CCNP ENARSI v8 Chapters 6 – 10: OSPF Test Online

Hint

The area 0 authentication message-digest command configures OSPF MD5 authentication globally. The router ospf 10 , default-information originate , and ip ospf priority 255 commands configure general OSPF routing functions that are not related to authentication.

Hint

OSPF neighbors that reach the Full state are converged and can exchange routing information.

Hint

The first preference for an OSPF router ID is an explicitly configured 32-bit address. This address is not included in the routing table and is not defined by the network command. If a router ID that is configured through the router-id command is not available, OSPF routers next use the highest IP address available on a loopback interface, as loopbacks used as router IDs are also not routable addresses. Lacking either of these alternatives, an OSPF router will use the highest IP address from its active physical interfaces.

Hint

The route metric and neighbor next hop address are routing table entries that are displayed by the show ip route command. The show ip ospf neighbor command does not display the OSPF process ID or the OSPF area number.

Hint

R2 will be elected DR because it has the highest priority of 255, all of the others have a priority of 1. R3 will be elected BDR because it has the numerically highest router-ID of 192.168.1.4. The R4 router-ID is 172.16.1.1 because it is the IPv4 address attached to the loopback 0 interface.

Hint

Frame Relay and Layer 2 VPN topologies can use a point-to-multipoint OSPF network type. This type of network is connected in a hub-and-spoke configuration and uses the same IP subnet.

Hint

OSPF uses the shortest path first algorithm to select the best path to forward packets. Each path is labeled with a value for cost and this algorithm accumulates costs along each path, from source to destination. The path chosen from Net A to Net B is R1-R3-R5-R7, that is equal to 5+7+14+2=28, the shortest cost.

Hint

The area type flag in hello packets must match between adjacent OSPF routers. For this reason, all routers in the stub area must be configured as a stub area.

Hint

When two Layer 3 devices are each configured with the area area-id virtual-link endpoint-RID command, a virtual link is formed. To verify that the virtual link is up, use the show ip ospf virtual-links command. The virtual link also appears as an OSPF VL interface within the output of the show ip ospf interface [brief] command.

Hint

Because the LSDBs on every router within an area must be the same, OSPF summarization can only be done between areas, or between autonomous systems. For this reason, only an ABR or an ASBR can be configured to perform summarization. An ABR can summarize interarea routes between areas, and an ASBR can summarize external routes as they enter the OSPF domain.

Hint

The topology table on an OSPF router is a link-state database (LSDB) that lists information about all other routers in the network, and represents the network topology. All routers within an area have identical link-state databases, and the table can be viewed using the show ip ospf database command. The EIGRP topology table contains feasible successor routes. This concept is not used by OSPF. The SPF algorithm uses the LSDB to produce the unique routing table for each router which contains the lowest cost route entries for known networks.

Hint

Type 5 LSAs are used for external routes and are generated by an ASBR. They are forwarded into other areas by ABRs.

Hint

On router R1, the network 192.168.10.0/30 is defined in the wrong area (area 1). It has to be defined in area 0 in order to establish adjacency with router R2, which has the network 192.168.10.0/30 defined in area 0.

Hint

The administrative distance does not matter in this case because (1) there is not an equal route in OSPF and (2) because the company is not running OSPF with the ISP. The interface is the correct one and using an exit interface is acceptable on a serial link. No network statements are required for static routes. A link must be up in order for a static route that uses the link to be shown in the routing table.

Hint

There may be several reasons why two routers running OSPF will fail to form an OSPF adjacency, including these: The subnet masks do not match, causing the routers to be on separate networks. OSPF Hello or Dead Timers do not match. OSPF network types do not match. There is a missing or incorrect OSPF network command. Mismatched IOS versions, the use of private IP addresses, and different types of interface ports used on a switch are not causes for an OSPF adjacency failing to form between two routers.

Hint

The show ip ospf interface command will display routing table information that is already known. The show running-configuration and show ip protocols commands will display aspects of the OSPF configuration on the router but will not display adjacency state details or timer interval details.

