20 Feb
2019
20 Feb
'19
11:23 a.m.
On Feb 20, 2019, at 2:13 PM, Ken Seefried via cctalk
<cctalk at classiccmp.org> wrote:
...
You can bridge between TR (and FDDI) and ethernet on a Cisco,
generally for non-routable protocols (e.g. NetBIOS); see:
'translational bridging'. If you're trying to get these protocols
across an intermediary 'alien' network (like the corp FDDI backbone,
or the Internet), there are things like DLSw.
Please note that among LANs, there is Token Ring (802.5) and there is everything else.
FDDI is like Ethernet and like 802.4. Token Ring is the oddball because (a) it
doesn't have proper multicast addresses, and (b) for some reason IBM invented
source-routed bridging and tied that to Token Ring.
FDDI is in no way at all like Token Ring. The only thing the two have in common is
"token" and "ring". The MAC protocol is utterly different; the
closest relative is 802.4 Token Bus. And as far as addressing is concerned, FDDI is like
802.4 and Ethernet, with real multicast and general use of normal transparent bridges.
The only complication with FDDI (and 802.4, if you could find it) is that it only has
802.2 frames, not classic-Ethernet (with 16 bit protocol types). So an FDDI to Ethernet
bridge has to translate Ethernet frames to an 802.2 based encapsulation. That is done by
converting them to SNAP frames, as described in RFC 1042. Bridges like the DECbridge 500
and DECbridge 900 will do that; I assume Cisco does likewise.
FDDI didn't live all that long because 100 Mb Ethernet replaced it, but while it was
out there it made a fine backbone for Ethernet-based LANs.
paul