6 Open Shortest Path First (OSPF)

Question 1

What is Open Shortest Path First (OSPF) ?

Answer 1

Open Shortest Path First is an open standard routing protocol which works by using the Dijkstra algorithm to initially construct a shortest path tree and follows that by populating the routing table with the resulting best paths. It supports multiple, equal-cost routes to the same destination, and also supports both IPv4 and IPv6 routed protocols.

Question 2

(OSPF) What is a Link ?

Answer 2

A link is a network or router interface assigned to any given network. When an interface is added to the OSPF process, it’s considered to be a link. This link, or interface, will have up or down state information associated with it as well as one or more IP addresses.

Question 3

(OSPF) What is a Router ID ?

Answer 3

The router ID (RID) is an IP address used to identify the router. Cisco chooses the router ID by using the highest IP address of all configured loopback interfaces. If no loopback interfaces are configured with addresses, OSPF will choose the highest IP address out of all active physical interfaces. To OSPF, this is basically the “name” of each router.

Question 4

(OSPF) What is a Neighbor ?

Answer 4

Neighbors are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link. OSPF neighbors must have a number of common configuration options to be able to successfully establish a neighbor relationship, and all of these options must be configured exactly the same way:

■ Area ID
■ Stub area flag
■ Authentication password (if using one)
■ Hello and Dead intervals

Question 5

(OSPF) What is Adjacency ?

Answer 5

An adjacency is a relationship between two OSPF routers that permits the direct exchange of route updates. Unlike EIGRP, which directly shares routes with all of its neighbors, OSPF is really picky about sharing routing information and will directly share routes only with neighbors that have also established adjacencies. And not all neighbors will become adjacent—this depends upon both the type of network and the configuration of the routers. In multi-access networks, routers form adjacencies with designated and backup designated routers. In point-to-point and point-to-multipoint networks, routers form adjacencies with the router on the opposite side of the connection.

Question 6

(OSPF) What is a Designated Router ?

Answer 6

A designated router (DR) is elected whenever OSPF routers are connected to the same broadcast network to minimize the number of adjacencies formed and to publicize received routing information to and from the remaining routers on the broadcast network or link. Elections are won based upon a router’s priority level, with the one having the highest priority becoming the winner. If there’s a tie, the router ID will be used to break it. All routers on the shared network will establish adjacencies with the DR and the BDR, which ensures that all routers’ topology tables are synchronized.

Question 7

(OSPF) What is a Backup Designated Router ?

Answer 7

A backup designated router (BDR) is a hot standby for the DR
on broadcast, or multi-access, links. The BDR receives all routing updates from OSPF adjacent routers but does not disperse LSA updates.

Question 8

(OSPF) What is Hello Protocol ?

Answer 8

The OSPF Hello protocol provides dynamic neighbor discovery and maintains neighbor relationships. Hello packets and Link State Advertisements (LSAs) build and maintain the topological database. Hello packets are addressed to multicast address 224.0.0.5

Question 9

(OSPF) What is Neighborship Database ?

Answer 9

The neighborship database is a list of all OSPF routers for which Hello packets have been seen. A variety of details, including the router ID and state, are maintained on each router in the neighborship database.

Question 10

(OSPF) What is Topological Database ?

Answer 10

The topological database contains information from all of the Link State Advertisement packets that have been received for an area. The router uses the information from the topology database as input into the Dijkstra algorithm that computes the shortest path to every network.

Question 11

(OSPF) What is Link State Advertisement ?

Answer 11

A Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information that’s shared among OSPF routers. There are different types of LSA packets. An OSPF router will only exchange LSA packets with routers it has established adjacencies for.

Question 12

(OSPF) What are OSPF areas ?

Answer 12

An OSPF area is a grouping of contiguous networks and routers. All routers in the same area share a common area ID. Because a router can be a member of more than one area at a time, the area ID is associated with specific interfaces on the router. This allows some interfaces to belong to area 1 while the remaining interfaces can belong to area 0. All of the routers within the same area have the same topology table. When configuring OSPF with multiple areas, you’ve got to remember that there must be an area 0 and that this is typically considered the backbone area. Areas also play a role in establishing a hierarchical network organization—something that really enhances the scalability of OSPF!

Question 13

(OSPF) What is a Broadcast (multi-access) ?

Answer 13

Broadcast (multi-access) networks like Ethernet allow multiple devices to connect to or access the same network, enabling a broadcast ability so a single packet can be delivered to all nodes on the network. In OSPF, a DR and BDR must be elected for each broadcast multi-access network.

Question 14

(OSPF) What is Non-broadcast multi-access ?

Answer 14

Nonbroadcast multi-access (NBMA) networks like Frame Relay, X.25, and Asynchronous Transfer Mode (ATM) allow for multi-access without broadcast ability like Ethernet. NBMA networks require special OSPF configuration to work.

Question 15

(OSPF) What is Point-to-point ?

