Single-Area OSPFv2

What it is

OSPF (Open Shortest Path First) is a dynamic routing protocol that lets routers find each other and learn routes automatically.

Routers share what they know and each calculate the best path to every destination. OSPFv2 is the version for IPv4.

OSPF is a link-state protocol. Each router describes its own links to the others, so that every router ends up with the same map of the area.

Each router runs the shortest path first algorithm on that map to build its routing table. The metric is cost, based on bandwidth.

In a single-area design, all routers belong to the same area, which is area 0 (the backbone). This is the simplest setup and the one the CCNA focuses on.

How it works

Routers pick an ID, become neighbours and, on shared segments, elect a DR.

Router ID

Every OSPF router needs a unique 32-bit identifier, written like an IPv4 address. It is chosen in this order:

1. A manually configured router-id.
2. Otherwise, the highest IP address on an up loopback interface.
3. Otherwise, the highest IP address on an active physical interface.

Setting it by hand is best practice.

Becoming neighbours

OSPF routers find each other by sending hello packets to the multicast address 224.0.0.5.

On broadcast and point-to-point links, hellos go out every 10 seconds, and a neighbour is declared dead after 40 seconds without hearing from it. These are the hello and dead timers. Non-broadcast network types use 30 and 120 seconds.

For two routers to become neighbours, several settings must agree on both:

• the area
• the hello and dead timers
• the subnet and mask of the link
• any authentication

The router IDs must be different. A mismatch in any of these prevents the adjacency.

A neighbour relationship moves through states:

A healthy neighbour shows FULL. Neighbours stuck in INIT, 2-WAY or EXSTART often signal a problem.

Network types and the DR/BDR

On a point-to-point link, only two routers share it. They become fully adjacent with each other, and no election is needed.

On a broadcast network, such as an Ethernet segment with several routers, full adjacency between every pair would create a lot of overhead. OSPF avoids this by electing a Designated Router (DR) and a Backup Designated Router (BDR).

Every other router, called a DROTHER, forms a full adjacency only with the DR and the BDR. Between DROTHERs the state stays at 2-Way.

The DR and BDR use multicast address 224.0.0.6, and the DR sends updates to everyone.

The election works like this:

• The router with the highest OSPF priority becomes the DR. The default priority is 1.
• A tie is broken by the highest router ID.
• The second best becomes the BDR.
• A priority of 0 means the router can never be DR or BDR.

The election is not pre-emptive. If a router with a better priority joins later, it does not take over until the DR fails or OSPF restarts.

Ethernet interfaces default to the broadcast type. If only two routers share an Ethernet link, you can change it to point-to-point on both routers and avoid the election:

interface GigabitEthernet0/0
 ip ospf network point-to-point

Configuration

router ospf 1
 router-id 1.1.1.1
 network 10.1.1.0 0.0.0.255 area 0
 network 192.168.10.0 0.0.0.255 area 0
 passive-interface GigabitEthernet0/2

• router ospf 1 starts the process. The number is the process ID, with only local meaning, so it need not match on other routers.
• network ADDRESS WILDCARD area 0 enables OSPF on any interface whose address falls inside that range. Notice the wildcard mask, which is the inverse of a subnet mask.
• passive-interface stops hellos on an interface, such as one facing users. The network is still advertised, but no neighbours form there.

An alternative is to turn OSPF on directly under the interface:

interface GigabitEthernet0/0
 ip ospf 1 area 0

A default route can be shared with the others using default-information originate. The cost of an interface can be set with ip ospf cost, or the reference bandwidth can be adjusted with auto-cost reference-bandwidth, which should then be set the same on all routers.

Verifying

show ip ospf neighbor
show ip ospf interface brief
show ip protocols
show ip route ospf

show ip ospf neighbor lists each neighbour, its router ID, its state (such as FULL/DR) and the interface. show ip ospf interface brief shows the area, cost and state of each OSPF interface. show ip route ospf shows the learned routes, marked with O.