From: Vytautas Valancius (valas@mail.lt)
Date: Mon Nov 29 2004 - 14:34:24 GMT-3
Hi Tim!
If lab states that there are new pc's on particular ports in that VLAN, i
guess you should place 'spanning-tree portfast' on those ports.
The thing is that some new pc's (especially with WinXP) boot-up very
quickly. Because of blocked port they might not get first/second DHCP reply.
Other related thing is arping for default gateway on XP machines. Because
for 30 sec port is in blocking state, quickPC+XP is unable to arp default
gateway MAC. Whithout right MAC workstations think that gateway is
unreachable and starts arping for every destination (they asume that maybe
someone is running proxy-arp feature). I had one customer who had high CPU
utilization because of ARP in residential metro network. We decided to
switch of proxy-arp and thousands of customers with new PCs stoped reaching
internet :)
cheers,
Vytautas Valancius
----- Original Message -----
From: "ccie2be" <ccie2be@nyc.rr.com>
To: "Group Study" <ccielab@groupstudy.com>
Sent: Monday, November 29, 2004 8:00 PM
Subject: DHCP and the 3550
> Hi guys,
>
> I would like to just confirm something regarding the above.
>
> Assume a router's ethernet interface is connected to a 3550 port which is
vlan
> X along with a bunch of hosts.
>
> Also, the router is configured as dhcp server for the hosts in vlan X.
>
> Am I correct in thinking that nothing related to dhcp must be configured
on
> the 3550?
>
> The way I understand it, when a host in vlan X powers up and sees it needs
an
> ip address, it will broadcast a dhcp request. The 3550, when it gets that
> broadcast, will flood the broadcast out all ports in vlan X. Thus, the
router
> acting as a dhcp server will get the broadcast dhcp request. After
getting
> the request, the router will respond using the source mac address of the
> request as the destination mac address.
>
> When the 3550 gets the response, it will forward it to the host.
>
> In other words, the default behavior of the 3550 is such that nothing
related
> to dhcp (such as dhcp snooping, dhcp relay, etc) is required for dhcp to
work
> properly in the above scenario.
>
> I'd hate to lose points for dhcp on the lab if I correctly configure it on
the
> router and forget to configure some stupid little thing on the 3550.
>
> TIA, Tim
>
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