300-410 OSPF Troubleshooting (v2/v3) — 15 Questions

Question 1

A network engineer is troubleshooting an OSPFv2 adjacency issue between two directly connected routers, R1 and R2, both running IOS-XE. The link is a point-to-point Ethernet link. The engineer issues 'show ip ospf neighbor' on R1 and sees no neighbors. 'show ip ospf interface GigabitEthernet0/0' on R1 shows 'Network Type BROADCAST', but the link is actually a point-to-point link. Both routers have 'ip ospf 1 area 0' configured on the interface. What is the most likely cause of the adjacency not forming?

Accepted Answer

The routers have duplicate OSPF router IDs.. The default OSPF network type on Ethernet interfaces is BROADCAST, which requires a DR/BDR election and uses multicast 224.0.0.5 and 224.0.0.6. On a point-to-point link, if both routers do not agree on the network type, they may not form an adjacency because the hello packets are sent differently. However, since both are BROADCAST, they should form an adjacency. The real issue is likely a mismatch in OSPF network type if one side is manually set to POINT-TO-POINT, but here both are BROADCAST. A more common cause is that the interface is administratively down or there is a Layer 1 issue, but the stem indicates the interface is up. Another common cause is that the router IDs are not configured, leading to OSPF not starting. The most likely cause in this scenario is that the routers have duplicate router IDs, which prevents adjacency formation. The stem does not mention router IDs, so the best answer is that the engineer should check for duplicate router IDs.

Answer

The OSPF network type mismatch between the two routers (one is BROADCAST, the other is POINT-TO-POINT).

Answer

The interface is configured with 'ip ospf passive-interface'.

Answer

The OSPF process is not enabled globally; 'router ospf 1' is missing.

Question 2

A network engineer is troubleshooting OSPFv3 on a dual-stack network. Routers R1 and R2 are connected via a serial link. Both routers have OSPFv3 configured for IPv6. The engineer runs 'show ipv6 ospf neighbor' on R1 and sees R2 as FULL/DR. However, R1 cannot ping the IPv6 address of R2's loopback interface. 'show ipv6 route ospf' on R1 does not show any OSPF routes. What is the most likely cause?

Accepted Answer

The interface on R1 does not have an IPv6 address configured.. In OSPFv3, the network type on serial interfaces defaults to POINT-TO-POINT, but if the interface is configured as BROADCAST (e.g., using 'ipv6 ospf network broadcast'), a DR/BDR election occurs. However, the adjacency is FULL, so the issue is not adjacency. The lack of OSPF routes in the routing table suggests that the routes are not being installed. One common reason is that OSPFv3 requires an IPv6 address on the interface, but the router may have an IPv4-only configuration or the interface may not have an IPv6 address. Another possibility is that the OSPFv3 process is not redistributing connected routes. The most likely cause is that the interface does not have an IPv6 address configured, which is required for OSPFv3 to advertise prefixes.

Answer

The OSPFv3 process has 'no ipv6 unicast-routing' enabled globally.

Answer

The interface on R2 is configured as passive under OSPFv3.

Answer

The OSPFv3 process is configured with 'default-information originate always' but no default route exists.

Question 3

A network engineer is troubleshooting OSPFv2 route redistribution. R1 is an ASBR redistributing static routes into OSPF. R2, an internal router, receives the redistributed routes but they appear as O E2 routes. However, R1 also has a directly connected network 10.1.1.0/24 that is not being advertised as an OSPF route. 'show ip ospf database external' on R2 shows the redistributed static routes but not the connected network. What is the most likely cause?

Accepted Answer

The ASBR is missing the 'redistribute connected' command under the OSPF process.. By default, OSPF does not redistribute connected routes unless explicitly configured. The ASBR must use the 'redistribute connected' command under the OSPF process to advertise directly connected networks. The static routes are being redistributed because they are matched by the 'redistribute static' command, but the connected network is not part of the static route set unless it is also a static route. The engineer likely forgot to add 'redistribute connected' or use the 'subnets' keyword.

Answer

The connected network is not included in the redistribution because the engineer used 'redistribute static' without the 'subnets' keyword.

