IBM 3174 C 6.4 Microcode Disks?

Paul Koning paulkoning at
Wed Feb 20 13:23:52 CST 2019

> On Feb 20, 2019, at 2:13 PM, Ken Seefried via cctalk <cctalk at> 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.


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