OSPF: NBMA mode on NBMA network type

Hello,
Let's say we have an NBMA network (e.g. Frame Relay) and we configure NBMA mode on it.
Jeremy Ciaora (CBT Nuggets) says that there must be full-mesh connectivity in an NBMA mode in order to have IP communication between all routers.
However, Cisco Student Guide says that we must have VC connectivity between each non-DR and DR. In addition, it states that NBMA mode is for full-mesh and partial mesh topologies.

any idea?

Find more posts tagged with

Save $250 on 2025 certification boot camps from Infosec!

Book now with code EOY2025

Button

Comments

dtlokee

If you're using inverse arp you need to have a full mesh for it to work properly. You can configure NBMA in either a full or partial mesh but you need to put in your own static IP to DLCI entries to make it work. This will cause issues with the routing protocols such as split horizion on the hub router.

scheistermeister

dtlokee wrote:

If you're using inverse arp you need to have a full mesh for it to work properly. You can configure NBMA in either a full or partial mesh but you need to put in your own static IP to DLCI entries to make it work. This will cause issues with the routing protocols such as split horizion on the hub router.

+1 Jeremy talks about it later in the video and shows how to make it work using static maps.

ccnpninja

Jeremy talks about it later in the video and shows how to make it work using static maps.

but , in the video, that makes the network topology full meshed.

kpjungle

shadowman724 wrote:

Jeremy talks about it later in the video and shows how to make it work using static maps.

but , in the video, that makes the network topology full meshed.

But the routing protocol is not aware that it was not fully meshed before the static maps. If you use point to multipoint, the routing protocol will not have the limited knowledge.

kryolla

NBMA there is a DR election which the DR must have reachability to all routers in the segment. So in a hub and spoke the hub is the DR and the spokes does not participate on the election. Non-broadcast means it does not support broadcast thus you need a unicast (neighbor command). When using NBMA the next hop does not change or get modified by the hub router so you dont have an layer 3 to layer 2 resolution to the remote networks. You use a map command to reach the other networks and when you do this inverse arp is disable so you need a map command to reach the hub and a map command to reach all the other networks. Or use an network type that modifies the next hop. Hope this helps

scheistermeister

ccnpninja wrote:

Jeremy talks about it later in the video and shows how to make it work using static maps.

but , in the video, that makes the network topology full meshed.

Making static maps does not make it fully meshed. Fully meshed means each router has a physical connection to every other router. The map just tells the router how to reach the other router.

The exception is BGP where you do a full mesh with neighbor commands.

ccnpninja

Fully meshed means each router has a physical connection to every other router.

A little correction here: We're speaking of logical full mesh or partial mesh topologies, i.e. whether or not there are PVCs between the routers.
So does creating static maps (in the case of Frame Relay) - like Jeremy did in the video- upgrade a partial meshed network to a full meshed one?

scheistermeister

ccnpninja wrote:

Fully meshed means each router has a physical connection to every other router.

A little correction here: We're speaking of logical full mesh or partial mesh topologies, i.e. whether or not there are PVCs between the routers.

Say what now? Are you talking about a logical link or a physical one? A PVC would be a frame relay link while a logical one would be something like a tunnel interface.

ccnpninja wrote:

So does creating static maps (in the case of Frame Relay) - like Jeremy did in the video- upgrade a partial meshed network to a full meshed one?

Either way a map does not make a link. All the map does it tell the router which interface it needs to go out of to reach a specific IP. It is still a partial mesh, logical and physical. Each spoke router is only connected to the hub physically and logically. Now if you use the virtual-link command you could make a logical full mesh.

_maurice

I'm also studying for the BSCI. Just to clarify, is the map command you all are talking about this:

frame-relay map ip x.x.x.x dlci ??

Out of curiosity, are there other map comands that don't start with frame-relay?

kryolla

a map command is for layer 3 to layer 2 resolution for multipoint interfaces. In order to have full mesh the FR switch would need PVC built between the end points and since customers pay per pvc it is more costly to have a full mesh.

ccnpninja

Scheistermeister,

A PVC would be a frame relay link while a logical one would be something like a tunnel interface.
...Now if you use the virtual-link command you could make a logical full mesh.

I didn't mention neither tunnel interfaces, nor virtual links. These are out of topic.
Say we have 3 routers. One hub, 2 spokes. The neighbor command is configured on the hub router only.
With correct IP addressing and after enabling OSPF, the hub can reach both spokes. However, the spokes can not reach each other.
So what did Jeremy do? He added 2 static Frame Relay maps, one on each spoke router. Right? (check the video please).
So here is my question again: do these static mappings make the network full meshed?

networker050184

No it does not make it a full mesh. The only thing the maps do is make the other spoke router reachable THROUGH the hub.

kryolla

ccnpninja wrote:

Scheistermeister,

A PVC would be a frame relay link while a logical one would be something like a tunnel interface.
...Now if you use the virtual-link command you could make a logical full mesh.

I didn't mention neither tunnel interfaces, nor virtual links. These are out of topic.
Say we have 3 routers. One hub, 2 spokes. The neighbor command is configured on the hub router only.
With correct IP addressing and after enabling OSPF, the hub can reach both spokes. However, the spokes can not reach each other.
So what did Jeremy do? He added 2 static Frame Relay maps, one on each spoke router. Right? (check the video please).
So here is my question again: do these static mappings make the network full meshed?

No the static mapping does NOT make it a full mesh. A FR switch with PVC built to all the end points make it a full mesh. Have you tried to set up a FR switch?

scheistermeister

ccnpninja wrote:

So what did Jeremy do? He added 2 static Frame Relay maps, one on each spoke router. Right? (check the video please).
So here is my question again: do these static mappings make the network full meshed?

As I stated in the lower part of my post and as others after me have said, nope, the static maps do not make it a full mesh.

ccnpninja

thanks guys.