Answer

The connected network is not being advertised because it is not part of the OSPF process; the engineer must configure 'network 10.1.1.0 0.0.0.255 area 0' under router ospf.

Answer

The connected network is a loopback interface, and OSPF does not advertise loopback networks by default.

Question 4

A network engineer is troubleshooting an OSPFv2 adjacency issue between two routers across a Frame Relay network. R1 and R2 are connected via a point-to-point subinterface. The engineer configures 'ip ospf network point-to-point' on both subinterfaces. However, the adjacency does not form. 'show ip ospf interface' on R1 shows the interface is up and OSPF is enabled, but no neighbors are seen. What is the most likely cause?

Accepted Answer

The Frame Relay map is missing or the DLCI is not assigned to the subinterface.. On Frame Relay point-to-point subinterfaces, the OSPF network type should be point-to-point, but the adjacency may still fail if the Layer 2 mapping is incorrect. The subinterface must be mapped to the correct DLCI. If the 'frame-relay interface-dlci' command is missing or misconfigured, the router cannot send packets to the neighbor, even though OSPF is enabled. The engineer should verify the DLCI mapping.

Answer

The OSPF network type is set to broadcast, causing a DR/BDR election that fails on a point-to-point subinterface.

Answer

The subinterface is not configured with an IP address.

Answer

The OSPF hello and dead timers are mismatched between R1 and R2.

Question 5

A network engineer is troubleshooting an OSPFv3 issue where a router R1 is not learning routes from a neighbor R2. The adjacency is FULL, but 'show ipv6 route ospf' on R1 shows only a default route. R2 is an ASBR redistributing connected routes into OSPFv3. 'show ipv6 ospf database external' on R1 shows the external routes, but they are not installed in the routing table. What is the most likely cause?

Accepted Answer

The router R1 has a distribute-list in the OSPFv3 process that filters out the external routes.. In OSPFv3, external routes are not installed if the router has a route to the forwarding address that is not reachable. However, the more common cause is that the OSPFv3 process on R1 has 'no ipv6 unicast-routing' or the router is missing a default route to reach the external destinations. Another possibility is that the external routes have a metric that is too high or the route is suppressed by a filter. The most likely cause is that the router has a route map or distribute list that is filtering the external routes from being installed.

Answer

The external routes have a metric of 16777214, which is considered infinite.

Answer

The router R1 does not have IPv6 unicast routing enabled.

Answer

The external routes are type 5 LSAs but the router is in a totally stubby area.

Question 6

A network engineer is troubleshooting an OSPFv2 route flapping issue. The router R1 is learning a route to 192.168.1.0/24 via two different paths: one through R2 and one through R3. The route is flapping between the two paths every few seconds. 'show ip ospf interface' shows that both interfaces are stable. What is the most likely cause?

Accepted Answer

The routers R2 and R3 have the same OSPF router ID.. Route flapping can be caused by a number of issues, but if the interfaces are stable, the problem is likely due to a routing loop or a metric issue. One common cause is that the routers are using the same router ID, causing OSPF to see the same route from two different neighbors as equal-cost paths, but the route may be flapping due to a change in the metric. Another cause is that the link between R1 and R2 is flapping at Layer 1, but the stem says the interfaces are stable. The most likely cause is that the routers have duplicate router IDs, which can cause OSPF to treat the same route as two different routes and cause instability.

Answer

The route is being redistributed by both R2 and R3 with different metrics.

Answer

The OSPF network type on the interfaces is set to broadcast, causing DR/BDR instability.

Answer

The link between R1 and R2 has a high error rate causing intermittent packet loss.

Question 7

A network engineer is troubleshooting an OSPFv2 adjacency issue between two routers connected via a VLAN trunk. R1 and R2 are in different VLANs but are connected through a Layer 2 switch. The engineer has configured 'ip ospf 1 area 0' on the subinterfaces. The adjacency forms but is stuck in INIT state. What is the most likely cause?

Accepted Answer

The routers have the same OSPF router ID.. The INIT state indicates that the router has received a hello packet from the neighbor but the neighbor has not seen its own router ID in the hello packet. This is often due to a mismatch in the OSPF network type or a unicast communication issue. In a VLAN trunk scenario, if the subinterfaces are not in the same VLAN, the routers may not be able to communicate at Layer 2. However, the most common cause is that the routers are using the same router ID, which causes the hello packet to be ignored because the router sees its own router ID in the neighbor list.

