Gary,
Using the "ip ospf network broadcast" statement on an NBMA interface just
tells OSPF to treat this interface the same as a broadcast multiaccess
network type, no matter what sort of interface it actually is. In other
words, you are lying to the OSPF process about the underlying layer-two
technology/topology. OSPF expects that all interfaces on a broadcast
multiaccess network can all hear each other's hello packets so they will
dynamically try to discover their neghbors by means of hellos and they will
elect a designated router for that subnet.
To use the broadcast network type for OSPF on an NBMA interface the router
needs to be able to make a separate copy of each broadcast/multicast packet
for each PVC defined on the interface. If you allow the frame-relay
inverse arp process to create your mappings dynamically, you will get this
by default.
The critical difference between NBMA and broadcast multiaccess (ethernet,
FDDI, token ring) is that on a broadcast multiaccess topology every
broadcast packet sent by any interface can be heard by every interface on
the segment. For this to be the case in NBMA, you need fully-meshed PVCs
among all the router interfaces. If you had a "hub-and-spoke" NBMA
topology, for instance (no PVCs directly connecting the "spoke" routers to
each other), the "spoke" routers would see only the hello packets from the
"hub" router. They would never see hello packets from the other "spoke"
routers--the "hub" router is not allowed to take a broadcast/multicast from
a neighbor on one PVC and reflect it back to a neighbor on a different PVC.
If the "spoke" routers never see each other's hello packets they will never
be able to form neighbor relationships with each other. This could cause
bizzarre connectivity problems with OSPF if a "spoke" router were to become
designated router for the NBMA subnet.
Pamela
At 01:30 AM 12/28/00 -0800, gary gary wrote:
>hi guys:
> I am confused by the "ip ospf network broadcast "
>in NBMA,
>what is difference between the multiaccess like
>ethernert with it,
>as the definition of NBMA,it has no broadcast
>ability,but why cisco can use
>ip ospf network broadcast,if i am wrong ,please
>correct me
_________________________________
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