Answer 15

Point-to-point is a type of network topology made up of a direct connection between two routers that provides a single communication path. The point-to-point connection can be physical—for example, a serial cable that directly connects two routers—or logical, where two routers thousands of miles apart are connected by a circuit in a Frame Relay network. Either way, point-to-point configurations eliminate the need for DRs or BDRs.

Question 16

(OSPF) What is Point-to-multipoint ?

Answer 16

Point-to-multipoint is a network topology made up of a series of connections between a single interface on one router and multiple destination routers. All interfaces on all routers share the point-to-multipoint connection and belong to the same network. Point-to-multipoint networks can be further classified according to whether they support broadcasts or not. This is important because it defines the kind of OSPF configurations you can deploy.

Question 17

OSPF operation is divided into these three categories:

Answer 17

■ Neighbor and adjacency initialization
■ LSA flooding
■ SPF tree calculation

Question 18

What is Neighbor and adjacency initialization ?

Answer 18

When OSPF is initialized on a router, the router allocates memory for itself, as well as for the maintenance of both neighbor and topology tables. Once the router determines which interfaces have been configured for OSPF, it’ll check to see if they’re active and begin sending Hello packets. The Hello protocol is used to discover neighbors, establish adjacencies, and maintain relationships with other OSPF routers. Hello packets are periodically sent out of each enabled OSPF interface and in environments that support multicast. The address used for this is 224.0.0.5, and the frequency with which Hello packets are sent out depends upon the network type and topology. Broadcast and point-to-point networks send Hellos every 10 seconds, but non-broadcast and point-to-multipoint networks send them every 30 seconds.

Question 19

What is LSA Flooding ?

Answer 19

LSA flooding is the method OSPF uses to share routing information. Through Link State Updates (LSU) LSU packets, LSA information containing link-state data is shared with all OSPF routers within an area. The network topology is created from the LSA updates, and flooding is used so that all OSPF routers have the same topology map to make SPF
calculations with. Efficient flooding is achieved through the use of a reserved multicast address: 224.0.0.5 (AllSPF Routers). LSA updates, which indicate that something in the topology has changed, are handled a bit differently. The network type determines the multicast address used for sending updates. Point-to-multipoint networks use the adjacent router’s unicast IP address. Once the LSA updates have been flooded throughout the network, each recipient must acknowledge that the flooded update has been received. It’s also important for recipients to validate the LSA update.

Question 20

What is SPF Tree Calculation ?

Answer 20

Within an area, each router calculates the best / shortest path to every network in that same area. This calculation is based upon the information collected in the topology database and an algorithm called shortest path first (SPF). Picture each router in an area constructing a tree—much like a family tree—where the router is the root and all other networks are arranged along the branches and leaves. This is the shortest path tree used by the router to insert OSPF routes into the routing table. Understand that this tree only contains networks that exist in the same area as the router itself does. If a router has interfaces in multiple areas, then separate trees will be constructed for each area. One of the key criteria considered during the route selection process of the SPF algorithm is the metric or cost of each potential path to a network. But this SPF calculation doesn’t apply to routes from other areas.

Question 21

What are OSPF Metrics ?

Answer 21

OSPF uses a metric referred to as cost. A cost is associated with every outgoing interface included in an SPF tree. The cost of the entire path is the sum of the costs of the outgoing interfaces along the path. Because cost is an arbitrary value as defined in RFC 2338, Cisco had to implement its own method of calculating the cost for each OSPF-enabled interface. Cisco uses a simple equation of 108 / bandwidth, where bandwidth is the configured bandwidth for the interface. Using this rule, a 100 Mbps Fast Ethernet interface would have a default OSPF cost of 1 and a 1,000 Mbps Ethernet interface would have a cost of 1. This value can be overridden with the ip ospf cost command. The cost is adjusted by changing the value to a number within the range of 1 to 65,535. Because the cost is assigned to each link, the value must be changed on the specific interface you want to
change the cost on.

Question 22

Enabling OSPF

Answer 22

The easiest, although least scalable way to configure OSPF, is to just use a single area. Doing this requires a minimum of two commands. The first command used to activate the OSPF routing process is:

Router(config)# router ospf ?

<1-65535> Process ID
A value in the range from 1 to 65,535 identifies the OSPF process ID. It’s a unique number on this router that groups a series of OSPF configuration commands under a specific running process. Different OSPF routers don’t have to use the same process ID to communicate. It’s a purely local value that doesn’t mean a lot, but you still need to remember that it can’t start at 0 because that’s for the backbone. It has to start at a minimum of 1. You can have more than one OSPF process running simultaneously on the same router if you want, but this isn’t the same as running multi-area OSPF. The second process will maintain an entirely separate copy of its topology table and manage its communications independently of the first one. It comes into play when you want OSPF to connect multiple ASs together.

Question 23

Configuring OSPF Areas

Answer 23

After identifying the OSPF process, you’ve got to identify the interfaces that you want to activate OSPF communications on as well as the area in which each resides. This will also
configure the networks you’re going to advertise to others.

