By implementing this new standard, OSPF gained the ability to forward multicast packets from one IP network to another. A new link-state Advertisement (LSA) is used to determine the exact location of all the Autonomous Systems members. This RFC provides the information necessary to understand the operation of this new feature and its specific LSA, the group-membership-LSA. Also presented is how the link-state database operates to include the building, pruning, and caching of routes.
A potential area of concern was seen as a result of this new feature. When OSPF forwards multicasts between areas, incomplete routes are built; this may lead to routing inefficiency. To correct that problem, OSPF summary link advertisements or OSPF AS external link advertisements are used to approximate the neighbors needed for routing. The RFC provides a very good description of this issue and the resulting methodology needed to compensate.
Discussion is provided on the compatibility between network devices running MOSPF and non-multicast OSPF. To include some of the issues surrounding the networks operation if this topology will be in place.
Additional practical information on this subject can be found in RFC 1585, MOSPF: Analysis and Experience.
RFC 1585: MOSPF: Analysis and Experience
This RFC immediately followed the 100+ page RFC 1584 that fully detailed all relevant information regarding the ability of allowing OSPF to perform multicasting. This RFC is rather short and was written to fulfill the requirements imposed by the Internet Engineering Task Force (IETF) Internet Routing Protocol Standardization Criteria as detailed in RFC 1264.
A brief discussion surrounding the basic operation of the MOSPF and how it uses the Internet Group Management Protocol (IGMP) to monitor multicast group membership. This information is retrieved from the LAN and then forwarded out by the router by the OSPF flooding protocol through the use of the new group-membership-LSA. The specific benefits that result from this process and detailed operation is provided.
The six primary characteristics of the multicast datagrams path are also provided as well as some of the more interesting miscellaneous features.
The RFC further details the testing the author conducted and how MOSPF was implemented during these tests. Further discussion is provided on the scaling characteristics of MOSPF and some of the known difficulties surrounding it.
RFC 1586: Guidelines for Running OSPF over Frame Relay Networks
This RFC specifies a set of guidelines for implementing the Open Shortest Path First (OSPF) routing protocol to bring about improvements in how the protocol runs over Frame Relay networks. The authors show the techniques that can be used to prevent the fully meshed connectivity that had been required by OSPF until the publication of this RFC. The benefits of following the guidelines detailed in this RFC allow for more straightforward and economic OSPF network designs. This RFC differs from many of the others in that it does not require changes to be made to the protocol itself but rather better ways to configure it.
The reason behind this RFC is that OSPF considers Frame Relay networks as non-broadcast multiple access (NBMA). OSPF does this because Frame Relay (FR) can support more than two connected routers but Frame Relay does not offer any broadcast capabilities. The following quote from the RFC addresses this issue.
These rather expensive limitations can result in reducing the value and cost effectiveness of Frame Relay as network size increases. The RFC proposes a set of solutions that do not greatly increase the complexity of OSPFs configuration. A brief list of their recommendations is provided, though I recommend further reading in the actual RFC if more in depth information is required.
One of the recommendations is to expand the operation of an OSPF interface to allow the protocol to understand its function (point-to-point, broadcast, NBMA). In other words, allow OSPF to support both logical and physical interfaces.
The other recommendation proposed by the RFC is to use the NBMA model as OSPFs mode of operation for small homogenous networks.