OSPF

The OSPF hello timer has been set to 15 seconds on a router in a point-to-point network. By default, what is the dead interval on this router? 60 seconds

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

What is used to create the OSPF neighbor table? adjacency database

The adjacency database is used to create the OSPF neighbor table. The link-state database is used to create the topology table, and the forwarding database is used to create the routing table.

Key concept

1. The adjacency database is responsible for creating the neighbor table. 2. The Dijkstra algorithm is responsible for computing the cost for each route. 3. The Link-state database is responsible for creating the topology table. 4. The Forwarding database is responsible for creating the routing table.

Which statement is correct about multiarea OSPF? Arranging routers into area partitions a large autonomous system in order to lighten the load on routers.

A company with one large autonomous system or AS can be divided into smaller areas. When this occurs and the OSPF routing protocol is implemented, the design is called multi-area OSPF. Multi-area OSPF decreases the frequency of the SPF calculation, thus lightening the load on the router. In a single area OSPF design, all the routers are located in area 0 or the backbone area.

(Data Structures) OSPF messages are used to create and maintain three OSPF databases, as follows:

Adjacency database - This creates the neighbor table. command: (show ip ospf neighbor) Link-state database (LSDB) - This creates the topology table. command:(show ip ospf database) Forwarding database - This creates the routing table. command: (show ip route)

What does an OSPF area contain? Routers that have the same link-state information in their LSDBs

An OSPF area contains one set of link-state information, although each router within the area will process that information individually to form its own SPF tree. OSPF process IDs are locally significant and are created by the administrator. Router IDs uniquely identify each router.

Why is a DR and BDR election necessary? Multiaccess networks can create two challenges for OSPF regarding the flooding of LSAs, as follows:

Creation of multiple adjacencies - Ethernet networks could potentially interconnect many OSPF routers over a common link. Creating adjacencies with every router is unnecessary and undesirable. It would lead to an excessive number of LSAs exchanged between routers on the same network. Extensive flooding of LSAs - Link-state routers flood their LSAs any time OSPF is initialized, or when there is a change in the topology. This flooding can become excessive.

Synchronizing OSPF Databases

Decide First Router-In the ExStart state, the two routers decide which router will send the DBD packets first. The router with the higher router ID will be the first router to send DBD packets during the Exchange state. In the figure, R2 has the higher router ID and sends its DBD packets first. Exchange DBDs-In the Exchange state, the two routers exchange one or more DBD packets. A DBD packet includes information about the LSA entry header that appears in the LSDB of the router. The entries can be about a link or about a network. Each LSA entry header includes information about the link-state type, the address of the advertising router, the cost of the link, and the sequence number. The router uses the sequence number to determine the newness of the received link-state information. Send an LSR-R1 compares the information received with the information it has in its own LSDB. If the DBD packet has a more current link-state entry, the router transitions to the Loading state. For example, in the figure, R1 sends an LSR regarding network 172.16.6.0 to R2. R2 responds with the complete information about 172.16.6.0 in an LSU packet. Again, when R1 receives an LSU, it sends an LSAck. R1 then adds the new link-state entries into its LSDB.

What happens immediately after two OSPF routers have exchanged hello packets and have formed a neighbor adjacency? Tehy echange abbreviated lists of their LSDBs

During the exchange of hello packets, OSPF routers negotiate the election process and set the OSPF parameters. DBD packets are exchanged after that step has been completed. DBD packets contain abbreviated lists of link-state information. After that information has been exchanged, OSPF routers exchange Type 3 LSR packets to request further information.

Link-State Operation

Establish Neighbor Adjacencies Exchange Link-State Advertisements Build the Link State Database Execute the SPF Algorithm Choose the Best Route

(Routing Protocol Messages)Routers running OSPF exchange messages to convey routing information using five types of packets. These packets, as shown in the figure, are as follows:

Hello packet Database description packet Link-state request packet Link-state update packet Link-state acknowledgment packet

What is the order of packet types used by an OSPF router to establish convergence?

