Top 100 OSPF Interview Questions
The purpose of this post is to get you equipped with the best set of questions asked for OSPF in interview:
Q1: Why are loopbacks advertised as /32 host routes in OSPF? A1: Loopbacks are advertised as /32 host routes in OSPF to ensure uniqueness and stability, enabling precise reachability to individual loopback interfaces.
Q2: How do I change the reference bandwidth in OSPF? A2: You can change the OSPF reference bandwidth using the "auto-cost reference-bandwidth" command followed by the desired bandwidth value.
Q3: How does OSPF calculate its metric or cost? A3: OSPF calculates its metric/cost based on the bandwidth of the link. Cost = Reference bandwidth / Interface bandwidth.
Q4: What algorithm is used by OSPF if equal cost routes exist? A4: OSPF uses the Shortest Path First (SPF) algorithm and load-balances over equal cost routes.
Q5: Are OSPF routing protocol exchanges authenticated? A5: Yes, OSPF exchanges can be authenticated using methods like plain text, MD5, or IPsec.
Q6: What is the link-state retransmit interval, and how do I set it? A6: The link-state retransmit interval controls how often OSPF retransmits LSAs. Set it with the "ip ospf retransmit-interval n" command. retransmit-interval=5sec by default
Q7: Does the static option of the virtual link in OSPF allow discontiguous networks? A7: Yes, the static option of the virtual link allows discontiguous networks in OSPF.
Q8: Does Cisco OSPF support IP TOS-based routing? A8: Yes, Cisco OSPF supports IP TOS-based routing using the "ip ospf cost" command.
Q9: Does the offset-list subcommand work for OSPF? A9: Yes, The offset-list subcommand is used to modify the link-state advertisement (LSA) age, not for filtering.
Q10: Can an OSPF default be originated into the system based on external information on a router without a default? A10: Yes, an OSPF default can be originated based on external info using the "default-information originate" command.
Q11: Can I use the distribute-list in/out command with OSPF to filter routes? A11: No, the "distribute-list" command doesn't filter OSPF routes.
Q12: How can I give preference to OSPF inter-area routes over intra-area routes? A12: Use OSPF area types like "not-so-stubby-area" or "totally-stubby-area" to influence inter-area route preference.
Q13: When routes are redistributed between OSPF processes, are SPF metrics preserved? A13: OSPF redistributes routes with the same metrics as external routes, preserving the original metrics.
Q14: Which address-wild-mask pair is used for assigning an unnumbered interface to an area? A14: Use the pair "0.0.0.0 0.0.0.0" to assign an unnumbered interface to an OSPF area.
Q15: Can one side be numbered and the other unnumbered in OSPF? A15: Yes, OSPF allows one side of a point-to-point link to be numbered and the other unnumbered.
Q16: What causes the "unknown routing protocol" error in OSPF configuration? A16: This error might occur due to OSPF configuration issues or software anomalies.
Q17: What do the states DR, BDR, and DROTHER mean in "show ip ospf interface" output? A17: In OSPF, DR (Designated Router), BDR (Backup DR), and DROTHER are roles in a multi-access network.
Q18: Why don't I see OSPF neighbors as FULL/DR or FULL/BDR on my serial link? A18: FULL/DR and FULL/BDR states apply to multi-access networks, not serial links.
Q19: Do I need special commands to run OSPF over asynchronous links? A19: No, OSPF can run over asynchronous links without special commands.
Q20: Which Cisco IOS release supports per-interface OSPF authentication type? A20: Cisco IOS Software release 12.0 and later support per-interface OSPF authentication type.
Q21: Can I control the P-bit when importing external routes into an NSSA? A21: Yes, OSPF NSSA areas can control the P-bit for external routes using the "no-summary" option.
Q22: Why are OSPF show commands slow to respond? A22: Slow OSPF show command responses might be due to hardware limitations or high CPU usage.
Q23: What does the "clear ip ospf redistribution" command do? A23: The command clears redistributed routes from the OSPF database.
Q24: How often does OSPF send out LSAs? A24: OSPF sends out link-state advertisements (LSAs) whenever there is a topology change.
Q25: How can I prevent individual interfaces from forming OSPF adjacencies? A25: Use the "passive-interface" command to prevent OSPF adjacencies on specific interfaces.
Q26: Which path is chosen when there are two Type 5 LSAs for the same external network in OSPF? A26: OSPF chooses the path with the lowest cost when there are multiple Type 5 LSAs for the same external network.
