA networks are multiaccess; they can connect multiple routers to the same segment. To reduce n*(n−1)/2 adjacency complexity, OSPF elects a DR and BDR on such segments. On point-to-point links (two routers only), there is no election because only a single adjacency exists, and the overhead of elections is unnecessary. Step-by-Step Solution: Classify network types: broadcast and NBMA are multiaccess; point-to-point is not.Apply OSPF behavior: DR/BDR elections occur on multiaccess networks.Select broadcast multi-access and NBMA as the correct types. Verification / Alternative check: Running show ip ospf interface on Ethernet (broadcast) or Frame Relay (NBMA) shows DR/BDR roles; on point-to-point serial links no DR/BDR is elected. Why Other Options Are Wrong: B/C: Point-to-point links do not run DR/BDR elections. D: False; OSPF often elects DR/BDR on multiaccess networks. E: Incomplete; broadcast multi-access is also included. Common Pitfalls: Assuming any Ethernet link is point-to-point; switched Ethernet is broadcast multi-access from OSPF’s perspective and elects DR/BDR. Final Answer: Broadcast multi-access and Non-broadcast multi-access

Question 12

EIGRP data structures — where are successor routes stored? In Enhanced IGRP (EIGRP), identify where successor routes (the best next hops) are retained during normal operation.

Accepted Answer

Introduction / Context: EIGRP maintains three core data structures: the neighbor table (adjacent routers), the topology table (all learned routes with metrics), and the routing table (best routes chosen for forwarding). Understanding which table holds which routes is essential for troubleshooting convergence and reachability. Given Data / Assumptions: Successor routes are the current best next hops to each destination. EIGRP may also keep feasible successors as backups in the topology table. We are focusing on where successor routes reside during steady state. Concept / Approach: By design, EIGRP installs successor routes into the routing table for forwarding, and also records them in the topology table along with other candidates. The neighbor table does not store routes; it stores adjacency information (IP, hold time, SRTT, RTO, queue counts). Feasible successors remain in the topology table and can be promoted if the successor fails and feasibility condition is met. Step-by-Step Solution: Identify successor routes as the currently selected best paths.Confirm their presence in the routing table for actual forwarding.Recognize that the topology table contains all learned paths including the active successor.Therefore, successor routes are in both the routing table and the topology table. Verification / Alternative check: Use show ip route eigrp to view installed successors and show ip eigrp topology to see successors and feasible successors with metrics such as feasible distance and reported distance. Why Other Options Are Wrong: A/C: Each lists only one table; EIGRP uses both. B/E: The neighbor table holds adjacency statistics, not routes. Common Pitfalls: Confusing the neighbor table with the topology table, or assuming feasible successors are always installed in the routing table (they are not unless promoted). Final Answer: In the routing table and the topology table

Question 13

Why design OSPF hierarchically — benefits of multiple areas What are the main reasons for deploying OSPF in a hierarchical, multi-area design?

Accepted Answer

Introduction / Context: As networks grow, link-state protocols can experience large databases and frequent recalculation. OSPF counters this by partitioning the network into areas, with area 0 as the backbone. Understanding the advantages of a hierarchical design is essential for scaling and operational stability. Given Data / Assumptions: We consider standard OSPF multi-area designs (ABRs, area 0). Goals typically include performance, stability, and manageability. Results can vary by topology, but core benefits are well established. Concept / Approach: Hierarchical OSPF localizes flooding and SPF recalculation. When a link changes within an area, only that area’s routers recalculate; ABRs summarize and limit LSAs across area boundaries. This reduces routing overhead, speeds convergence in the wider domain, and confines instability so that a single area’s churn does not impact others. Step-by-Step Solution: Partition the network into multiple areas with area 0 as the core.Use ABRs to limit LSA flooding and optionally summarize routes between areas.Observe reduced LSDB sizes per area and faster SPF runs after localized changes.Note that failures are contained within an area, improving domain-wide stability. Verification / Alternative check: Compare show ip ospf database sizes and SPF timers in single-area vs. multi-area pilots; multi-area deployments show smaller LSDBs per router and fewer domain-wide recomputations after localized events. Why Other Options Are Wrong: B: Understates benefits; convergence and overhead do improve. C/D: Overstate by suggesting universal simplicity; hierarchical design helps but does not guarantee trivial configuration. E: Irrelevant; static routing is not a goal of hierarchical OSPF. Common Pitfalls: Believing any multi-area split automatically simplifies configuration; poor area design can complicate things. Benefits rely on sensible summarization and stable ABR placement. Final Answer: To decrease routing overhead, speed up convergence, and confine instability to single areas

Question 14

EIGRP successor routes — properties and behavior Which statements about EIGRP successor routes are correct?

