NBMA Non-broadcast / broadcast

Hi,

When using nbma non-broadcast or broadcast network types do all routers paritcipating need to be in a full mesh?

Cheers
Waru

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Comments

Paul Boz

I'm going to assume you mean with regards to some sort of routing protocol.

In that case no, there are several solutions to this. Prior to Ios 10.0 the only option was to use "neighbor" statements on each router to specify the OSPF neighbors. In such a case you want to ensure that the hub router is set to be the permanent DR by setting priority on the spokes to zero. For a large WAN network this is a pain in the butt and can become unruly quickly.

Beyond IOS 10.0, the "ip ospf network broadcast" command became available. In this case you need to specify the "broadcast" keyword after your frame-relay maps so that the broadcast and multicast traffic gets passed through the network.

waru

oops. yes I meant in reference to ospf. In the book im reading it says nonbroadcast and broadcast need full mesh topology but other things I had read led me to believe this wasnt the case.

Paul Boz

You are correct in assuming that. Out of curiosity what book are you reading?

In fact, I just re-read the TCP/IP Vol1 passage on OSPF NBMA and every example they have are for partial mesh

dtlokee

You can have a hub and spoke design as long as the DR/BDR are ONLY on the hubs(s). You need to set the priority on any of the spoke routers to 0. If using a partial mesh, again the make sure the DR and BDR are located so they they ahve a direct L2 connection to all of the other routers on the segment.

waru

Cheers Dtlokee for clarifying. Paul, The book is CCNP BSCI Official Exam Certification Guide, Fourth Edition. It has a table describing the features of each ospf network type where it mentions the full mesh.

Paul Boz

That's interesting. I'm going to check to see if that volume is in the library at work when I come in in a few hours. I need to investigate.

Grym

In the Authorized self study guide 3rd edition, it says:

Broadcast - Full or partial mesh

Non Broadcast - Full or Partial mesh

point to multipoint - Partial or star

I think the key word you might be missing is typical as it also said that it is "typically used with fully-meshed networks"

I know this isn't the same book, but i think it confirms what Paul and dt have said, no it does not have to be a full mesh.

Page 194 and 195 of the above guide....

dtlokee

Ok, so OSPF broadcast and non-broadcast both use a DR and in both cases all routers will need DIRECT L2 connectivity to the DR (and if one exists to the BDR). In a frame-relay hub and spoke or partial mesh this means the DR must be the hub or the router with a PVC to all other routers. If there is no router with a PVC to all other routers you cannot use broadcast or non-broadcast as the network type. This leaves us with point-to-multipoint [non-broadcast]. In this case the routers will advertise a host route to the other routers on the segment and therefore all routers will have a way to get to the next hop of the route and life is good.

Alternate solutions can include using tunnel interfaces or ppp over frame relay to fix our network, but this is not going to be the optimal way to do things. Typically the best way to configure a partial mesh is using point-to-point subinterfaces, that solves most of the headaches.

Paul Boz

I see more and more GRE w/ IPSec and VPN connections carrying LAN traffic over the WAN infrastructure every day. Either option is great for resolving the nonbroadcast configuration as well.

arden

I've written up a set of tutorial on the OSPF network types here:

Broadcast & Non-Broadcast - http://ardenpackeer.com/routing-protocols/tutorial-ospf-network-types-and-frame-relay-part-1/
Point-to-Multipoint & Point-to-Multipoint non-broadcast - http://ardenpackeer.com/routing-protocols/tutorial-ospf-network-types-and-frame-relay-part-2/
Point-To-Point & Loopback - http://ardenpackeer.com/routing-protocols/tutorial-ospf-network-types-and-frame-relay-part-3/
Mixing & Matching Types - http://ardenpackeer.com/routing-protocols/tutorial-ospf-network-types-and-frame-relay-part-4/

It goes through all the different network types, hopefully come in handy for anyone studying for the CCNP cert. There is a Dynagen .net files so you can play along. Hope it helps guys!

cisco_trooper

waru wrote:

Cheers Dtlokee for clarifying. Paul, The book is CCNP BSCI Official Exam Certification Guide, Fourth Edition. It has a table describing the features of each ospf network type where it mentions the full mesh.

You should really check out Routing TCP/IP Volume 1 Second Edition by Doyle. This will go a LONG way toward clarifying anything related to OSPF, but not just OSPF, routing protocols in general.