Understanding the network.pdf - Back to Home

it is possible to exchange packets between the DCLIs, by default only unicast
packets are sent. FR does not natively support broadcast and multicast
transmissions. Therefore, in order for the messages to be properly relayed, the
multipoint router must send a unicast copy of these packets to each endpoint. To
enable broadcast simulation, add the [broadcast] keyword to the end of the
or interface configuration
command.
NOTE
With multipoint grouping, a collection of FR endpoints function as if they are
connected over a local shared transport. The FR provider provisions the multipoint
group (up to 32 nodes) by associating all the endpoint members'local DCLIs to a
multipoint group. Each endpoint is assigned an address reflecting a common
network address (the same IP subnet or equivalent) with the DCLI functioning in
effect as a MAC address. From a conceptual perspective, the endpoints function as
DTE peers connected to a common virtual DCE.
NOTE
Not all Layer 3 protocols can operate over a point-to-multipoint topology. The
reason for this is that not all protocols will permit datagrams to be relayed out of the
same interface from which they are received. In addition, the multipoint
sub-interface designation also enables "split-horizon" (disabled by IOS when
frame-relay encapsulation is selected) on any IP routing protocol.
Point-to-Point VC Topology
Typically, multi-VC FR implementations (partial and full mesh) are configured using
partially or fully meshed point-to-point sub-interfaces. Although quite effective,
they are costly in comparison to multipoint implementations because each link
requires its own PVC. In addition to cost, the actual configuration is fairly complex,
requiring that some real thought be paid to addressing the various point-to-point
PVC sub-interfaces. Good documentation is a big help when debugging problems
(so be sure to write some). Figure 9.5 illustrates a fully meshed FR topology using
point-to-point PVC. Following the illustration are interface configuration examples
from two of the routers (batman and robin).