Q27: Why doesn't my Cisco 800 router run OSPF? A27: Some Cisco 800 routers might not support OSPF due to hardware or software limitations.
Q28: Should I use the same process number for OSPF on multiple routers in the same network? A28: Each router in an OSPF network should have a unique process number.
Q29: Who performs load balancing with CEF and OSPF in a router? A29: Cisco Express Forwarding (CEF) performs load balancing for OSPF paths.
Q30: How does OSPF use two Multilink paths for packet transfer? A30: OSPF uses equal cost paths in a Multilink bundle for load balancing.
Q31: How are topological changes detected rapidly in OSPF? A31: OSPF uses Hello and Dead timers to detect topological changes quickly.
Q32: Does the 3825 Series Router support the OSPF Stub feature? A32: Yes, the Cisco 3825 Series Router supports the OSPF Stub feature.
Q33: What does the "%OSPF-4-FLOOD_WAR" error message indicate? A33: This message indicates a warning related to LSA re-originations in OSPF.
Q34: Can OSPF run over a GRE tunnel? A34: Yes, OSPF can run over a GRE tunnel.
Q35: Can Type 3 LSAs be manipulated for two areas to be sent to a non-backbone area? A35: Yes, OSPF allows summarization of Type 3 LSAs at the ABR for route control between areas.
Q36: What is the purpose of "variable IP-OSPF-Transmit-Delay"? A36: This command sets the delay for OSPF packet transmission.
Q37: Does changing an NSSA area from "nssa no-summary" to "nssa" cause neighbor adjacencies to reset? A37: Yes, changing OSPF area type might lead to neighbor adjacencies being reset.
Q38: What does "SeqNumberMismatch" mean in an OSPF adjacency error message? A38: "SeqNumberMismatch" indicates a sequence number discrepancy during OSPF adjacency formation.
Q39: What's the maximum OSPF processes on 7600/6500 platforms? A39: The maximum OSPF processes vary depending on platform capabilities.
Q40: How does ISPF improve the OSPF network? A40: Incremental SPF (ISPF) speeds up OSPF network convergence by calculating only necessary parts of the SPF tree.
Q41: Is there a way to compare OSPF commands between NX-OS/IOS? A41: Different Cisco OS versions might have slightly different OSPF command syntax.
Q42: Are there OSPF features for quick convergence and slow re-convergence of routes? A42: OSPF inherently provides quick convergence, but re-convergence time depends on network conditions.
Q43: What are the different neighbor states in OSPF? A43: OSPF neighbor states include Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading, and Full.
Q44: How can I define the cost of a propagated default route in OSPF? A44: You can define the cost of a propagated default route using the "default-information originate" command with the "metric" option.
Q45: What roles do topology and routing table play in OSPF? A45: OSPF builds the SPF tree using the topology database and derives the routing table from it.
Q46: Why do I see FULL/DR and FULL/BDR for OSPF neighbors but 2-WAY/DROTHER for others? A46: FULL/DR and FULL/BDR indicate OSPF neighbors with specific roles in multi-access networks, while 2-WAY/DROTHER applies to the rest. it indicates that the router has formed a two-way adjacency but is not a DR or BDR.
Q47: What are the types of LSAs in OSPF? A47: OSPF has Router LSAs, Network LSAs, Summary LSAs, and External LSAs.
Q48: Why do I get a "cannot allocate router id" error in OSPF configuration? A48: This error may occur due to conflicts in router ID assignment.
Q49: What are the OSPF network types and examples for each? A49: OSPF network types include Broadcast e.g. Ethernet, Non-broadcast (NBMA) e.g. X.25, Frame Relay, and ATM., Point-to-Multipoint (Broadcast/Non-Broadcast) e.g. Frame Relay, Point-to-Point e.g. 1.544Mbps leased line, Loopback, Virtual link
Q50: How do I create a totally stubby area in OSPF? A50: Use the command "area x stub no-summary" to create a totally stubby area.
Q51: What tables does OSPF maintain? A51: OSPF maintains the Neighbor Table, Link-State Database, and Routing Table.
Q52: Which multicast addresses are used in OSPF? A52: OSPF uses two IP multicast addresses on broadcast and point-to-point networks: 225.0.0.5 for all OSPF routers and 224.0. 0.6 for all DR/BDR (designated router/backup designated router) routers..
Q53: On which router is the area default cost defined? A53: The area default cost is defined on the ABR (Area Border Router).
