NBMA networks are those networks that
support many (more than two) routers, but have no broadcast
capability. Neighboring routers are maintained on these nets using
OSPF's Hello Protocol. However, due to the lack of broadcast
capability, some configuration information may be necessary to aid in
the discovery of neighbors. On non-broadcast networks, OSPF protocol
packets that are normally multicast need to be sent to each
neighboring router. An X.25 Public Data Network (PDN) is an
example of a non-broadcast network.
Note the following:
NBMA networks, such as the one in Figure , pose a special
problem for OSPF and DR election. On a Cisco router, these networks
can be configured to act as a single broadcast interface with multiple
connections.
Because Router A is using a single multipoint interface (a Frame
Relay interface using inverse ARP or frame-map configurations to
separate the traffic between the permanent virtual circuits [PVC's])
when Router A broadcasts a packet, all the other routers receive it.
But when Routers B or F broadcast a packet, the only router that
receives the packet is Router A.
Because all the routers connected to this multi-access network
assume it is a single broadcast domain, they will attempt,
unsuccessfully, to elect a BDR and DR. Assuming that all routers are
connected to the link at the same time, the following scenario will
occur:
-
Routers A and B will elect Router A as the DR and Router B as
the BDR.
-
Routers A and F will elect Router F as the DR and Router A as
the BDR.
-
Router B will not receive Router F's hellos.
-
Router F will not receive Router B's hellos.
Essentially, this is broken; there is no way to determine what the
final outcome will be. It may actually work for some time until a link
flaps or one of the routers on the network goes down.
There are 3 possible solutions to this problem:
-
Set all remote sites to OSPF priority 0, and the hub or core
router to anything else.
-
Use point-to-point subinterfaces
-
Configure the network as a point-to-multipoint network type
The first solution - configuring the OSPF router priorities - was
the only solution for some time (before the availability of
point-to-point subinterfaces or network type point-to-multipoint).
Some network administrators, however, configure the remote routers
with a low priority rather than a priority of 0, which works but can
still cause problems because the BDR status will be in question. It is
best to simply configure the remote routers to be ineligible to become
a DR or BDR.
The second solution - using point-to-point subinterfaces - has been
available for some time now and has many advantages. It has one
disadvantage that many administrators don't like though: A separate
network address must be used for each serial link. If a network has a
lot of remote sites connected to distribution or access layer routers
in this fashion, this can become a major administrative nightmare.
The final solution - network type point-to-multipoint - is a recent
development. Instead of the hub router treating the NBMA network as a
broadcast domain, it treats each PVC as a point-to-point link,
building full adjacencies with each router. This technique is
effective, but it results in the creation of host routes for each
remote router on the NBMA network.
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