Answer

The OSPF hello interval is mismatched between the two routers.

Answer

The subinterfaces are in different VLANs, preventing Layer 2 communication.

Answer

The OSPF network type is set to point-to-point on one side and broadcast on the other.

Question 8

A network engineer is troubleshooting an OSPFv2 issue where a router R1 is not receiving a specific route for 10.0.0.0/8 from a neighbor R2. The adjacency is FULL, and 'show ip ospf database' on R1 shows the LSA for 10.0.0.0/8 as a type 5 LSA. However, the route is not in the routing table. 'show ip route 10.0.0.0' shows no route. What is the most likely cause?

Accepted Answer

The router R1 has a distribute-list applied to the OSPF process that filters the route 10.0.0.0/8.. If the LSA is in the database, the route should be installed unless there is a filtering mechanism or a routing table issue. One common cause is that the router has a route map that filters the route from being installed. Another possibility is that the router has a static route that is being preferred, but the stem says no route is present. The most likely cause is that the router has a distribute-list that filters the route.

Answer

The route is a type 5 LSA, but the router is in a stub area.

Answer

The router has a higher administrative distance for OSPF routes, causing it to prefer another routing protocol.

Answer

The route has a metric of 16777214, which is considered infinite.

Question 9

A network engineer is troubleshooting an OSPFv3 adjacency issue between two routers over an IPv6-only link. The engineer configures OSPFv3 on both routers and sees that the adjacency is stuck in EXSTART state. 'debug ipv6 ospf packet' shows that the routers are exchanging DBD packets but not progressing. What is the most likely cause?

Accepted Answer

The OSPFv3 interface MTU is mismatched between the two routers.. The EXSTART state indicates that the routers are trying to establish the master/slave relationship and exchange DBD packets. If they are stuck, it is often due to a mismatch in the OSPF MTU or the interface MTU. If the MTU is mismatched, the DBD packets may be too large to be sent, causing the routers to keep retransmitting. Another cause could be a mismatch in the OSPF network type, but that usually causes issues earlier. The most likely cause is an MTU mismatch.

Answer

The routers have different OSPFv3 router IDs.

Answer

The OSPFv3 process is configured with 'ipv6 ospf 1 area 0' on the interface, but the interface is not IPv6-enabled.

Answer

The link is a point-to-point link, but the OSPF network type is set to broadcast.

Question 10

A network engineer runs the following command on Router R1:

R1# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.1.1.2         1   FULL/DROTHER    00:00:35    192.168.12.2    GigabitEthernet0/0
10.1.1.3         1   FULL/DR         00:00:32    192.168.13.3    GigabitEthernet0/1

Based on this output, which statement is correct?

Accepted Answer

Router R1 has a full adjacency with both neighbors and is in a stable state.. The output shows two OSPF neighbors. The neighbor with state FULL/DROTHER indicates it is not the DR or BDR on that segment. The neighbor with state FULL/DR is the designated router. The dead times are within normal range, indicating the adjacency is stable.

Answer

Router R1 is the DR on the segment connected to GigabitEthernet0/0.

Answer

Router R1 is experiencing a neighbor timeout on GigabitEthernet0/1.

Answer

Router R1 is not receiving hello packets from 10.1.1.2.

Question 11

A network engineer runs the following command on Router R1:

R1# show ip ospf database

OSPF Router with ID (10.1.1.1) (Process ID 1)

Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.1.1.1        10.1.1.1        120         0x80000002 0x00A1B2 3
10.1.1.2        10.1.1.2        200         0x80000001 0x00B2C3 2
10.1.1.3        10.1.1.3        150         0x80000003 0x00C3D4 4

Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.12.2    10.1.1.2        180         0x80000001 0x00D4E5

Based on this output, what can be concluded?

Accepted Answer

Router 10.1.1.2 is the DR on the segment 192.168.12.0/24.. The database shows router link states for three routers and one net link state. The net link state is advertised by 10.1.1.2, indicating it is the DR on the segment 192.168.12.0/24. The absence of other net link states suggests only one broadcast segment exists.