Router# config t
Router(config)# router ospf 1
Router(config-router)# network 10.0.0.0 0.255.255.255 area ?
<0-4294967295> OSPF area ID as a decimal value
A.B.C.D OSPF area ID in IP address format
Router(config-router)# network 10.0.0.0 0.255.255.255 area 0

Remember, the OSPF process ID number is irrelevant. It can be the same on every router on the network, or it can be different—doesn’t matter. It’s locally significant and just enables the OSPF routing on the router. The arguments of the network command are the network number (10.0.0.0) and the wildcard mask (0.255.255.255). The combination of these two numbers identifies the interfaces that OSPF will operate on and will also be included in its OSPF LSA advertisements. Based on the example configuration, OSPF will use this command to find any interface on the router configured in the 10.0.0.0 network and will place any interface it finds into area 0. You can also label an area using an IP address format. The final factor is the area number, which indicates the area that the interfaces identified in the network and wildcard mask belong. Remember that OSPF routers will become neighbors only if their interfaces share a network that’s configured to belong to the same area number. The format of the area number is either a decimal value from the range 0 to 4,294,967,295 or a value represented in standard dotted-decimal notation. For example, area 0.0.0.0 is a legitimate area and is identical to area 0.

Question 24

Wildcards:

Answer 24

A 0 octet in the wildcard mask indicates that the corresponding octet in the network must match exactly. On the other hand, a 255 indicates that you don’t care what the corresponding octet is in the network number. A network and wildcard mask combination of 1.1.1.1 0.0.0.0 would match an interface configured exactly with 1.1.1.1 only, and nothing else. This is really useful if you want to activate OSPF on a specific interface in a very clear and simple way. If you want to match a range of networks, the network and wildcard mask combination of 1.1.0.0 0.0.255.255 would match any interface in the range of 1.1.0.0 to 1.1.255.255. Because of this, it’s simpler and safer to stick to using wildcard masks of 0.0.0.0 and identify each OSPF interface individually. Once configured, they’ll function exactly the same.

Question 25

OSPF and Loopback Interfaces

Answer 25

You’ve got make sure to configure loopback interfaces when using OSPF. In fact, Cisco suggests using them whenever you configure OSPF on a router for stability. Loopback interfaces are logical interfaces, which means they’re virtual, software-only interfaces, not actual, physical router interfaces. A big reason we use loopback interfaces with OSPF configurations is because they ensure an interface is always active and available for OSPF processes. Loopback interfaces also come in very handy for diagnostic purposes as well as for OSPF configuration. Understand that if you don’t configure a loopback interface on a router, the highest active IP address on a router will become that router’s RID during bootup!

Question 26

OSPF router ID (RID)

Answer 26

The RID is not only used to advertise routes; it’s also used to elect the designated router (DR) and the backup designated router (BDR). These designated routers create adjacencies when a new router comes up and exchanges LSAs to build topological databases.

Question 27

The show ip ospf Command

Answer 27

The show ip ospf command is what you’ll need to display OSPF information for one or all OSPF processes running on the router. Information contained therein includes the router ID, area information, SPF statistics and LSA timer information.

Question 28

The show ip ospf database Command

Answer 28

Using the show ip ospf database command will give you information about the number of routers in the internetwork (AS) plus the neighboring router’s ID—the topology database
I mentioned earlier. Unlike the show ip eigrp topology command, this command reveals the OSPF routers, but not each and every link in the AS like EIGRP does.

Question 29

The show ip ospf interface Command

Answer 29

The show ip ospf interface command reveals all interface-related OSPF information. Data is displayed about OSPF information for all OSPF-enabled interfaces or for specified interfaces.

■ Interface IP address
■ Area assignment
■ Process ID
■ Router ID
■ Network type
■ Cost
■ Priority
■ DR/BDR election information (if applicable)
■ Hello and Dead timer intervals
■ Adjacent neighbor information

Question 30

The show ip ospf neighbor Command

Answer 30

The show ip ospf neighbor command is really useful because it summarizes the pertinent OSPF information regarding neighbors and the adjacency state. If a DR or BDR exists, that
information will also be displayed.

Question 31

The show ip protocols Command

Answer 31

The show ip protocols command is also highly useful, whether you’re running OSPF, EIGRP, RIP, BGP, IS-IS, or any other routing protocol that can be configured on your router. It provides an excellent overview of the actual operation of all currently running protocols!

Question 32

Compare OSPF and RIPv1

Answer 32

OSPF is a link-state protocol that supports VLSM and classless routing; RIPv1 is a distance-vector protocol that does not support VLSM and supports only classful routing.

Question 33

How do OSPF routers become neighbors and/or adjacent ?

Answer 33

OSPF routers become neighbors when each router sees the other’s Hello packets.

Question 34

administrative distance of OSPF ?

Answer 34

By default, the administrative distance of OSPF is 110.

Question 35

Hello packets are addressed to ?

Answer 35

Hello packets are addressed to multicast address 224.0.0.5.