Mailinglist Archive: opensuse (1188 mails)
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Re: [opensuse] DHCP lease requests
- From: Greg Freemyer <greg.freemyer@xxxxxxxxx>
- Date: Fri, 6 Jan 2012 12:25:00 -0500
- Message-id: <CAGpXXZ+cr6bJ8FaagmbcdNi3nNSfj6W+0+w7C3UDEtxT7_5wKA@mail.gmail.com>
On Fri, Jan 6, 2012 at 12:08 PM, Dave Howorth
<dhoworth@xxxxxxxxxxxxxxxxx> wrote:
In theory it could do that, but it doesn't seem to work out in
practice. I don't know why in detail.
=== background for anyone interested
At the switch level all communications is actually MAC to MAC, not IP to IP.
So when you add a new machine to a LAN segment, the local switch
network has to add your MAC to the tree-spanning tables which are
maintained by all the switches.
But when it does that, it makes a concerted effort to map your MAC to
all the other MACs by the shortest path.
If you have a class C (/24) then there aren't that many options so it
takes no time to build the spanning tree.
If you have a full class A (ie. /8 or 10.x.x.x) then you conceivably
have 10's of thousands (or more) MACs to potentially keep up with and
setup spanning trees for.
In that case a haphazard interconnection of all the switches can cause
loops which complicate the process of bringing up a new machine.
The traditional solution is to introduce smaller lan segments and then
have routers between the lan segments. ie. 10.0.0.x, 10.0.1.x,
10.0.2.x are all in use at my office and we have a routers setup that
send traffic between those lan segments. You can get a cheap router
these days, so it is not expensive to do that if it makes sense.
But as I said, I have one client with 80 24-port switches in one huge
lan segment. So that would be about 2000 ports / MAC addresses the
switches have to map and track with various paths from one port to
another. In their case it takes about 45 seconds to add a new machine
(MAC) to the lan segment (switch network) and get a IP via DHCP.
They are in the middle of new network design now to reduce that time
significantly.
Greg
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<dhoworth@xxxxxxxxxxxxxxxxx> wrote:
Greg Freemyer wrote:
When they bring up a new machine and do dhcp, it takes about 45
seconds to get the new IP. I'm convinced most of the time is the
switches playing games trying to figure out the most efficient way to
connect the newly introduced machine to the dhcp server.
Isn't that any easy problem to solve? Whichever switch the new machine
is connected to just looks in its tables and asks "how did I send
traffic to <the dhcp server> last time somebody asked me?" The answer
doesn't depend in any way on there being a new machine.
But I'm no expert, so perhaps I'll learn something!
In theory it could do that, but it doesn't seem to work out in
practice. I don't know why in detail.
=== background for anyone interested
At the switch level all communications is actually MAC to MAC, not IP to IP.
So when you add a new machine to a LAN segment, the local switch
network has to add your MAC to the tree-spanning tables which are
maintained by all the switches.
But when it does that, it makes a concerted effort to map your MAC to
all the other MACs by the shortest path.
If you have a class C (/24) then there aren't that many options so it
takes no time to build the spanning tree.
If you have a full class A (ie. /8 or 10.x.x.x) then you conceivably
have 10's of thousands (or more) MACs to potentially keep up with and
setup spanning trees for.
In that case a haphazard interconnection of all the switches can cause
loops which complicate the process of bringing up a new machine.
The traditional solution is to introduce smaller lan segments and then
have routers between the lan segments. ie. 10.0.0.x, 10.0.1.x,
10.0.2.x are all in use at my office and we have a routers setup that
send traffic between those lan segments. You can get a cheap router
these days, so it is not expensive to do that if it makes sense.
But as I said, I have one client with 80 24-port switches in one huge
lan segment. So that would be about 2000 ports / MAC addresses the
switches have to map and track with various paths from one port to
another. In their case it takes about 45 seconds to add a new machine
(MAC) to the lan segment (switch network) and get a IP via DHCP.
They are in the middle of new network design now to reduce that time
significantly.
Greg
--
To unsubscribe, e-mail: opensuse+unsubscribe@xxxxxxxxxxxx
To contact the owner, e-mail: opensuse+owner@xxxxxxxxxxxx
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