jeff@NMSU.Edu (Jeff Harris) (03/08/91)
At NMSU, we currently have most of our ethernet segments connected to a
backbone using filter repeaters (MAC layer bridges). We are inerested
in replacing those with routers as funds permit. The question is how to
phase in the movement from bridges to routers. We have always given
different buildings a range of IP's consistant with Class C subnetting,
so our current environment looks like this:
128.123.1.x 128.123.2.x
| |
MAC Bridge MAC Bridge
| |
----------------------------------------------------128.123.x.x
| |
MAC Bridge MAC Bridge
| |
128.123.3.x 128.123.4.x
As we break the backbone into pieces, our system might look like this:
128.123.1.x 128.123.2.x
| |
MAC Bridge MAC Bridge
| |
---------------------| Router |----------------------
| |
MAC Bridge MAC Bridge
| |
128.123.3.x 128.123.4.x
No we have the problem of what the routing tables look like for the
various machines. The machines that are attached are mainly Suns and
PCs, with a few VAXes and other things thrown in for good measure. And
while only four segments of 128.123 are shown there are actually 30
segments involved.
Do we tell each machine that it is on a Class C network? If so, do we
have to statically assign routes for each subnet (since there are not
real routers between all segments to provide routing info)? Is there
someway to tell the router that there are multiple Class C network on
each interface? That seems to be the easiest since RIP information
would be consistant. If the latter is possible, what happens to a packet
from 128.123.1.x to 128.123.3.x? Is it actually handled by the router,
our will the Suns send it between themselves as would be desired?
Has anyone else tried this that can provide some do/don't's etc? Any
help would be appreciated.
Thank you
Jeff Harris
Network Manager
Computer Center - Room 133E
Box 30001 / Dept 3AT
New Mexico State University
Las Cruces, New Mexico 88003-0001
Internet: jeff@NMSU.Edu Voice : (505) 646-5110
UUCP : sun!sunpeaks!sunnmex!nmsu!jeff FAX : (505) 646-5278
Bitnet : jeff@nmsurobel2@mythos.ucs.indiana.edu (Allen Robel) (03/18/91)
>someway to tell the router that there are multiple Class C network on >each interface? That seems to be the easiest since RIP information >would be consistant. If the latter is possible, what happens to a packet >from 128.123.1.x to 128.123.3.x? Is it actually handled by the router, >our will the Suns send it between themselves as would be desired? cisco supports multiple subnets on each interface with the interface config command IP ADDRESS <address><subnet-mask> SECONDARY We've had up to 4 subnets off a single interface with no problems. There's no mention in the docs as to what the maximum number that cisco supports. IP devices on different subnets on the same interface will use the cisco to talk to one another even though they are on the same physical cable. This is consistant with the various Laws of the Universe that deal with IP. regards, Allen Robel robel2@mythos.ucs.indiana.edu University Computing Services ROBELR@IUJADE.BITNET Network Research & Planning voice: (812)855-7171 Indiana University FAX: (812)855-8299
haas%basset.utah.edu@cs.utah.edu (Walt Haas) (03/18/91)
In article <33369@boulder.Colorado.EDU> Allen Robel <robel2@mythos.ucs.indiana.edu> writes: >>someway to tell the router that there are multiple Class C network on >>each interface... what happens to a packet >>from 128.123.1.x to 128.123.3.x? > >IP devices on different subnets on the same interface will >use the cisco to talk to one another even though they >are on the same physical cable. Note that the two networks specified in the example are *NOT* actually Class C networks, they are subnets of the same Class B network. In this particular case (which we support a lot), it is possible to give all hosts on the physical cable a subnet mask of 255.255.0.0 . If you do this these hosts will ARP for any destination on the Class B network, and the cisco will give a proxy ARP response for any host not on the same cable. Therefore the cisco will not have to duplicate packets between hosts that can talk directly. The downside, of course, is that there will be more broadcast packets on that cable. -- Walt Haas haas@ski.utah.edu
barmar@think.com (Barry Margolin) (03/19/91)
In article <33369@boulder.Colorado.EDU> robel2@mythos.ucs.indiana.edu (Allen Robel) writes: >cisco supports multiple subnets on each interface with the >interface config command >IP ADDRESS <address><subnet-mask> SECONDARY > >We've had up to 4 subnets off a single interface with no >problems. There's no mention in the docs as to what >the maximum number that cisco supports. I'm using this, and I've run into a couple of minor problems. One problem is that there is still only one broadcast address per interface, so you have to use 255.255.255.255 as the broadcast address, rather than the subnet broadcast addresses. This means that hosts on both logical subnets will receive broadcasts intended only for those on just one of the subnets. In particular, this happens with RIP route broadcasts. Our Unix hosts' consoles and logs are full of messages "packet from unknown router, 131.239.32.250", which is the address of the router on the other logical subnet. -- Barry Margolin, Thinking Machines Corp. barmar@think.com {uunet,harvard}!think!barmar