🚀 go-pugleaf

In article <1992Nov19.193651.21854@umr.edu>, rfranken@mcs213f.cs.umr.edu (Richard Brett Frankenberger) writes:
> In article <sige6ao@rhyolite.wpd.sgi.com> vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:

> The hassels of source-routing? No big deal ... I manage a WANof about 30      
> TR's allsource-routed togethor (now being converted to routers).  Source
> routing is no major problem (certainly less confusing that setting up
> IP routing).  BUT, if you don't like it, you don't have to use it.
> Anyway, source-routing is more like bridging than routnig (in several
> ways), and in a large WAN, you are better off with routers anyway.
>

Most IBM LAN systems run NetBIOS directly on top of the Data Link. Because
there is no Network Layer, routers are NOT possible. This is, in fact,
the reason Source Routing exists in the first place. It is a "mini-Network"
Layer protocol embedded into the Data Link.

I strongly discourage the use of Source Routing over WANs, since the
Explorer (route discovery) traffic is incredibly bandwidth-intensive.
  
> On ether net, you can get between segments one of two ways.  Transparent
> bridges or Routers.  On token-ring, you have those two options PLUS  
> Source-Routing.  Source-Routing is just another option.  You don't have
> to use it. 
>

In practice, it is NOT possible to use transparnet bridges in a Token
Ring environment. First, they are not made by many manufacturers (although
this is an implementation, not an architecvtural problem). More important,
IBM Hosts are generally configured with multiple FEPs (3745s) connected
to redundant backbones for high availability and load sharing. IBM 
configures (actually, the users configure according to IBM recommendations)
the two FEPs to have the same MAC address. This is what provides the 
redundancy and load sharing. When a route discovery is made to the host, the
station may be connecting to either FEP, without regard. This is the reason
why IBM makes such widespread use of the Locally administered address
space, rather than global addressing used by Ethernet.

Transparent bridges require that all devices have unique MAC addresses,
otherwise the table learning algorithms are unstable. This is one of
the key invariants of the underlying LAN that Transparent bridges
demand.
  

-- 
Rich Seifert                    Networks and Communications Consulting
seifert@netcom.com              (408) 996-0922
                                (408) 996-2860 FAX
"... specialists in Local Area Networks and Data Communications systems"