Hint

If a LAN network is not advertised using OSPFv2, a remote network will not be reachable. The output displays a successful neighbor adjacency between router R1 and R2 on the interface S0/0 of both routers.

Hint

In order to form OSPFv2 neighbor adjacencies, two connected router interfaces must share the same subnet. Router R2 is shown in the topology with an IP address of 192.168.20.5 and does not exist on the same subnet as the 192.168.20.1 /30 IP address of S0/0 on router R1.

Hint

OSPFv3 uses type 5, AS external LSAs, to announce default routes and routers learned through redistribution from other routing protocols.

Hint

Two new LSA types are added to OSPFv3, type 8, link-local LSA, and type 9, interarea prefix LSA. These two LSAs advertise unicast prefixes and prevent the need for OSPF calculations when interface addresses are added or changed.

Hint

OSPFv3 runs directly over IPv6 and inter-router communication for both IPv4 and IPv6 address families is handled by IPv6 link-local addressing. OSPV3 uses a 32-bit router ID for IPv4 and IPv6 address families routing. IPv6 addressing is not enabled by default on Cisco router interfaces.

Hint

The show ospfv3 interface brief command displays associated process ID, area, address family (IPv4 or IPv6), interface state, and neighbor count. The state of OSPFv3 neighbor routers and the router IDs of OSPFv3 neighbor routers are displayed by the show ospfv3 ipv6 neighbor command. Hello and other timer intervals are displayed by the regular form of the show ospfv3 interface command.

Hint

The interarea prefix LSAs sent by an OSPFv3 Area Border Router advertise and describe routes that belong to other areas in the OSPF domain.

Hint

The summarization commands in OSPFv3 reside under the address-family configuration mode.

Hint

There are two methods that can be used to change the router ID of an OSPF router. The router can be rebooted or the OSPF process can be cleared.

Hint

The output of show ipv6 ospf interface fa0/0 shows that OSPFv3 is not enabled on the interface even though the command is in the running configuration. This is because no router ID has been configured on this router. The router ID in OSPFv3 is a 32 bit number, similar to the ID in OSPFv2. If the router ID is not manually specified then an IPv4 address from one of the interfaces is used instead. This router has no IPv4 addresses configured on it so a router ID cannot be automatically chosen. The router-id command must be configured under ipv6 router ospf 1 in order to fix this problem.

Hint

To form a neighbor relationship between OSPF speaking routers, the routers should reside on the same network and have the same Hello and dead intervals. Others criteria that must match are area ID and network type. In the topology displayed, the two routers are in two different areas, thus causing an area mismatch error.

Hint

Unlike OSPFv2, OSPFV3 does not use the network command to advertise directly attached networks. OSPFv3 is enabled directly on the interface. Once the command ipv6 ospf process_id area area_id is entered on the interface, that particular network will be included in OSPFv3 advertisements.

Hint

The OSPFv3 process is enabled on an interface-by-interface basis in interface configuration mode with the ospfv3 pid { ipv4 | ipv6 } area area-id command.

Hint

If the output of the show ipv6 ospf neighbor command does not show a state of FULL or 2WAY , the two routers have not formed an OSPFv3 adjacency. If two routers do not establish an adjacency, the link-state information will not be exchanged, and routes to destination networks may not exist.

Hint

By default, the dead interval is calculated as 4 times the hello interval.

Hint

The ABR for the Area 1 routing table will not change. The ABR connects to the backbone area as well as Area 1. The ABR router continues to receive external routes and intra-area routes through the interface that connects to the backbone area.

Hint

The command show ip ospf interface verifies the active OSPF interfaces. The command show ip interface brief is used to check that the interfaces are operational. The command show ip route ospf displays the entries that are learned via OSPF in the routing table. The command show ip protocol s checks that OSPF is enabled and lists the networks that are advertised.

Hint

The network prefix 2001:db8:20::/44 will summarize the 2001:db8:21:10::/64 and 2001:db8:22:10::/64 networks.