OSPF Neighbors

Dynaice

Hi All,

I have what maybe a silly question, but wanted to see if I could get some clarification on the OSPF neighbor states.

From what I understand, to be a neighboring OSPF router, you must be in the same subnet among other requirements, so wouldn't this mean that only directly connected routers would be neighbors? For some reason I gathered that another router NOT directly connected to the router could be a neighbor, but when I think about it it, I don't see how that's possible seeing that routers separate networks.

Maybe I was getting it confused with a router doesn't have to be directly connected to be in the same area, which would make sense.

If anyone could help clear this up with me, I would appreciate it.

Thanks

Roguetadhg

They don't need to be directly connected to be in an area. Areas are there to help disperse routing loops. So a rather large collection of routers could be all in Area 0.

Each router in an Area knows where it's at, in terms of it's area. It also calculates where to go to get to other networks.

Dynaice

But they would need to be directly connected to be neighbors, correct? As in directly connected is the only way to be in the same subnet as I understand.

Roguetadhg

They would need to be in the same subnetwork, directly connected to become neighbors. Among other items.

NetworkVeteran

Dynaice wrote: »

From what I understand, to be a neighboring OSPF router, you must be in the same subnet among other requirements

There are exceptions to this rule for point-to-point links and virtual links.

so wouldn't this mean that only directly connected routers would be neighbors?

The routers could be separated by L2 devices (hub or switch), a WAN (dial-up, frame relay, mpls), or a tunnel (gre) and still share the same network mask and be OSPF neighbors.

Roguetadhg

good time to lab some ospf thanks Op.

Dynaice

Great! Thanks to both of you. This clears up questions precisely.

Dynaice

Roguetadhg wrote: »

good time to lab some ospf thanks Op.

Haha, ya I was playing with it on packet tracer, but you never know when a simulator just won't do certain things. I haven't had the chance to try it on my real routers yet.

Roguetadhg

Well, Im reading the ospf section in my NP book trying to keep the study jucies flowing (More time to absorb the OSPF/EIGRP information)

On a direct connection serial link (HDLC/PPP) neighborships will form with the next router over. Broadcast networks - Ethernet, Hubs, Switches - elect a Designated router (DR) and a BDR (Backup DR). All routers will form an adj with the B/DRs. all other routers on the lan segment will be named DRothers.

Instead of giving their updates to everyone, broadcasting it to all routers. The DROTHERS, non-B/DRs, will unicast to the B/DRs. This is to ensure that the routers on the lan have the same database. DROthers will form a Two-way relationship with their DROTHER buddies.

networker050184

Think of it this way, what is the TTL in a standard OSPF hello packet? How far will that get it?

Roguetadhg

1. Thank you google.

vishaw1986

Hi Dynaice

In OSPF we use two terms

1> Neighbors : Two ospf router which share a common physical interface and match certain parameter in the Hello Packet (Hello and hold timers , Area ID , Same Primary Subnet , Same area type ) are said to be neighbors .

2> Adjacency : when two router share the complete LSDB , they are adjacent .

Very good explanation I got on Internet :

Lets take a broadcast access medium topology like a LAN for example, to illustrate the differences.

As you know, in a LAN environment you have a DR election. After this election, the DR and BDR are elected, and all other routers( DROthers) form adjacencies with the DR/BDR. Meaning, they pass the 2WAY state into ex-start and start exchanging LSA headers to begin the route exchange.

However, between two DROther routers, you will never pass beyond 2WAY. That is to say, they will exchange hello packets, but will never enter exstart, never exchange LSA headers, never converge into adjaceny neighbors.