In a point-to-multipoint network, a hub router
is directly connected to multiple spoke routers, but all the WAN
interfaces are addressed on the same subnet.
You saw this logical topology earlier in the chapter, but you learned
that OSPF does not work properly as an NBMA OSPF network type. By manually
changing the OSPF network type to point-to-multipoint, you can make this
logical topology work. Routing between RTA and RTC will go through the
router that has virtual circuits to both routers, RTB. Note that it is not
necessary to configure neighbors when using this feature (Inverse ARP will
discover them).
Point-to-multipoint networks have the following properties:
- Adjacencies are established between all neighboring routers. There
is no DR or BDR for a point-to-multipoint network. No network LSA is
originated for point-to-multipoint networks. Router priority is not
configured for point-to-multipoint interfaces or for neighbors on
point-to-multipoint networks.
- When originating a router LSA, the point-to-multipoint interface is
reported as a collection of point-to-point links to all the
interface's adjacent neighbors, together with a single stub link
advertising the interface's IP address with a cost of 0.
When flooding out a nonbroadcast interface, the LSU or LSAck packet
must be replicated to be sent to each of the interface's neighbors.
To configure point-to-multipoint, you must manually override the
detected OSPF network type with the following syntax:
router(config-if)#ip ospf network
point-to-multipoint
You should also configure the interface with a
frame-relay map ip
command, as in the following syntax:
router(config-if)#frame-relay map ip
address dlci broadcast
The
broadcast
keyword permits the router to send broadcasts via the
specified DLCI to the mapped neighbor or neighbors. If you apply the
point-to-multipoint configuration to the example network ,
you would have to configure two separate
frame-relay map
statements on the
hub router, RTB. Partial configurations for each router are shown in
Figure .
In a point-to-multipoint configuration, OSPF treats all
router-to-router connections on the nonbroadcast network as if they were
point-to-point links. No DR is elected for the network. Neighbors can be
manually specified using the
neighbor
command or can be dynamically
discovered using Inverse ARP.
Ultimately, point-to-multipoint OSPF offers efficient operation without
administrative complexity.
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