Hello, DBD,LSR,LSU,LSAck

OSPF Packets

Hello-Discovers neighbors and builds adjacencies between them Database Description (DBD-)Checks for database synchronization between routers Link-State Request (LSR)-Requests specific link-state records from router to router Link-State Update (LSU)-Sends specifically requested link-state records Link-State Acknowledgment (LSAck)-Acknowledges the other packet types

What are two reasons for creating an OSPF network with multiple areas? (Choose two.) to reduce use of memory and processor resources to reduce SPF calculations

If a router is not running OSPF, it is not configurable with an OSPF area. OSPF areas have no direct relationship with the Internet. Routers that run OSPF can connect to the Internet, but multiple OSPF areas are not required for this purpose. OSPF areas help to decrease the demand for router memory and processing power by limiting OSPF protocol traffic, keeping link-state databases small, and requiring fewer SPF recalculations. Multiarea OSPF requires additional steps to configure and therefore does not simplify the configuration process.

answers

In the Two-Way State on multiaccess networks, the routers elect a Designated Router (DR) and a Backup Designated Router (BDR). 2. In the Exchange State, routers send each other DBD packets. 3. In the Init State, Hello packets are received from neighbors, containing the sending Router ID. 4. In the ExStart State on point-to-point networks, the routers decide which router initiates the exchange of DBD packets. 5. In the Full State, routers have converged. 6. In the Down State, no Hello packets are received. 7. In the Loading State, routes are processed using the SPF algorithm.

Which OSPF packet contains the different types of link-state advertisements? LSU

Link-state update (LSU) packets contain different types of link-state advertisements (LSAs). The LSUs are used to reply to link-state requests (LSRs) and to announce new information.

What function is performed by the OSPF designated router? dissemination of LSAs

OSPF designated routers are elected on multiaccess networks to disseminate LSAs to other OSPF routers. By having a single router disseminate LSAs, the exchanging of LSAs is more efficient.

At which OSPF state are neighbor routers converged and able to exchange routing updates? Full

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

What is a feature of the OSPF routing protocol?​ It scales well in both small and larger networks.

OSPF uses the SPF algorithm to choose the best path. Routing changes trigger routing updates (no 30-second updates). In IPv4, OSPF uses MD5 authentication between two neighboring OSPF routers. In IPv6, OSPFv3 does not include any authentication capabilities of its own. Instead it relies entirely on IPsec to secure communications between neighbors. Routers can be grouped into areas to support a hierarchical system.​

OSPF (Open Shortest Path First) single-area and multiarea

OSPFv2 is used for IPv4 networks. OSPFv3 is used for IPv6 networks

A router is participating in an OSPFv2 domain. What will always happen if the dead interval expires before the router receives a hello packet from an adjacent DROTHER OSPF router? OSPF will remove that neighbor from the router link-state database

On Cisco routers the default dead interval is 4 times the hello interval, and this timer has expired in this case. SPF does not determine the state of neighbor routers; it determines which routes become routing table entries. A DR/DBR election will not always automatically run; this depends on the type of network and on whether or not the router no longer up was a DR or BDR.

Multiarea OSPF

Smaller routing tables Reduced link-state update overhead Reduced frequency of SPF calculations

What is a function of OSPF hello packets? to discover neighbors and build adjacencies between them

The OSPF hello packet serves three primary functions: discover OSPF neighbors and establish adjacencies, advertise parameters that OSPF neighbors must agree on, and elect the DR and BDR.

answer

The Type 2 DBD packet contains an abbreviated list of the LSDB of the sending router. 2. The Type 4 LSU packet is used by routers to announce new information. 3. The Type 3 LSR packet is used by routers to request more information. 4. The Type 1 Hello packet is responsible for establishing and maintaining adjacency with other OSPF routers. 5. The Type 5 LSAck packet is used to confirm receipt of another OSPF packet type. 6. The Router ID uses an 8-bit field in the OSPFv2 hello packet.

Algorithm

The router builds the topology table using results of calculations based on the Dijkstra shortest-path first (SPF) algorithm. The SPF algorithm is based on the cumulative cost to reach a destination. The SPF algorithm creates an SPF tree by placing each router at the root of the tree and calculating the shortest path to each node. The SPF tree is then used to calculate the best routes. OSPF places the best routes into the forwarding database, which is used to make the routing table.

Which three statements describe features of the OSPF topology table? when converged, all routers in an area have identical topology tables It is a link-state database that represents the network topology The table can be viewed via the show ip ospf database command

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.

What is identical on all OSPF routers within a single area? link-state database

When the LSP flooding process completes, all OSPF routers will learn the same link-state information in the routing area. This information is used to build a complete link-state database, which will be the same on all OSPF routers within that specific area.