Accepted Answer

Introduction / Context: EIGRP uses the Diffusing Update Algorithm (DUAL) to select loop-free paths. Understanding the distinction among successor routes, feasible successors, the topology table, and the neighbor table is key to interpreting outputs and convergence behavior. Given Data / Assumptions: Successor = best next-hop path currently used for forwarding. Feasible successor = precomputed backup that satisfies the feasibility condition. Neighbor table = adjacency/transport stats, not routes. Concept / Approach: The successor route is installed in the routing table and also present in the topology table. If a feasible successor exists (reported distance < feasible distance), DUAL can switch to it immediately upon failure of the successor, avoiding queries. Neither successors nor feasible successors live in the neighbor table; neighbors are peers, not routes. Route states are typically passive in steady state; active indicates a recomputation is in progress. Step-by-Step Solution: Identify successor as the forwarding path in steady state.Confirm that feasible successors may back up the successor.Exclude neighbor table from route storage; it holds peer info (SRTT, RTO, Q count).Exclude “active in routing table”; passive is the steady state. Verification / Alternative check: Use show ip eigrp topology to see successors (via code P for passive) and feasible successors, and show ip route eigrp to verify installed successors used for forwarding. Why Other Options Are Wrong: B: Successors are actively used for forwarding, not just backups. C: Successors are passive when healthy; active indicates reconvergence. D/E: Neighbor table is not a routing repository, and successors exist in steady state, not only during queries. Common Pitfalls: Confusing the meanings of passive/active, or assuming a neighbor entry implies a route. DUAL state applies to routes, not to peers. Final Answer: A successor route is used by EIGRP to forward traffic to a destination, and it may be backed up by a feasible successor route.

Question 15

OSPF adjacencies without DR/BDR elections With which OSPF network type do routers form adjacencies but do not run a DR/BDR election process?

Accepted Answer

Introduction / Context: OSPF behavior varies with the underlying network type. DR/BDR elections occur on multiaccess networks to reduce adjacency overhead. On links that inherently have only two endpoints, there is no need for a DR/BDR, yet full adjacencies still form. Recognizing these differences aids design and troubleshooting. Given Data / Assumptions: Network types considered: point-to-point, broadcast multi-access, NBMA. Adjacency formation is required in all OSPF network types. The question asks specifically where DR/BDR is not elected. Concept / Approach: DR/BDR election is only needed when more than two routers share a segment, as on Ethernet (broadcast) or Frame Relay (NBMA). On point-to-point links (for example, serial PPP links or point-to-point subinterfaces), only two routers exist; therefore, OSPF forms a full adjacency without any DR/BDR roles. Step-by-Step Solution: Classify network type: point-to-point means two routers only.Apply OSPF rule: DR/BDR elections occur only on multiaccess segments.Conclude: point-to-point links have no DR/BDR election but do form full adjacencies. Verification / Alternative check: Command show ip ospf interface on a serial PPP link reports “Network Type POINT_TO_POINT, No designated router on this network,” confirming the behavior. Why Other Options Are Wrong: B: Area 0 is a logical construct; DR/BDR depends on network type, not area number. C/D: Both are multiaccess and do run DR/BDR elections. E: Virtual links traverse areas but do not change the underlying network-type rules. Common Pitfalls: Assuming all Ethernet links are point-to-point; Ethernet is broadcast multi-access in OSPF, and elections occur. Final Answer: Point-to-point

Question 16

Inspecting EIGRP adjacencies — neighbors, SRTT/RTO, and queues You must view the IP addresses of devices that have formed EIGRP adjacencies with your router and check the retransmit (RTO) and queue counts per neighbor. Which single command provides …

