CCNA Interview Questions and Answers -10
1. When I have two type 5 link-state advertisements (LSAs) for the same external network in the OSPF database, which path should be installed in the IP routing table?
When you have two type 5 LSAs for the same external network in the OSPF database, prefer the external LSA that has the shortest path to the Autonomous System Boundary Router (ASBR) and install that into the IP routing table. Use the show ip ospf border-routers command to check the cost to the ASBR.
2. Why is it that my Cisco 1600 router does not recognize the OSPF protocol?
3. Why is it that my Cisco 800 router does not run OSPF?
Cisco 800 routers do not support OSPF. However, they do support Routing Information Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). You can use the Software Advisor ( registered customers only) tool for more information on feature support.
4. Should I use the same process number while configuring OSPF on multiple routers within the same network?
OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing Protocol (EIGRP), does not check the process number (or autonomous system number) when adjacencies are formed between neighboring routers and routing information is exchanged. The only case in which the OSPF process number is taken into account is when OSPF is used as the routing protocol on a Provider Edge to Customer Edge (PE-CE) link in a Multiprotocol Label Switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute derived from the OSPF process number to indicate whether the route originated within the same OSPF domain or from outside it. If the OSPF process numbering is inconsistent on PE routers in the MPLS VPN, the domain-id OSPF mode command should be used to mark that the OSPF processes with different numbers belong to the same OSPF domain.
This means that, in many practical cases, you can use different autonomous system numbers for the same OSPF domain in your network. However, it is best to use consistent OSPF-process numbering as much as possible. This consistency simplifies network maintenance and complies with the network designer intention to keep routers in the same OSPF domain.
5. I have a router that runs Cisco Express Forwarding (CEF) and OSPF, who does loadbalancing when there are multiple links to a destination?
CEF works by performing the switching of the packet based on the routing table which is populated by the routing protocols such as OSPF. CEF does the load-balancing once the routing protocol table has been calculated. For more details on load balancing, refer to How does load-balancing work?
6. How does OSPF use two Multilink paths to transfer packets?
OSPF uses the metric aCost, which is related to the bandwidth. If there are equal cost paths (the same bandwidth on both multilinks), OSPF installs both routes in the routing table. The routing table tries to use both links equally, regardless of the interface utilization. If one of the links in the first multilink fails, OSPF does not send all the traffic down the second multilink. If the first multilink peaks 100%, OSPF does not send any traffic down the second multilink because OSPF tries to use both links equally, regardless of the interface utilization. The second is used fully only when the first multilink goes down.
7. How can you detect the topological changes rapidly?
In order to have a rapid fault detection of topology changes, the hello timer value needs to be set to 1 second. The hold timer value, which is is four times that of the hello timer, also needs to be configured. There is a possibility of more routing traffic if the hello and hold timer values are reduced from their default values.
Note: Tuning OSPF Timers might result in network as well device resource overhead. Cisco recommends to use Bidirectional Forwarding Detection (BFD) instead of tuning the routing protocol timers. BFD also gives sub-second convergence. Refer toOSPF Support for BFD over IPv4 for more information.
8. Does the 3825 Series Router support the OSPF Stub feature?
Yes, the 3800 Series Router that runs Advanced IPServices image supports the OSPF Stub feature.
9. What does the error message %OSPF-4-FLOOD_WAR: Process process-id reoriginates LSA ID ip address type-2 adv-rtr ip address in area area id means?
The error message is due to the some router that is flushing the network LSA because the network LSA received by the router whose LSA ID conflicts with the IP address of one of the router’s interfaces and flushes the LSA out of the network. For OSPF to function correctly the IP addresses of transit networks must be unique. If it is not unique the conflicting routers reports this error message. In the error message the router with the OSPF router ID reported as adv-rtr reports this message.
10. Can we have OSPF run over a GRE tunnel?
Yes, refer to Configuring a GRE Tunnel over IPSec with OSPF.
11. Is there a way to manipulate and prefer the Type 3 LSAs to originate from two different areas sent to the non-backbone area?
Type 3 LSA is originated by the Area Border Router (ABR) as a summary route. Manipulating the summary route is not possible in an ABR router.
12. Is there a drop/flap of an OSPF neighborship when changing an OSPF area type from nssa no-summary to nssa?
When the NSSA ABR is configured to move from nssa no-summary to nssa, the OSPF neighborship does not flap.
13. In the %OSPF-5-ADJCHG: Process ID, Nbr [ip-address] on Port-channel31 from FULL to EXSTART, SeqNumberMismatcherror message, what does SeqNumberMismatch signify?
The OSPF neighbor was changed state from FULL to EXSTART because of the receipt of a Database Description (DBD) packet from the neighbor with an unexpected sequence number.
Seq Number Mismatch means that a DBD packet during OSPF neighborship negotiation has been received that either:
has an unexpected DBD sequence number
unexpectedly has the Init bit set
has an Options field differing from the last Options field received in a Database Description packet.
What is the maximum number of OSPF processes (VRF aware) on 7600/6500 platforms?
Cisco IOS has a limit of 32 routing processes. Two of these are saved for static and directly connected routes. The Cisco 7600 router supports 28 OSPF processes per VRF.
