Routing

Hi Experts

What is the difference between OSPF interface type nbma and multipoint and in which scenario what we should use? I believe both are non-broadcast multiaccess.

Thanks

  You could configure a ptmp as nbma or broadcast, if needed.

  NBMA mode is used when the physically interface dont have broadcast capabilities, so you can't automatically discover neighbors. Of course you can configure an Ethernet link as nbma, if for security you dont want to broadcast ospf hellos.

  NBMA require to manually configure neighbor relationships, so extra configuration. More secure if you want to use this, because there is no ospf hellos.

  For example, if you have a frame relay link, but dont use its broadcast capabilities, will have to use nbma and static neighbors.

   Or if you have a shared Ethernet with other customers, and you want to hide you ospf routers, or you have several ospf routers in the same Ethernet lan but in different areas that should not interfere (you will get lots of error msg if the routers have not a common area and use broadcast/multicast hellos).

Great and practical explaination !!! Just last thing, What is the concept of pure P2MP and P2MP as collection of point to point connectivity?

   P2MP is a Hub and Spoke topology. Usually all neighbors are under the same ip network, and are able to send traffic to each other, but always through the hub router. Neighbors could auto discover each others.

  Under a pure PTP, hub neighbors can only "speak" with the hub, and have no knowledge about other neighbors, and no way to discover this.

    P2MP is useful if you want to automatically discover new hub neighbors. Under Frame Relay you could request to add a new dlci to your hub link, install a new router in a remote site with a new fr interface, this neighbor will automatically discover the dlci it have to use, hub router will discover a new dlci and neighbor, and neighbor would autoconfigure its ip address through dhcp from the hub router, and will automatically be connected to the hub.

    Dlci are automatically discovered through lmi, and its broadcast capability allow to discover the other side ip address, use ppp over fr, etc. All these methods will give an ip address to the new router.

   With few configuration you could add remote sites.

  PTP, need explicit configuration of new neighbors. You need new subnets for every new router, and of course know the dlci this ptp link is going to use. More secure because if you dont configure the new ptp interface there will not be connectivity through new dlci. But you have to do all the configuration.

  Of course you hub could be configured under ospf as PTMP, and your spokes as PTP, but this is not related to the physical or logical interface configuration, have only sense under ospf. Have to be carefull because each link type (broadcast, nonbroadcast, ptp, ptmp) have its own hello, retransmit, dead timers, etc. Mismatch will refuse new neighbor relationships.

  Br

  Alex.

Thanks for the great explaination !!  So in P2MP (not pure P2MP), neighbours can discover each other through HUB, so they can make OSPF neighbourship with each other through HUB?

   No, there will not be ospf relationships between spoke neighbors, because ospf hellos have  TTL =  1. This way as soon the hello reach the hub, this is processed and discarded.

   Usually hello packets, multicasts, broadcasts, used for routing updates only have the TTL set to 1. There is a concept called routing by rumor, each router sends its routing information to other router directly connected, this to other and so on, this is why ttl is 1.

   But you could configure static routes with other neighbor next hop, or in the case of bgp, eigrp or rip, you will see as next hop the "real" ip address neighbor.

   Of course you could configure a peer relationship with neighbor ip address, but becareful of the TTL, usually will not work except for bgp.

   The hub and spoke is a easy way to add new peers, but could be confusing on the routing protocols adjacencies. You have to fully understand how routing protocols works, how this establish and maintain adjacencies.

    A PTMP is likely a ethernet segment, every router could send traffic to each other without a routing protocol, because all ip address are under the same ip network. But broadcast and multicast would be forwarded only if its TTL are 2 or more.

    Under OSPF the Hub should be always the Designated Router (DR) in that PTMP network, and no neighbor should be Backup Designated Router.

Thanks again for the explaination. You really know routing protocol operations 🙂 I read this topic in books but there is no practical explaination there