OSPF over NBMA an overview

OSPF over NBMA is weird. You will see 😉 We must know some foundation concept to know how this weirdness works. So lets hop into it

OSPF supports many network types like:

  • Broadcast, multi access networks
    • Ethernet, Token ring
    • single operation mode
    • DR/BDR election, 10 sec hellos
  • Point-to-Point networks
    • T1, ISDN BRI/PRI
    • single operation mode
    • NO DR/BDR, 10 sec hellos,
  • Non-Broadcast multi-access (NBMA) networks
    • Frame Relay, ATM, MPLS
    • Five modes of operation, that you can choose to run ospf over it 😀

NBMA topologies

NBMA can have all kind of connections. You can imagine as bunch of routers connected together via some frame relay cloud. How are the routers logically connected depends on how much you pay to your service provider. If you dont have money you will have hub and spoke topology. If you want to have a trade-off you will have partial mesh and in case you are rich you will have full mesh.  (we are not speaking about MPLS that is always full mesh 😉 )

How OSPF handles these 5 modes of operations at NBMA? First here are the modes:

  • NBMA – RFC Standard
  • Point-to-Multipoint – RFC Standard
  • Point-to-point – Cisco proprietary – cisco adopted the point-to-point network behavior of OSPF into NBMA
  • Broadcast – Cisco proprietary – cisco adopted the broadcast multi-access network behavior of OSPF into NBMA.
  • Point-to-Multipoint, non-broadcast – Cisco proprietary – cisco has created new way of handling it in point-to-multipoint topology.

Each of these modes gives you different style of communication. You just needs to look at it and choose one that suits for you best.

NBMA mode

  • RFC standard
  • Default for x.25, FR, ATM
  • Neighbors statically configured – Why? you can use broadcast keyword! Because routers assume you cannot use broadcasts so broadcast keyword doesn’t help you. Verified by lab 😀
  • Must be one subnet!
  • Acts like a LAN environment but BC and MC are not allowed (the very name of this is non-broadcast MULTI ACCESS (multi access = lan))
  • DR/BDR elected
  • You must tune this mode based on topology you got. The DR and BDR election for example if you got full mesh you dont care, its like ethernet. But in case of hub and spoke not everyone sees each other cause broadcast and multicast are forbidden so DR must be definitely the HUB.

Point-to-multipoint mode

  • fixes the issues with the NBMA mode
  • requires single subnet
  • treats to series of links as a point to point links – no DR/BDR elected, however hello and dead timers are 30, 120 sec
  • neighbors automatically form (as long as you allow pseudo broadcasts)
  • easy to setup unless you got topology that supports this. If you got some mess topology with hub and spoke and then some point to point and then some partial mesh, you will totally have more subnets so this doesnt supports it.

Point-to-point mode

  • uses separate sub-interfaces (can have more subnets)
  • requires different subnets
  • no DR/BDR elected
  • neighbors automatically form
  • you got point to point logical connection to every one at Frame relay even you dont have full mesh connectivity, hello timers 10 and 40 sec.
  • only drawback is that it is greedy for IP addresses.

Brodcast – same as Ethernet behaviour and Point-to-Multipoint non-broadcast is same as point-to-multipoint but the neighbors must be statically configured.

Note these modes are chosen automaticaly based on type of interface. When you configure serial as point-to-point it will be in point-to-point mode. By default all serial interfaces are in NBMA mode.