14. How does ISPF impact or improve the OSPF network?.
Incremental SPF is more efficient than the full SPF algorithm, thereby allowing OSPF to converge faster on a new routing topology in reaction to a network event. The incremental SPF is designed in such a way that it only updates the affected nodes without rebuilding the whole tree. This results in a faster convergence and saves CPU cycles because the unaffected nodes do not need to be processed. Concerning the best practice ISPF would make more of a difference for a large OSPF domain.
Incremental SPF provides greater improvements in convergence time for networks with a high number of nodes and links. Incremental SPF also provides a significant advantage when the changes in the network topology are further away from the root of the SPT; for example, the larger the network the more significant the impact. A segment of 400-1000 nodes should see improvements. However, it might be hard to verify in a deployed production network without some kind of facility or tool to measure the end-to-end delay. For more information, refer to OSPF Incremental SPF.
15. Is there a way to compare Cisco NX-OS/IOS OSPF commands?
Yes, refer to Cisco NX-OS/IOS OSPF Comparison.
16. Is there any feature of OSPF protocol for quick convergence and a slow reconvergence of routes?
The OSPF Shortest Path First Throttling feature makes it possible to configure SPF scheduling in millisecond intervals and to potentially delay SPF calculations during network instability. SPF is scheduled to calculate the Shortest Path Tree (SPT) when there is a change in topology.
Syntax of the command under OSPF:
timers throttle spf [spf-start] [spf-hold] [spf-max-wait]
spf-start—Initial delay to schedule an SPF calculation after a change, in milliseconds. Range is from 1 to 600000.
spf-hold—Minimum hold time between two consecutive SPF calculations, in milliseconds. Range is from 1 to 600000.
spf-max-wait—Maximum wait time between two consecutive SPF calculations, in milliseconds.
Range is 1 to 600000.
For more information on the OSPF Throttling feature, refer to OSPF Shortest Path First Throttling.
17. What does BADSEQNUM in the %OSPF-5-NBRSTATE: ospf-101  Process 101, Nbr 10.253.5.108 on Vlan7 02 from FULL to EXSTART, BADSEQNUM OSPF log message mean?
This message is related to the DBD exchange process, which uses a sequence number for the synchronization of the database. For some reason a bad sequence number was reported in the DBD packet. This might occur because of transient conditions, which includes packet loss or packet corruption.
18. Are the multicast IP addresses mapped to MAC-level multicast addresses?
OSPF sends all advertisements using multicast addressing. Except for Token Ring, the multicast IP addresses are mapped to MAC-level multicast addresses. Cisco maps Token Ring to MAC-level broadcast addresses.
19. Does the Cisco OSPF implementation support IP TOS-based routing?
Cisco OSPF only supports TOS 0. This means that routers route all packets on the TOS 0 path, eliminating the need to calculate nonzero TOS paths.
20. Does the offset-list subcommand work for OSPF?
The offset-list command does not work for OSPF. It is used for distance vector protocols such as Interior Gateway Routing Protocol (IGRP), Routing Information Protocol (RIP), and RIP version 2.
21. Can an OSPF default be originated into the system based on external information on a router that does not itself have a default?
OSPF generates a default only if it is configured using the command default-information originate and if there is a default network in the box from a different process. The default route in OSPF is 0.0.0.0. If you want an OSPF-enabled router to generate a default route even if it does not have a default route itself, use the command default-information originate always.
22. When I issue the show ip ospf neighbor command, why do I only see FULL/DR and FULL/BDR, with all other neighbors showing 2-WAY/DROTHER?
To reduce the amount of flooding on broadcast media, such as Ethernet, FDDI, and Token Ring, the router becomes full with only designated router (DR) and backup designated router (BDR), and it shows 2-WAY for all other routers.
23. Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on my serial link?
This is normal. On point-to-point and point-to-multipoint networks, there are no designated routers (DRs) or backup designated routers (BDRs).
24. Do I need any special commands to run OSPF over BRI/PRI links?
In addition to the normal OSPF configuration commands, you should use the dialer map command. When using the dialer map command, use thebroadcast keyword to indicate that broadcasts should be forwarded to the protocol address.
25. Do I need any special commands to run OSPF over asynchronous links?
In addition to the normal OSPF configuration commands, you should use the async default routing command on the asynchronous interface. This command enables the router to pass routing updates to other routers over the asynchronous interface. Also, when using the dialer map command, use the broadcast keyword to indicate that broadcasts should be forwarded to the protocol address.
26. Which Cisco IOS Software release began support for per-interface authentication type in OSPF?
Per-interface authentication type, as described in RFC 2178 Description: leavingcisco.com , was added in Cisco IOS Software Release 12.0(8).
27. Can I control the P-bit when importing external routes into a not-so-stubby area (NSSA)?
When external routing information is imported into an NSSA in a type 7 link-state advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The P-bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated. Only those LSAs with the P-bit set are translated. When you redistribute information into the NSSA, the P-bit is automatically set. A possible workaround applies when the Autonomous System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR can then summarize with the not-advertise keyword, which results in not advertising the translated type 7 LSAs.
28. Why are OSPF show commands responding so slowly?
You may experience a slow response when issuing OSPF show commands, but not with other commands. The most common reason for this delay is that you have the ip ospf name-lookup configuration command configured on the router. This command causes the router to look up the device Domain Name System (DNS) names for all OSPF show commands, making it easier to identify devices, but resulting in a slowed response time for the commands. If you are experiencing slow response on commands other than just OSPF show commands, you may want to start looking at other possible causes, such as the CPU utilization