An OSPF question

But router knows interface type, of course. serial interfaces are treated as p2p , ethernet as broadcast, e.g.

Thanks, I use fast ethernet.

Broadcast should work over Ethernet - as long as both ends are conifgured consistently. Make sure they are both in the same subnet, same mask, same area.

you may want to turn on more detailed OSPF logging to see if there are any diagnostics available via the internal log.

e:

=FD=D3=C3=CE=C4=D7=D6 -

Thanks, I may not give a good descrption of my question. Two routers (not cisco) direct connect via fast ethernet using ospf. How do the routers know it is broadcast or p2p? TIA, /s

Once again, router knows which interface is installed.

If router detects an ethernet interface it knows that a broadcastable medium is connected. Broadcast is essential for ethernet.

If router detects an serial interface it knows there is no broacastable medium.

If your questions refers to configuration statement "ip ospf network broadcast" have a look at this great document

Especially pay attention on sections Neighbors and Adjacencies. It's all explained herein very well.

Andre

rote:

=D2=FD=D3=C3=CE=C4=D7=D6 -

Thanks, I have checked the cisco url you list above. There is a command to assign network type "ip ospf network {broadcast | non- broadecast | point to multipoint} My question is if two routers connected via ethernet, Do I should use broadcast or non-broadcast or either way? /s

document

>

This command is used in interface configuration context and is used to change default interface network type. You don't need it. There's no difference between direct connected router or ones who are connected via a switch. Broadcast will work either.

Andre

document

>

it depends.

some boxes (not cisco AFAIR) do not allow you to choose, as they limit which options work on Ethernet.

I have had major hassles with cisco to foundry links on GigE which will not work with some permutations of the choices - but do not bother to log why they do not form an adjacency......

the "classic" argument is that point to point uses a bit less resources in the router, so is "better" if you have a router with lots of OSPF interface - but this needs more config. Reality is if you are that close to the edge, your network is already broken, but you havent provoked it enough yet......

However - you imply this is "just point to point" - with many OSPF devices actually being switches and sometimes using adjacencies on VLANs, you can end up with more than 2 devices in a subnet even when all the pysical connections are point to point. Here, making a special case for when there are only 2 devices may make life more complicated.

personally - mimimum config and being consistent seems to cause fewer problems so is the way to go. So - broadcast is my preferred setup.

document

On a broadcast network, OSPF need to perform DR election.

This not required on a point-to-point link when OSPF network type is configured as point-to-point and thus the OSPF adjacency should form more quickly.

It does, no need for DR/BDR selection. Also, you remove Type2 LSAs so on a large network, it can help. Granted, it's not night and day difference, but I'm all for cleaning things up even at the cost a few more config lines.