Accepted Answer

Introduction / Context: Effective EIGRP troubleshooting starts with adjacency health. The neighbor table displays each peer's address, uptime, smooth round-trip time (SRTT), retransmission timeout (RTO), and queue counts, indicating whether updates are backing up. Selecting the right show command is fundamental. Given Data / Assumptions: EIGRP is running and neighbors have formed. You need peer IPs, RTO, and Q counts. Standard Cisco IOS show commands are available. Concept / Approach: The neighbor table is the authoritative source for adjacency-state telemetry. show ip eigrp neighbors lists each neighbor with its IP address, interface, hold time, SRTT, RTO, and queue statistics. The topology table lists routes and metrics; interface outputs show per-interface settings; route tables show installed paths, not neighbor health. Step-by-Step Solution: Identify the needed data fields: neighbor IPs, RTO, Q counts.Map fields to EIGRP data structures: neighbor table contains them.Choose the command that displays the neighbor table: show ip eigrp neighbors. Verification / Alternative check: Run show ip eigrp neighbors; verify columns for Address, Interface, Hold, Uptime, SRTT, RTO, and Q. Rising Q values may indicate congestion or slow neighbors. Why Other Options Are Wrong: A: Not a standard IOS command; adjacency info is under neighbors. B: Displays routes and metrics, not neighbor telemetry. C: Lists interfaces participating in EIGRP, not per-neighbor stats. E: Routing table entries, not adjacency health. Common Pitfalls: Confusing topology and neighbor outputs or ignoring sustained nonzero queues which can signal retransmission issues. Final Answer: show ip eigrp neighbors

Question 17

OSPF troubleshooting: A network administrator configures: Router(config)# router ospf 1 Router(config-router)# network 10.0.0.0 255.0.0.0 area 0 but still sees no OSPF routes in the routing table. What configuration mistake was made?

Accepted Answer

Introduction / Context: This question evaluates understanding of OSPF configuration syntax and the distinction between subnet masks and wildcard masks. OSPF requires a wildcard mask, not a standard subnet mask, when configuring networks under the routing process. Given Data / Assumptions: Configuration: Router(config)# router ospf 1 Router(config-router)# network 10.0.0.0 255.0.0.0 area 0 OSPF not forming adjacencies, no routes visible Concept / Approach: The “network” command in OSPF uses a wildcard mask, which is the inverse of a subnet mask. For network 10.0.0.0/8, the subnet mask is 255.0.0.0 but the wildcard mask should be 0.255.255.255. Step-by-Step Solution: Entered mask = 255.0.0.0 (interpreted as wildcard).OSPF expects 0.255.255.255 for network 10.0.0.0/8.Because of incorrect wildcard, OSPF fails to match any interfaces.Correct configuration: network 10.0.0.0 0.255.255.255 area 0. Verification / Alternative check: Run show ip ospf interface to see if interfaces are participating. With wrong wildcard mask, no interfaces will appear under OSPF. Why Other Options Are Wrong: OSPF area: Area 0 is valid for backbone. Process ID: Locally significant only; any number works. AS configuration: Not relevant; OSPF is IGP, not dependent on AS configuration. Router ID: If missing, router automatically selects highest IP; not the cause here. Common Pitfalls: Confusing wildcard mask with subnet mask; forgetting that OSPF requires inverse masks. Final Answer: The wildcard mask is incorrect.

Question 18

EIGRP diagnostic: Which command displays all feasible successor routes known to a router under EIGRP?

Accepted Answer

Introduction / Context: Enhanced Interior Gateway Routing Protocol (EIGRP) maintains successor and feasible successor routes. The feasible successor acts as a backup path if the primary fails. Given Data / Assumptions: Task: Identify command showing feasible successors Possible EIGRP monitoring commands are provided Concept / Approach: The EIGRP topology table stores successors and feasible successors. Successor routes appear in the routing table, but feasible successors remain in the topology table until needed. Step-by-Step Solution: Check routing table: only shows active successors, not feasible ones.Check summary: shows neighbor counts and AS but not topology.Check adjacency: shows neighbor relationships, not routes.Correct command: show ip eigrp topology. Verification / Alternative check: Run the command in a lab; feasible successors are displayed with FD (feasible distance) metrics. Why Other Options Are Wrong: show ip routes: Does not display backup routes. summary: High-level data only. adjacencies: Only neighbor info. protocols: Shows routing processes, not routes. Common Pitfalls: Thinking feasible successors appear in the routing table—they do not until activated. Final Answer: show ip eigrp topology

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