OSPF on broacast links

[opers13]

Hi all,

why would I use the command "ip ospf network point-to-point" on broadcast links and not have DR and BDR election?

I know the links would form adjacencey with each other but... any other reason??

thanks

[opers13]

This router has more than 3 speaking OSPF neighbors, here's a sh ip ospf neighbnor output:

RTR1#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

xxx.xx.1.4 0 FULL/ - 00:00:35 xxx.xx.130.6 GigabitEthernet1/24

xxx.xx.1.2 0 FULL/ - 00:00:35 xxx.xx.130.14 GigabitEthernet5/1

xxx.xx.1.3 0 FULL/ - 00:00:30 xxx.xx.130.2 GigabitEthernet5/2

xxx.xx.1.3 0 FULL/ - 00:00:32 xxx.xx.131.10 GigabitEthernet1/3

xxx.xx.4.161 0 FULL/ - 00:00:32 xxx.xx.131.1 GigabitEthernet1/1

xxx.xx.4.165 0 FULL/ - 00:00:38 xxx.xx.131.5 GigabitEthernet1/2

xxx.xx.4.169 0 FULL/ - 00:00:32 xxx.xx.133.21 GigabitEthernet1/4

xxx.xx.4.173 0 FULL/ - 00:00:34 xxx.xx.133.25 GigabitEthernet1/5

xxx.xx.2.17 0 FULL/ - 00:00:37 xxx.xx.0.6 GigabitEthernet1/7

xxx.xx.2.13 0 FULL/ - 00:00:34 xxx.xx.0.2 GigabitEthernet1/6

RTR1#

[Kevin Dorrell]

Wow! I suppose all the other routers on the VLAN must have something similar. That makes 55 adjacencies going on just for this single Ethernet. Doesn't that have an negative impact on the SPF calculation time, as well as the LSA flooding?

[bilousand]

Now try to connect any two of them with direct layer 3 link and take a look what will happen. ;)

[opers13]

these are all direct connect over L3 links...what you mean?

[bilousand]

Looks like we're speaking about different things for quite some time then...

[Kevin Dorrell]

Ah! I think I've just fallen in! These 10 neighbors are on L3 interfaces. I had thought they were on the same VLAN. Of course, if they were on the same broadcast segment, the interface would say VLAN n, and not GigbitEthernet ... . This is a layer-3 distribution switch.

Do you have any Ethernet segments where there are more than 2 routers? If not, then point-to-point is fine. Effectively you are using each Ethernet segment as a P2P link.

Have a nice weekend!

Kevin Dorrell

Luxembourg

[aravindhs]

Hi Kevin,

I just tried to put three routers'fa0/0 on one broadcast segment (a L2 switch) and put all of them on point-to-point ospf network type. It threw up all sorts of logs and won't form an adjacency. It seems it will accept no-nonsense on a point-to-point link and just hold one neighbor on interfaces with type set to p2p.

Cheers

Arav

[Kevin Dorrell]

Thanks Arav, with this sort of question it all boils down to "how does a real router behave?". It's great that on NetPro there are so many people who are passionate about the subject that they will lab up the scenarios. Thanks.

So, looking at the original posting, I think we have established that these were layer-3 distribution links. If they were no switchport to no switchport, then they might as well be treated as point-to-point.

There is one thing to say about (real) point-to-point links: they detect link failures at both ends of the link simultaneously, whereas an Ethernet network does not - it has to wait for the hold time on the OSPF or the BFD to expire. That has nothing to do with the OSPF logical network type, but more to do with the nature of the layer-1 connection. In this respect, a no switchport to no switchport connection behaves more similarly to a P2P serial link than to an Ethernet. Maybe that is the rationale behind using the P2P logical network type for the OSPF.

Kevin Dorrell

Luxembourg

[opers13]

"a no switchport to no switchport connection behaves more similarly to a P2P serial link than to an Ethernet. Maybe that is the rationale behind using the P2P logical network type for the OSPF."

makes sense...thanks for the help!

alex