Q54: With which network type will OSPF establish router adjacencies but not perform the DR/BDR election process? A54: OSPF establishes adjacencies without DR/BDR election in point-to-point network types.
Q55: What are the Route Types in OSPF? A55: OSPF has Intra-Area (O), Inter-Area (O IA), External Type 1 (E1), External Type 2 (E2), NSSA Type 1 (N1), and NSSA Type 2 (N2) route types.
Q56: What is the administrative distance of OSPF? A56: The administrative distance of OSPF is 110.
Q57: What are the default hello and dead timers in OSPF? A57: The default hello timer is 10 seconds, and the default dead timer is 40 seconds.
Q58: What are the Area Types in OSPF? A58: OSPF has Standard / Normal Area, Backbone Area, Stub Area, Totally Stubby Area, Not So Stubby Area (NSSA).
Q59: What are the different States in OSPF? A59: OSPF states include Down, Init, Attempt, 2-Way, Exstart, Exchange, Loading, and Full.
Q60: What must match before an OSPF adjacency forms? A60: OSPF adjacencies form when routers have matching area IDs, authentication, subnet masks, and priority values.
Q61: How can I perform Inter-Area Summarization in OSPF? A61: Perform inter-area summarization using the "area x range" command on the ABR.
Q62: How can I perform External Summarization in OSPF? A62: External route summarization in OSPFv3 is achieved by using “summary-prefix <summary IPv6 prefix / prefix length>” command on ASBR or using the " summary-address { ip-address mask | prefix mask } " command.
Q63: What are Virtual-Links in OSPF? A63: An OSPF virtual link is a shortcut that allows an isolated OSPF area to connect to the backbone area 0. The virtual link helps network administrators extend their OSPF network while maintaining OSPF design requirements. Virtual links connect non-backbone areas using a transit area.
Q64: Why are Virtual Links needed in OSPF? A64: Virtual links are used to restore connectivity in cases where a discontiguous backbone area exists. Its a OSPF design/behaviour that all areas need to be connected to OSPF backbone area(i.e Area0). If in case the area is not connected to area0 there are two methods which can be used and they are (Virtual Link & Tunnel).
Q65: What is a Sham-Link in OSPF? A65: A sham-link connects two parts of an NSSA area through the backbone, maintaining connectivity.
Q66: Explain "area 1 stub no-summary" syntax in OSPF. A66: This syntax configures Area 1 as a stub area, but retains external route information.
Q67: Why might OSPF neighbors be "stuck in 2-Way"? A67: The OSPF neighbor 2-way state is not a cause for concern in Broadcast and Non-Broadcast MultiAccess (NBMA) networks but in other network types it might be configuration issue like mismatched hello/dead timers or subnet configurations.
Q68: Why might OSPF neighbors be "stuck in Exstart"? A68: Neighbors stuck in Exstart could indicate a sequence number mismatch.
Q69: Why might OSPF neighbors be "stuck in Init"? A69: Neighbors stuck in Init could result from OSPF network type mismatches.
Q70: Why might OSPF neighbors be "stuck in Loading"? A70: Neighbors stuck in Loading could indicate synchronization issues.
Q71: What algorithm does OSPF use? A71: OSPF uses the Dijkstra algorithm for its SPF (Shortest Path First) calculations.
Q72: What is OSPF Router ID and how is it elected? A72: OSPF Router ID is a unique 32-bit identifier. It's manually configured or elected based on the highest IP address on an active interface.
Q73: What is DR/BDR in OSPF, and how are they elected? A73: DR (Designated Router) and BDR (Backup DR) roles are elected on multi-access networks like Ethernet to minimize LSAs and manage flooding.
Q74: What happens when OSPF router priority is set to "zero"? A74: Setting OSPF router priority to zero prevents the router from becoming a DR or BDR.
Q75: Does DR/BDR election occur in OSPF Point-to-Point Network? A75: No, DR/BDR election doesn't occur in OSPF Point-to-Point networks.
Q76: What is the Backbone Area in OSPF? A76: The Backbone Area (Area 0) is the central area connecting all other areas in OSPF.
Q77: Can non-connected areas exchange OSPF advertisements? A77: OSPF areas must connect to Area 0 (Backbone) for direct advertisement. Virtual links can connect non-connected areas indirectly.
Q78: Explain the concept of Area Border Router (ABR) in OSPF. A78: An ABR connects different OSPF areas, facilitating routing between them.
Q79: Explain the concept of Autonomous System Border Router (ASBR) in OSPF. A79: An ASBR connects OSPF to external routing domains, redistributing external routes into OSPF.
