OSPF

What it is

OSPF (Open Shortest Path First) is a link state interior routing protocol. Each router floods descriptions of its links (LSAs), every router in an area builds the same link state database (LSDB), and each runs the SPF algorithm to compute loop free shortest paths.

OSPFv2 is for IPv4. It runs over IP with protocol number 89 and uses cost as its metric.

How it works

Router ID and areas

Every OSPF router needs a 32 bit router ID, written like an IPv4 address and unique in the domain. Configure it by hand with ospf 1 router-id 1.1.1.1. A changed router ID takes effect only after reset ospf process.

An area groups routers that share one LSDB. Area 0 is the backbone, and every other area must connect to it.

An ABR (area border router) has interfaces in area 0 and another area. An ASBR imports external routes into OSPF.

Packet types

All OSPF routers listen on 224.0.0.5. DR and BDR also listen on 224.0.0.6.

Neighbor states

Routers are neighbors at 2-Way and adjacent at Full. Two DROther routers on a LAN correctly stay in 2-Way.

To become neighbors, the Hello packets must agree on area ID, Hello and dead intervals, authentication, area type and, on broadcast networks, the network mask. Router IDs must be different.

Network types and timers

DR and BDR election

On broadcast and NBMA networks, a DR (designated router) and BDR reduce adjacencies. Every router forms Full adjacencies only with the DR and BDR.

The router with the highest interface DR priority wins (default 1, range 0 to 255). On a tie, the highest router ID wins. Priority 0 means the router never becomes DR or BDR.

The election is not preemptive. A router with higher priority that joins later does not take over until the DR and BDR fail or the OSPF process is reset.

Cost

Cost = reference bandwidth / interface bandwidth, with a minimum of 1. The Huawei default reference bandwidth is 100 Mbit/s, so FE and GE links both cost 1 and a 10 Mbit/s link costs 10. Change the reference with bandwidth-reference or set a cost per interface with ospf cost. The route cost is the sum of outbound interface costs along the path.

Typical configuration

[R1] ospf 1 router-id 1.1.1.1
[R1-ospf-1] area 0
[R1-ospf-1-area-0.0.0.0] network 10.0.12.0 0.0.0.255
[R1-ospf-1-area-0.0.0.0] network 1.1.1.1 0.0.0.0

The network command uses a wildcard mask (0 bits must match). Any interface whose address falls in the range runs OSPF in that area.

Verify with display ospf peer brief and display ip routing-table protocol ospf.

<R1> display ospf peer brief
         OSPF Process 1 with Router ID 1.1.1.1
                  Peer Statistic Information
 ----------------------------------------------------------------------------
 Area Id          Interface                        Neighbor id      State
 0.0.0.0          GigabitEthernet0/0/0             2.2.2.2          Full
 ----------------------------------------------------------------------------

A worked example

R1 and R2 share an Ethernet link but never become neighbors. R1 has ospf timer hello 5 on its interface and R2 uses the default 10 seconds.

Hello intervals must match, so each router ignores the other's Hellos. After undo ospf timer hello on R1, the neighbor reaches Full.

How the exam asks about this

• "What is the default OSPF Hello interval on an Ethernet interface?" → 10 seconds, with a dead interval of 40 seconds.
• "Two routers have DR priority 1. Which becomes DR?" → The one with the higher router ID.
• "In which state do two DROther routers on the same LAN remain?" → 2-Way.