OSPF in a Single Area

When configuring any OSPF router, you must establish what area assignment the interface will be enabled for. OSPF has some basic rules when it comes to area assignment. OSPF must be configured with areas. The backbone area 0, or 0.0.0.0, must be configured if you use more than one area assignment. If your OSPF design has only one area, it can have any number.

Table 2-11 Common OSPF Terms

Term

Description

Hello packet

Exchanged by the routers for neighbor discovery and forming adjacency, neighbor keep-alive, and DR/BDR election.

Link state

Information is shared between directly connected routers. This information propagates unchanged throughout the network and is also used to create a shortest path first (SPF) tree.

Area

A group of routers and links that share the same Area ID. All OSPF routers require area assignments. All routers within an area have the same database. Link state flooding is limited to an area.

Autonomous system (AS)

A network under a common network administration domain running common routing protocols.

Cost (OSPF Metric)

The routing metric used by OSPF. Lower costs are always preferred. You can manually configure the cost of an interface with the ip ospf cost command. By default, the cost is calculated by using the formula, cost = 108/bandwidth.

Router ID

Each OSPF router requires a unique router ID, which is the highest IP address configured on a Cisco router or the highest-numbered loopback address. You can manually assign the router ID.

Adjacency

When two OSPF routers have exchanged information between each other and have the same topology table. Adjacency can have a number of states or exchange states:

Init state—When Hello packets have been sent and are awaiting a reply to establish two-way communication.

Establish bidirectional (two-way) communication—Accomplished by the discovery of the Hello protocol routers and the election of a DR.

Exstart—Two neighbor routers form a master/slave relationship and agree upon a starting sequence that will be incremented to ensure that LSAs are acknowledged.

Exchange state—Database Description (DD) packets continue to flow as the slave router acknowledges the master's packets. OSPF is operational because the routers can send and receive LSAs between each other. DD packets contain information such as the router ID, area ID, checksum, if authentication is used, link-state type, and the advertising router. LSA packets also contain information such as router ID, and additionally include MTU sizes, DD sequence numbering, and any options.

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Table 2-11 Common OSPF Terms (Continued)

Term

Description

Adjacency (Continued)

Loading state—Link-state requests are sent to neighbors asking for recent advertisements that have been discovered in Exchange state but not received.

Full state—Neighbor routers are fully adjacent because their link-state databases are fully synchronized within the area. Routing tables begin to be populated.

Topology table

Also called the link-state table, this table contains every link in the entire network.

Designated Router (DR)

This router ensures adjacencies between all neighbors on a multiaccess network (such as Ethernet). This ensures that not all routers need to maintain full adjacencies with each other.

The DR is selected based on the priority. In a tie, the router with the highest router ID is selected.

Backup DR

A Backup Designated Router is designed to perform the same functions in case the DR fails.

Link-state advertisement (LSA)

A packet that contains all relevant information regarding a router's links and the state of those links.

Priority

Sets the router's priority so a DR or BDR can be correctly elected.

Router links

Describe the state and cost of the router's interfaces to the area. Router links use LSA type 1.

Summary links

Originated by Area Border Routers, these links describe networks in the AS. Summary links use LSA type 3 and 4.

Network links

Originated by DRs. Network links use LSA type 2.

External links

Originated by autonomous system boundary routers; they advertise destinations external to the AS or the default route external to the AS.

Area Border Router (ABR)

Router located on the border of one or more OSPF areas to connect those areas to the backbone network.

Autonomous system boundary router (ASBR)

An ABR located between an OSPF autonomous system and a non-OSPF network.

The configuration steps to enable OSPF in a single area are as follows:

Step 1 Start OSPF with the command router ospf process ID. The process ID is locally significant to the router.

Step 2 Enable the interfaces with the network command. For example, to place the Network 131.108.1.0 in area 1, the IOS command is network 131.108.1.0 area 1.

Step 3 Identify area assignments.

Step 4 (Optional) Assign the router ID with the router-id router-id IOS command under the OSPF process.

NOTE The following is a list of reasons OSPF (link-state) is considered a better routing protocol than RIPv1 (distance vector):

• OSPF has no hop count limitation. (RIP has a limit of 15 hops only.)

• OSPF understands VLSM and allows for summarization.

• OSPF uses multicasts (not broadcasts) to send updates.

• OSPF converges much faster than RIP because OSPF propagates changes immediately. OSPF is faster because it sends the link update and then calculates the local routing table. RIP calculates the local routing table and then sends an update.

• OSPF allows for load balancing with up to six equal-cost paths.

• OSPF has authentication available (RIPv2 does also, but RIPv1 does not).

• OSPF allows tagging of external routes injected by other autonomous systems.

• OSPF configuration, monitoring, and troubleshooting have a far greater IOS tool base than RIP.

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