Subject: DHCP and spanning tree on switches

From: "Paul Roberts" <[email-masked]>
Date: Thu, 29 Oct 1998 07:00:45 -0500 (EST)

Hi,

I'm not an expert on switches so I am wondering if anyone can help me (and
please correct me if I am wrong)...

The way I understand switches work with DHCP, is when the client sends a
DHCPDISCOVER it takes a while for the switch port to learn the topology of
the network and it runs the spanning tree alogorithm to make sure there are
no loops. This takes a few seconds during which time the port is blocked.
The client will send another DHCPDISCOVER, and hopefully the switch port has
now come up, learnt where the client is trying to get to and switches the
packet across, and everything works fine.

However, I am seeing a situation where laptops moving to a different subnet
are having difficulty obtaining the correct IP address for that subnet. What
I think is happening is that the client broadcasts a DHCPREQUEST (during
INIT-REBOOT) to try and renew the old IP address. Because the switch port is
blocking for a few seconds while it learns the topology, the DHCPREQUEST
never makes it to the DHCP server (via an ip-helper on the switch), so the
client never gets a DHCPNAK. The client then tries to use it's old IP
address, which of course is invalid now that it has changed subnets.

I have heard that Cisco switch ports can be set to "fast port" mode, which
disables spanning tree, but isn't disabling spanning tree a bit dangerous?
In this case there aren't single workstations on each port, but lan segments
(each in a different subnet, so the switch is effectively routing).

What is the solution in this case? I guess this isn't a problem with desktop
PC's that don't move around and stay on the same port, but I haven't got a
clue how to get this working for laptops that move around onto different
ports, other than disabling spanning tree, which I'm not sure is a safe
thing to do.

Regards,

Paul

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From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Thu, 29 Oct 1998 11:24:59 -0500 (EST)

Re: Paul Roberts question..

I was wondering why people wanted to disable the spanning tree
algorithm. Thank you for your example. It's not the algorithm
that's bothering them...it's the preforwarding delay.

It does seem dangerous to turn off the algorithm
completely. However, if you know that
the port does not contain a switch, but rather contains a laptop,
it should be fine to start forwarding immediately. Perhaps
there could be some sort of switch-switch protocol in which switches could
let each other know that they are switches. Of course if that
message got lost, you could still have temporary loops.

Also, I'd think that an alternative to disabling the spanning tree
algorithm would be to set pre-forwarding delay to zero. That way
the algorithm would still break loops if there were any, but
there might be temporary loops if you plugged switches into switches
into switches...

Is it possible to keep spanning tree on but just eliminate the
preforwarding delay (which I just put in because I was paranoid
about loops with no hop count and no limit to how many copies of
a packet could get spawned)? People survive loops today, because
they can be caused by a repeater coming up. It's possible that
the preforwarding delay is not really necessary, and certainly
way too conservatively set as the default.

Radia

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From: [email-masked] (by way of Ralph Droms)
Date: Fri, 30 Oct 1998 06:51:55 -0500 (EST)

message sent to [email-masked] by [email-masked] follows.
-----------------------------------------------------------------------

> -----Original Message-----
> From:	Paul Roberts [SMTP:[email-masked]]
> Sent:	Thursday, October 29, 1998 7:04 AM
> To:	Multiple recipients of list
> Subject:	DHCP and spanning tree on switches
>
> Hi,
>
	[Glancy, Terry]   Fast port does not disable Spantree, Just reduces
the listen time so the network login and DHCP can function. Not required in
shared segments with concentrators as the port is always active. If you are
not planning on connecting others switch to these ports there is NO danger.
New Cisco hardware such as the 1900 come with the client ports in fast mode
and the up links in learning mode
	We have now turned spanning tree completely off as we have true star
topology with no chance of loops

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From: [email-masked] (by way of Ralph Droms)
Date: Fri, 30 Oct 1998 06:52:15 -0500 (EST)

message sent to [email-masked] by [email-masked] follows.
------------------------------------------------------------------------

Spanning tree is most important if there's a possibility of a bridge
loop on your network.   If you don't have a bridge loop, then spanning
tree is still a nice thing, because it reduces congestion, but not
having it is not "dangerous," as long as your network isn't overly
congested.

			       _MelloN_