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Enterprise Networking, Security, and Automation ( Version 7.00) – Modules 1 – 2: OSPF Concepts and Configuration Exam 1. What is a function of OSPF hello packets? • to send specifically requested link-state records • to discover neighbors and build adjacencies between them • to ensure database synchronization between routers • to request specific link-state records from neighbor routers 2. Which OPSF packet contains the different types of link-state advertisements? • hello • DBD • LSR •LSU • LSAck 3. Which three statements describe features of the OSPF topology table? (Choose three.) • It is a link-state database that represents the network topology. • Its contents are the result of running the SPF algorithm. • When converged, all routers in an area have identical topology tables. • The topology table contains feasible successor routes. • The table can be viewed via the show ip ospf database command. • After convergence, the table only contains the lowest cost route entries for all known networks. 4. What does an OSPF area contain? • routers that share the same router ID • routers whose SPF trees are identical • routers that have the same link-state information in their LSDBs • routers that share the same process ID 5. What is used to facilitate hierarchical routing in OSPF? • the use of multiple areas • frequent SPF calculations • autosummarization • the election of designated routers 6. Which OSPF data structure is identical on all OSPF routers that share the same area? • forwarding database • link-state database Waarom is deze pagina onscherp?Dit is een Premium document. Word Premium om het volledige document te kunnen lezen. Waarom is deze pagina onscherp?Dit is een Premium document. Word Premium om het volledige document te kunnen lezen. Waarom is deze pagina onscherp?Dit is een Premium document. Word Premium om het volledige document te kunnen lezen.
Waarom is deze pagina onscherp?Dit is een Premium document. Word Premium om het volledige document te kunnen lezen. Waarom is deze pagina onscherp?Dit is een Premium document. Word Premium om het volledige document te kunnen lezen. IntroductionThis document describes how to repair a problem with Open Shortest Path First (OSPF) when routes in the database are not in the Routing Information Base (RIB) or routing table. PrerequisitesRequirementsReaders of this document must have knowledge of these topics:
Components UsedThe information in this document is based on these software and hardware versions:
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command. ConventionsFor more information on document conventions, see the Cisco Technical Tips Conventions. Background InformationThis document describes a common problem with tOpen Shortest Path First (OSPF) in which the routes that are in the database do not appear in the Routing Information Base (RIB) or routing table. In most cases OSPF finds a discrepancy in the database so it does not install the route in the routing table. Often, you can see the Adv Router is not-reachable message (which means the router that advertises the LSA is not reachable through OSPF) on top of the link-state advertisement (LSA) in the database when this problem occurs. Here is an example:
There are several reasons for this problem, most of which deal with mis-configuration or a broken topology. When the configuration is corrected the OSPF database discrepancy goes away and the routes appear in the routing table. This document explains some of the more common reasons that can cause the discrepancy in the database. Some of the commands used throughout this document for verification of OSPF behavior include the show ip ospf interface, show ip ospf database router, show ip ospf neighbor and the show ip ospf database external . If you have the output of any of these commands from your Cisco device, you can use Cisco CLI Analyzer to display potential issues and fixes. Reason 1: Network Type MismatchThe next network diagram is used as an example:
As you can see in the previous output, Router R4-4K is configured for broadcast, and Router R1-7010 is configured for point-to-point. This kind of network type mismatch makes the advertising router unreachable. You can see that for subnet 172.16.32.0/24, Router R1-7010 generates a point-to-point link and Router R4-4K generates a transit link. This creates a discrepancy in the link-state database, which means no routes are installed in the routing table. R1-7010(5)#show ip route 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks C 172.16.32.0/24 is directly connected, Serial1/0 C 172.16.30.1/32 is directly connected, Loopback0 SolutionTo solve this problem, configure both routers for the same network type. You can either change the network type of Router R1-7010 to broadcast, or change Router R4-4K serial interface to point-to-point. Note: If you have a situation where one side is a multipoint interface and the other side is a sub-interface then change the network type to broadcast on both sides. In this example we have removed the network-type broadcast statement on R4-4K because both sides are point-to-point High-Level Data Link Control (HDLC) encapsulated interfaces. R4-4K(4)#configure terminal R4-4K(4)(config)#interface serial 2 R4-4K(4)(config-if)#no ip ospf network broadcast R4-4K(4)(config-if)#end R4-4K(4)#show ip ospf interface serial 2 Serial2 is up, line protocol is up Internet Address 172.16.32.1/24, Area 0 Process ID 20, Router ID 172.16.33.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:04 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 172.16.32.2 Suppress hello for 0 neighbor(s) Reason 2: Wrong Address Assignment in Dual Serial Link SetupConsider this network diagram as an example:
You can see that the IP addresses are flipped in the previous configurations, which causes a discrepancy in the OSPF database. However, the routers still form neighbors in Cisco IOS version earlier than 12.1 because on a point-to-point link, OSPF routers do not verify that the neighboring router is on the same subnet. R4-4K(4)#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 172.16.32.2 1 FULL/ - 00:00:37 172.16.32.2 Serial2 172.16.32.2 1 FULL/ - 00:00:31 172.16.29.2 Serial3 From the previous output, you can see that Serial2 is used to form neighbors with IP address 172.16.32.2, which is not in the same subnet. Although neighbors are formed, no routes are installed in the routing table: R1-7010(5)#show ip route 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks C 172.16.32.0/24 is directly connected, Serial1/0 C 172.16.29.0/24 is directly connected, Serial1/1 C 172.16.30.1/32 is directly connected, Loopback0 SolutionTo solve this problem, either correctly assign the IP addresses or switch the serial cables. Here we have corrected the IP addresses:
R4-4K(4)#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 172.16.32.2 1 FULL/ - 00:00:36 172.16.32.2 Serial2 172.16.32.2 1 FULL/ - 00:00:39 172.16.29.2 Serial3 Now it shows the correct neighbor address on the Serial 2 interface. The routes are also in the routing table: Reason 3: One Side of Point-to-Point Link Included in Wrong Majornet or SubnetConsider this network diagram as an example:
This situation creates exactly the same behavior as the Wrong Address Assignment in Dual Serial Link Setup. To solve the problem, assign IP addresses in the same subnet on both routers. Reason 4: One Side Is Unnumbered and the Other Side Is NumberedConsider the next network diagram as an example:
R4-4K(4)#show interface serial 2 Serial2 is up, line protocol is up Hardware is cxBus Serial Interface is unnumbered. Using address of Loopback0 (172.16.35.1) R1-7010(5)# show interface serial 1/0 Serial1/0 is up, line protocol is up Hardware is cxBus Serial Internet address is 172.16.32.2/24 The previous output shows that R4-4K Serial 2 interface is unnumbered to Loopback0 (use address of Loopback0 172.16.35.1), whereas R1-7010's Serial 1/0 is a numbered interface. As you can see previously, the network-type in both cases is point-to-point. The problem is that one side is unnumbered and the other side is not, which creates a discrepancy in the database as shown below. You can see that R1-7010 generates an LSA for this point-to-point link with the Link Data field that contains its interface address, while R4-4K generates the LSA for the same link with the Link Data field that contains the MIBII IfIndex value. This creates a discrepancy in the link-state database, which means no routes are installed in the routing table. R1-7010(5)#show ip route 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks C 172.16.32.0/24 is directly connected, Serial1/0 C 172.16.30.1/32 is directly connected, Loopback0 SolutionTo solve this problem, configure both routers' serial interfaces as either numbered or unnumbered. In this example we have numbered the serial 2 interface of router R4-4K. Reason 5: Broken PVC in Fully Meshed Frame Relay EnvironmentConsider this network diagram as an example:
The broadcast model over Frame Relay works properly as long as the Frame Relay cloud is fully meshed. If any permanent virtual circuits (PVCs) are broken, it can create problems in the OSPF database, which in turn produces the Adv router not reachable message. In this example, the PVC between R9-2500 and R4-4K is broken, and R9-2500 link to the designated router (DR) is broken. As a result, R9-2500 declares all LSAs from R3-4K (which is not a DR), as unreachable. As you can see, R9-2500 does not generates a transit link for the serial interface attached to R3-4K; it generates a stub link instead because as far as R9-2500 is concerned there is no DR on this link. Refer to Problems with OSPF in NBMA and Broadcast Mode over Frame Relay for more detailed information about this problem. Reason 6: Forwarding Address Known via an External RouteConsider this network diagram as an example:
R2507 has 10.200.1.0/24 in its database but it has not installed it in the routing table because 10.3.4.4 is learned via an OSPF external route. Note: In this scenario the reason that the Forward Address was learned through an external route is due to the next caveat. With the fix of 'Cisco bug ID CSCdp72526' (registered customers only) , OSPF does not generate a type-5 link-state advertisement (LSA) of an overlapped external network; therefore, R2507 shows only a summary inter-area route of 10.0.0.0/8. Then, R2507 installs 10.200.1.0/24 with a forwarding address and it is reachable via an inter-area route 10.0.0.0/8, thus in compliance with RFC 2328. After the fix of the mentioned bug, output looks as follows:
If the forwarding address is also known via an external route, OSPF does not install that route in the routing table. For more detailed information about this problem, see Common Routing Problem with OSPF Forwarding Address. Reason 7: Routes Blocked by Distribute ListThe next network diagram is used as an example:
As you can see previously, R1-7010 has the distribute-list command configured and it only allows the 172.16.32.0/24 address range to be installed in the routing table. In link-state protocols you can not really filter an LSA with the distribute-list command. The LSA is still present in the database; however the LSA is not installed in the routing table. The distribute-list configuration command on R1-7010 filters the 172.16.33.1/32 network and prevents the subnet to be installed in the routing table. R1-7010(5)#show ip route 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks C 172.16.32.0/24 is directly connected, Serial1/0 C 172.16.30.1/32 is directly connected, Loopback0 SolutionTo solve this problem, configure R1-7010 and allow 172.16.33.0/24 in the access control list (ACL) so this network gets installed in the routing table. Related Information
Which OSPF database is identical on all converged routers within the?The link-state database lists information about all other routers within an area and is identical across all OSPF routers participating in that area.
Which OSPF component is identical on all routers in an OSPF area after convergence?When converged, all routers in an area have identical topology table, The table can be viewed via the show ip ospf database command. and It is a link-state database that represents the network topology.
Which of the following OSPF packets contains an abbreviated list of the Lsdb of the sending router?Type 2: Database Description (DBD) packet - Contains an abbreviated list of the sending router's LSDB and is used by receiving routers to check against the local LSDB.
What does an OSPF area contain?What does an OSPF area contain? The OSPF area contains OSPF internal routers. Their connections, which mean links, and through these links, OSPF neighborship allows OSPF Routers to exchange IP prefixes. In a given OSPF Area, every router has the same Link state database.
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Which OSPF database is identical on all converged routers within the same OSPF area
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