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7.6 | ![]() |
Integrated IS-IS Operation in a WAN Environment | |
7.6.4 | ![]() |
Frame Relay point-to-multipoint scenario with Integrated IS-IS |
In this scenario, all the Frame Relay
ports are configured as multipoint interfaces. They are configured either as a multipoint subinterface, on the hub router R4, or as a main interface on the
other routers. All interfaces share the same IP subnet in a multipoint
configuration. In a multipoint environment with IS-IS, it is important
that a full mesh be implemented. Therefore, all other routers will
also have VCs interconnecting them, although these are not shown in
the diagram. If this were a true hub-and-spoke environment, and the
spoke sites had no need to communicate to each other, this topology
could work with only the indicated DLCIs. In this case, the hub router
must become the DIS for the NBMA network, as it is the only router
visible to all others. A suitable IS-IS priority should be set on
the Frame Relay interface of the hub router using the
isis priority
command. Routes would be installed in each spoke router toward the
other spoke routers by way of their local IP addresses. However,
packets to these destinations would be dropped, as there are no direct
VCs between the spokes.
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This point-to-multipoint scenario shows the configuration of the
multipoint interface on the R4 hub router.
The debug isis adj-packet command again shows the neighbor relationship establishment. However this time the adjacency uses LAN IIH PDUs because this is a multipoint environment. Keep in mind that the preferred configuration for Integrated IS-IS in a WAN environment is to configure all interfaces as point-to-point subinterfaces. This avoids the full mesh required with the point-to-multipoint option. The point-to-multipoint option results in weak network stability. Having one PVC go down can have a domino effect on the WAN. This points out one major difference between OSPF and IS-IS configuration.
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