Restrict OSPF neighbor adjacencies on metro ethernet WAN?

[mmmikeh01]

I replaced 5 point to point T1s with a single Metro Ethernet WAN which has five endpoints and I use OSPF.

I'd like to understand the best way to limit unnecessary neighbor adjacencies between my routers. Most sites only need to see one or two other sites with the exception of one office which needs to see four sites. So there are several adjacencies which will never ever have any traffic between them and do not need to exchange routes. 

[paul driver · Accepted solution]

Hello

When using broadcast media such as ethernet you can accomplish your request by to manaually setting ospf to use unicast ajacencies instread of the default broadcast, this way you have control over the ospf peering.

In this example OSPF non Broadcast ( which require DR/BDR)

R1 (Central router)
int x/x
ip address x.x.x.x y.y.y.y
ip ospf network non-broadcast
ip ospf priority 100 (DR)
 

router ospf x
network x.x.x.x 0.0.0.0 area X
neighbor x.x.x..x ( R2)
neighbor x.x.x..x ( R3)
 

R2 (spoke)
int x/x
ip address x.x.x.x y.y.y.y
ip ospf network non-broadcast
ip ospf priority 0 (DROTHER)

router ospf x
network x.x.x.x 0.0.0.0 area X
 

R3 (spoke)
int x/x
ip address x.x.x.x y.y.y.y
ip ospf network non-broadcast
ip ospf priority 0 (DROTHER)
 

router ospf x
network x.x.x.x 0.0.0.0 area X

etc...

res
Paul
 

[mmmikeh01]

Thank you Paul.That is what I needed to know.

[paul driver]

Hello

Yes you can, And in fact a router can be a differant DR/BDR/Drother for each  peering

The OSPF non broadcast example above was mainly on the understanding you are using just one broadcast physical link to connect to all other rtrs and to avoid unecessary peering we can negate the multicast nature of OSPF

However if you have multiple physical  links to all rtrs then OSPF point-2-point would be beneficial also

res

Paul

[mmmikeh01]

I should have been more clear but the spokes and triangle I referred to were not of connectivity but neighbor-ship. So in that regard, using a single metro ethernet, I have a hub with four spokes and two of the spoke ends need a neighbor-ship with each other. 

[mmmikeh01]

Paul,

In the case where two of the spokes sites form a triangle with the central router, do neighbor statements get added on one or both of those routers too?

Mike

[MARK BAKER]

Hi Paul,

Can the neighbor command be used on the spokes pointing at the hub instead? We have standard spoke configs which would make it easier to do it the other way around when a spoke is added.

Thank you,

Mark

[paul driver]

Hello

Sorry I missed your additional question,

in theory they can- However for that to happen the spokes need to be DR/BDRs

Something you really don't won't them be in a hub spoke NBMA using broadcast links

res

Paul

[will]

I managed to make this work well, so just sharing my results/config! I was having some problems when neighbors both not defined on hub and spoke sides. and I inserted two neighbor IP's, one for the ospf router-ID (loopback) and one for the local broadcast subnet IP. it seemed to be randomly failing without both neighbors defined. I should note, that in my case, there ended up being just a DROTHER and not DR/BDR:

=== rtra - single point side (neighbor with rtr1, want to be "DR"):
int vlan100
 ip ospf network non-broadcast
 ip ospf priority 10  ! might not need this line
!
router ospf 100
 neighbor <rtr1 gi0/0/1>
 neighbor <rtr1 loop>

=== rtrb - single point side (neighbor with rtr1, want to be "DR"):
int vlan100
 ip ospf network non-broadcast
 ip ospf priority 5   ! might not need this line
!
router ospf 100
 neighbor <rtr1 gi0/0/1>
 neighbor <rtr1 loop>

=== rtr1 - multipoint side (want to pair with both rtra/rtrb):
interface GigabitEthernet0/0/1
 ip ospf network point-to-multipoint non-broadcast
 ip ospf priority 0    ! might not need this line
!
router ospf 100
 neighbor <rtra vlan100>
 neighbor <rtrb vlan100>
 neighbor <rtra loop>
 neighbor <rtrb loop>