From: syj@ecmwf.co.uk (Jean-Philippe Martin-Flatin)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Cisco Bridging
Date: 14 Oct 92 14:03:40 GMT
Organization: ECMWF

<*> [#1]
<*> 
<*> I have read through the doc on bridging with a Cisco.  It seems that
<*> Cisco's can either act as a bridge *OR* a router not both at the same
<*> time.  All I want to do is bridge two ethernets together and keep routing
<*> IP as I am now.  It seems like a fairly simple request.  Has anyone
<*> else had luck with this, or is this not possible with a Cisco router?
<*> 
<*> [#2]
<*> 
<*> I have confused everyone, as usual.   I want to route and bridge ip.
<*> I think I know why I cannot do this.  My bet is that cisco uses the same
<*> cache for routing and bridging.  Is this true?  I don't think this is an
<*> unreasonable thing to do.
<*> 
<*> ---------------------------------
<*> Tom Stephens
<*> Corporate Computing and Networks, Chippewa Falls, WI
<*> Cray Research Inc.  hens@cray.com

You can't. We have been lobbying CISCO to bridge or route the same
protocol on a per-interface basis, without any success. The problem is
not as simple as it looks: CISCO routers, like many others, are native
routers that may run in downgraded mode as bridges. They are not native
routers AND bridges, which explains why things become tricky when you
want to both bridge and route the same protocol.

At ECMWF, we have been bridging our Ethernet legs for 2 main reasons:

1) We did not want to change the IP address of a machine when it was moved
   from one Ethernet leg to another (machines move a lot here), as people
   moving machines have too little UNIX expertise to update an IP address
   under root.

2) We have SUN clients all over the building on different Ethernet legs,
   but we don't have one SUN server per Ethernet leg. As SUNs boot using
   subnet directed broadcast, bridging all Ethernets allowed us to have
   a unique subnet, and made life easy.

But we now need to route IP for 2 main reasons:

1) CISCO claim their routers are more efficient (throughput) in routing
   mode than in bridging mode, and we want to use them as close as possible
   to their maximum performance.

2) We need to route our Ethernets to our new FDDI (translation bridging
   is not appropriate in our context).

We are currently studying how to migrate to the routing solution. There
are several possible scenarios, and our favourite are:

1) The CISCOs will forward subnet directed broadcasts from/to appropriate
   Ethernet interfaces (ip helper-address & ip forward-protocol udp 111),
   so that SUN clients can boot from a server on another Ethernet/subnet.
   This assumes that SUN's boot protocol demands the client and server be
   on the same subnet only for the client's broadcast to reach the server,
   not at a later stage. We tested it was true for an NIS client to bind
   to an NIS server on a different subnet, so we hope it is also true for
   a client to boot from a server on a different subnet.

1bis) If above does not work, we can define a special subnet mask on SUNs,
      so that both the client and the server believe they are on the same
      subnet, though on different Ethernets. Allowing the forwarding of
      directed broadcast (ip directed-broadcast) on the CISCOs' Ethernet
      interfaces, and disabling it on the FDDI interface, should make it
      work. We also need proxy ARP enabled on Ethernet interfaces, and
      disabled on the FDDI interface, as SUNs now believe all Ethernets
      form a unique subnet.

2) SUNs will boot using RARP, so that an IP address change is made much
   easier to cope with: central repository of IP addresses, simple script
   to get the new location of the machine, office/subnet map table, and
   automated update of the SUN IP address in its boot procedure.

I may give you more details off-line if you are interested.

Hope this helps

/----------------------------------------------------\
| Jean-Philippe Martin-Flatin                        |
| European Centre for Medium-Range Weather Forecasts |
| Shinfield Park                                     |
| Reading RG2 9AX                                    |
| England                                            |
|----------------------------------------------------|
|  Disclaimer: I speak for myself, not my employer   |
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