Q80: What do "O E1" and "O E2" letters in OSPF represent? A80: "O E1" represents External Type 1 routes, considering both internal and external metrics. "O E2" represents External Type 2 routes, using only the external metric.
Q81: How does instability in one OSPF area impact others? A81: Instability in one OSPF area can affect other areas by potentially causing suboptimal routing paths. Unstable areas can impact the stability and convergence of other areas in OSPF.
Q82: What's the OSPF route preference order for different types? A82: preferred path list is Intra-Area (O) > Inter-Area (O IA) > NSSA Type 1 (N1) > External Type 1 (E1) > NSSA Type 2 (N2) > External Type 2 (E2)
Q83: What's the benefit of dividing a network into OSPF areas? A83: Dividing a network into OSPF areas reduces routing table size, enhances convergence, and limits LSA propagation.
Q84: What parameters must match for OSPF neighbors to form? A84: OSPF neighbors form when routers have matching hello/dead timers, Unique RID, same area, authentication, subnet masks.
Q85: What are the different OSPF neighbor states? A85: OSPF neighbor states include Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading, and Full.
Q86: Can OSPF form adjacencies with non-subnet neighbors? A86: OSPF forms adjacencies only with neighbors on the same subnet.
Q87: Explain OSPF LSAs, LSU, and LSR. A87: LSAs (Link-State Advertisements) describe router link state, LSU (Link-State Update) carries LSAs, and LSR (Link-State Request) asks for missing LSAs.
Q88: What's the difference between OSPF and EIGRP? A88: OSPF is a link-state protocol, while EIGRP hybrid protocol it uses the features of the both distance vector routing and link-state routing protocol. Difference between EIGRP and OSPF:
Q89: What's the difference between OSPF and IS-IS? A89: Both are link-state protocols. OSPF is used in IP networks, while IS-IS can be used in both IP and non-IP environments.
Q90: What are the OSPF table types? A90: OSPF maintains the Neighbor Table, Link-State Database, and Routing Table.
Q91: Is OSPF classless or classful? A91: OSPF is a classless protocol, supporting variable-length subnet masks.
Q92: What transport layer protocol does OSPF use? A92: OSPF uses IP protocol number 89 at the transport layer.
Q93: Can OSPF work without a Backbone Area? A93: No, OSPF requires an Area 0 (Backbone Area) for proper functioning.
Q94: Which RFC describes OSPF version 2? A94: OSPF version 2 is described in RFC 2328.
Q95: How many equal cost paths does OSPF load balance over by default? A95: OSPF load balances over up to 4 equal-cost paths by default.
Q96: What protocol number represents OSPF in IP packets? A96: OSPF is represented as protocol number 89 in IP packets.
Q97: What are the main functions of the DR (Designated Router)? A97: The DR minimizes adjacencies and reduces flooding by representing multiple routers on multi-access networks.
Q98: What parameters assist in DR election in OSPF? A98: Router priority and router ID help in DR/BDR (Backup Designated Router) election.
Q99: How can I advertise a default route in OSPF when it's absent in the RIB? A99: You can use the "default-information originate" command to advertise a default route in OSPF, even if it's not in the Routing Information Base (RIB).
Q100: What is stub area, Totally Stubby Area, Not So Stubby area (NSSA)? A100: Stub Area Rules: 1. In the OSPF Stub area, there should not be an ASBR. 2. The OSPF Stub area should not be Area 0. 3. No virtual link must be passing through that area. 4. All routers in that area must be configured as stub routers. A stub area is an isolated area that does not receive External LSA. Routers in Stub areas do not receive type 4 and type 5 LSAs it is replaced by a default route to an external autonomous system advertised by the area border router (ABR) Stub areas can have type 1, 2, and 3 OSPF LSAs. Totally stubby area External LSAs are stopped (El and E2) and summary LSAs are stopped (OIA routes). Replaced by a default route to other areas advertised by the ABR. Totally stubs is reducing the routing table to a minimum. This is a Cisco proprietary feature. NSSA breaks stub area rules. NSSA area creates a special type of link-state advertisement (LSA) known as type 7, type 7 LSAs is only exist in the NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, ABR sends default routes into NSSA instead of external routes from other ASBR. Totally NSSAs are similar to totally stubby areas, with the exception that the routers that are internal to the totally NSSAs need to have no knowledge of subnets outside of the area (with the exception of the routes injected by the NSSA ASBR)
Comments