Date: Thu, 9 May 96 14:27:40 EDT From: Milan Merhar To: p8021@hepnrc.hep.net Subject: VLANs and IP multicast Milan Momirov wrote: Is there an understanding as to how to process IP multicast packets that have to go to multiple VLANs, but onto a single physical segment? Presumably the router isn't expected to output onto the physical segment per-VLAN copies of each IP multicast packet. Would each IP multicast space be dynamically assigned its own VLAN id? There are two situations where multiple VLANs can map to a single physical segment. In the first case, the VLANs are intentionally kept distinct on that segment by using some form of frame tagging or encapsulation. In this situation, the forwarding device must replicate the original packet and send it multiple times, just as it would if the various VLANs were each on their own physical port. In the second case, two or more VLANs are represented on the physical segment in "native" or implicitly tagged form. (For example - these VLANs are defined over separate sets of source MAC addresses, and members of both sets are found on the same physical LAN.) In this case, the right thing to do is to forward the frame once onto the physical segment, even though it "represents" more than one VLAN. To do otherwise would make the forwarding device appear to duplicate packets, which is unacceptable bridge behavior. As to whether IP multicast groups should or would be mapped to VLANs, I think that would depend on what problem is being solved. Certainly, such a mapping is one way to insure that stations uninterested in the multicast won't be bothered by undesired messages, but that result may also be obtained without invoking VLANs. If membership in a certain VLAN is associated with other characteristics, such as a particular message priority or assurance of a certain quality of service, then in my opinion the VLAN approach has merit. - milan [ Good grief, two people named "Milan" on the same mailing list! ) ----------------- Milan J. Merhar, Consulting Engineer Whittaker Xyplex 295 Foster St. Littleton, MA 01460 USA Internet: milan@xyplex.com Phone:(508)952-4713 Fax:(508)952-4887