Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 292
MOSPF Differences

MOSPFdiffers from DVMRP and PIM is many ways, but it should be noted right up front that MOSPF does not broadcast the first multicast packet it receives. The protocol builds a source-rooted, shortest-path tree “on demand” when it receives the first multicast packet and then prunes the branches not associated with this group. Also, MOSPF does not allow for tunneling as DVMRP does. Multicast datagrams are sent in native mode and are not encapsulated (DVMRP uses IP-in-IP encapsulation for tunneling).

A difference between OSPF and MOSPF is that MOSPF does not allow for equal-cost multipaths. There are tie-breaking rules that have been identified for paths that are found to be equal when calculating the shortest-path tree. One of these rules is that, given an equal-cost path to a destination, the router or LAN with the higher IP address will be chosen. The second tie-breaking rule is that a broadcast-oriented network is always chosen over WAN routers (point-to-point links).

MOSPF Differences

•  MOSPF differs from DVMRP and PIM:
•  It does not broadcast the first multicast packet it receives
•  Multicast is an integral part of the protocol.
•  The protocol builds a source-rooted tree on-demand and then prunes branches.
•  Multicast members are part of the link state database.
•  Routes are computed like real routes and they are put into the forwarding cache.
•  MOSPF does not allow for equal cost multipath:
•  Tie-breaking algorithms have been put into place
•  MOSPF routes should be the DR and BDR in any mixed (i.e., non-MOSPF) environments.


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