Answer

Router R1 is the DR on the segment 192.168.12.0/24.

Answer

Router 10.1.1.3 has the highest router ID.

Answer

There is a network LSA for every broadcast segment in area 0.

Question 12

A network engineer runs the following command on Router R1:

R1# show ip ospf interface GigabitEthernet0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 192.168.12.1/24, Area 0
  Process ID 1, Router ID 10.1.1.1, Network Type BROADCAST, Cost: 10
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 10.1.1.2, Interface address 192.168.12.2
  Backup Designated router (ID) 10.1.1.1, Interface address 192.168.12.1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:03
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 10.1.1.2  (Designated Router)
  Suppress hello for 0 neighbor(s)

Based on this output, which statement is correct?

Accepted Answer

The dead timer is set to 40 seconds and is functioning correctly.. The interface is in state BDR, meaning Router R1 is the backup designated router. The DR is 10.1.1.2. The output shows one neighbor, which is the DR, and the adjacency is full.

Answer

Router R1 is the Designated Router on this segment.

Answer

Router R1 has a priority of 0, preventing it from becoming DR.

Answer

Router R1 is not receiving hello packets from the DR.

Question 13

A network engineer runs the following command on Router R1:

R1# show ip route ospf

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

O    10.2.2.0/24 [110/20] via 192.168.12.2, 00:12:34, GigabitEthernet0/0
O IA 10.3.3.0/24 [110/30] via 192.168.13.3, 00:10:21, GigabitEthernet0/1

Based on this output, what can be determined?

Accepted Answer

The route to 10.3.3.0/24 is learned from a different area.. The routing table shows two OSPF routes: one intra-area (O) and one inter-area (O IA). The intra-area route has a cost of 20, and the inter-area route has a cost of 30. Both are reachable via different next hops.

Answer

The route to 10.2.2.0/24 is an external OSPF route.

Answer

Router R1 has a default route configured.

Answer

Both routes have the same administrative distance.

Question 14

A network engineer runs the following command on Router R1:

R1# show ip ospf neighbor 10.1.1.2

Neighbor 10.1.1.2, interface address 192.168.12.2
    In the area 0 via interface GigabitEthernet0/0
    Neighbor priority is 1, State is FULL, 6 state changes
    DR is 10.1.1.2, BDR is 10.1.1.1
    Options is 0x12 (L L S R)
    Dead timer due in 00:00:36
    Neighbor is up for 00:15:42
    Index 1/1/1, retransmission queue length 0, number of retransmission 0
    First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
    Last retransmission scan length is 0, time is 0 msec
    Last retransmission scan time is 0 msec

Based on this output, what is the role of Router R1 on this segment?

Accepted Answer

Router R1 is the Backup Designated Router.. The output shows that the BDR is 10.1.1.1, which is Router R1's router ID. Therefore, R1 is the Backup Designated Router. The DR is 10.1.1.2.

Answer

Router R1 is the Designated Router.

Answer

Router R1 is a DROTHER router.

Answer

Router R1 has no special role on this segment.

Question 15

A network engineer runs the following command on Router R1:

R1# show ip ospf database router 10.1.1.2

OSPF Router with ID (10.1.1.1) (Process ID 1)

Router Link States (Area 0)

LS age: 150
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.1.1.2
  Advertising Router: 10.1.1.2
  LS Seq Number: 80000002
  Checksum: 0x1234
  Length: 48
  Number of Links: 2

Link connected to: a Transit Network
     (Link ID) Designated Router address: 192.168.12.2
     (Link Data) Router Interface address: 192.168.12.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.2.2.0
     (Link Data) Network Mask: 255.255.255.0
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

Based on this output, which statement is correct?

Accepted Answer

Router 10.1.1.2 is connected to a transit network and a stub network.. The router LSA from 10.1.1.2 shows two links: one transit network (to the DR address) and one stub network (10.2.2.0/24). The transit link indicates it is connected to a broadcast segment where it is the DR (since it lists its own interface address as DR address).

Answer

Router 10.1.1.2 is the DR on the transit network.

Answer

Router 10.1.1.2 has a cost of 20 to reach the stub network.

Answer

Router 10.1.1.2 is not advertising any external routes.