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From: bobneworleans on 15 Jun 2010 16:36
I want to configure VLAN 101 on three 3550 switches. Can the virtual
interface for VLAN 1 have the same IP address on all three switches?
(10.1.101.1) If so, why isn't there an IP address conflict?

If each switch needs it's own unique IP address for VLAN 101, how can
all the clients get the correct default gateway from DHCP?

If I configure the switches as a cluster, does this affect the way
VLANs are addressed or managed?
From: flamer die.spam on 15 Jun 2010 17:42
On Jun 16, 8:36 am, "bobneworle...(a)yahoo.com"
<bobneworle...(a)yahoo.com> wrote:
> I want to configure VLAN 101 on three 3550 switches.  Can the virtual
> interface for VLAN 1 have the same IP address on all three switches?
> (10.1.101.1)  If so, why isn't there an IP address conflict?
>
> If each switch needs it's own unique IP address for VLAN 101, how can
> all the clients get the correct default gateway from DHCP?
>
> If I configure the switches as a cluster, does this affect the way
> VLANs are addressed or managed?

Sounds like what you need is to run HSRP, one virtual IP address
shared by all the switches in the group, check the cisco manual below
to see how to config.


http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_12c_ea1/configuration/guide/swhsrp.html
From: bobneworleans on 15 Jun 2010 22:24
On Jun 15, 4:42 pm, "flamer die.s...(a)hotmail.com"
<die.s...(a)hotmail.com> wrote:
> On Jun 16, 8:36 am, "bobneworle...(a)yahoo.com"
>
> <bobneworle...(a)yahoo.com> wrote:
> > I want to configure VLAN 101 on three 3550 switches.  Can the virtual
> > interface for VLAN 1 have the same IP address on all three switches?
> > (10.1.101.1)  If so, why isn't there an IP address conflict?
>
> > If each switch needs it's own unique IP address for VLAN 101, how can
> > all the clients get the correct default gateway from DHCP?
>
> > If I configure the switches as a cluster, does this affect the way
> > VLANs are addressed or managed?
>
> Sounds like what you need is to run HSRP, one virtual IP address
> shared by all the switches in the group, check the cisco manual below
> to see how to config.
>
> http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/re...

As I understarnd HSRP, it's used when you need a backup device ready
to take over if the primary device fails. In my situation, I need the
ports on all three switches to be on all the time. No backup device
is needed in this application so HSRP does not apply.

I want to know how to configure the virtual interface on switches
hosting a common VLAN. Do they all need an IP address? If so, does
that address need to be unique on each virtual interface?
From: bod43 on 16 Jun 2010 05:00
On 16 June, 03:24, "bobneworle...(a)yahoo.com" <bobneworle...(a)yahoo.com>
wrote:
> On Jun 15, 4:42 pm, "flamer die.s...(a)hotmail.com"
>
>
>
> <die.s...(a)hotmail.com> wrote:
> > On Jun 16, 8:36 am, "bobneworle...(a)yahoo.com"
>
> > <bobneworle...(a)yahoo.com> wrote:
> > > I want to configure VLAN 101 on three 3550 switches.  Can the virtual
> > > interface for VLAN 1 have the same IP address on all three switches?
> > > (10.1.101.1)  If so, why isn't there an IP address conflict?
>
> > > If each switch needs it's own unique IP address for VLAN 101, how can
> > > all the clients get the correct default gateway from DHCP?
>
> > > If I configure the switches as a cluster, does this affect the way
> > > VLANs are addressed or managed?
>
> > Sounds like what you need is to run HSRP, one virtual IP address
> > shared by all the switches in the group, check the cisco manual below
> > to see how to config.
>
> >http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/re...
>
> As I understarnd HSRP, it's used when you need a backup device ready
> to take over if the primary device fails.  In my situation, I need the
> ports on all three switches to be on all the time.  No backup device
> is needed in this application so HSRP does not apply.
>
> I want to know how to configure the virtual interface on switches
> hosting a common VLAN.  Do they all need an IP address?  If so, does
> that address need to be unique on each virtual interface?

I think you need to give us a more complete explanation
of your requirements.

You won't be able to use the same address on three
seperate switches. Give them different addresses.
Chose one switch to do dhcp. Or of course two
with non-overlapping ranges - then you might have
additional resilience but some more management
complexity.

You need an IP address on a VLAN for two seperate
purposes. You can do one of these, both of these or
none of these depending on your requirements.

1.
To do IP routing.

If you are not doing IP routing then you don't *need* an
address at all since the switches will forward the traffic
at Layer 2.

2.
To permit remote switch management.

Please clarify what you need to do.

I have never fancied clustering much. Stacking of course
is a different matter due to the high speed bus.
From: bobneworleans on 16 Jun 2010 10:27
> I think you need to give us a more complete explanation
> of your requirements.
>
> You won't be able to use the same address on three
> seperate switches. Give them different addresses.
> Chose one switch to do dhcp. Or of course two
> with non-overlapping ranges - then you might have
> additional resilience but some more management
> complexity.
>
> You need an IP address on a VLAN for two seperate
> purposes. You can do one of these, both of these or
> none of these depending on your requirements.
>
> 1.
> To do IP routing.
>
> If you are not doing IP routing then you don't *need* an
> address at all since the switches will forward the traffic
> at Layer 2.
>
> 2.
> To permit remote switch management.
>
> Please clarify what you need to do.
>
> I have never fancied clustering much. Stacking of course
> is a different matter due to the high speed bus.

Hello Bod43,
Your explanation helped me a lot. Thanks!

Here are some parameters of the installation:
600 clients
4507 in MDF
20 3550 switches on 4 floors IDFs
wireless network

Here's what I plan to do. Does anything here go against standard
practices?

1. Set up a VLAN for each floor plus wireless plus management.
2. All routing between VLANs will be done by the 4507.
3, Set up every switch with the management VLAN plus floor vlan (or
wireless vlan).
4. Set up unique virtual addresses for each virtual interface on their
own subnet.
5. The 4507 will have a virtual interface for each floor VLAN
addressed as 10.1.<vlan>.1. All DHCP default routes will point to
virtual interfaces on the 4507.
6. Assign each port (access) to the appropriate vlan.
7. Trunks will include all necessary VLANs and exclude the rest.

I have some more questions:
The servers need to be reachable by clients on all floors. Should
they be on their own separate VLAN?
How will DHCP know which scope to use when assigning IP addresses to
clients from every floor?
Do I need a helper address configured on the VLAN virtual interface
for each floor switch so client traffic will reach the DHCP server?
Is it acceptable to use vlan 1 for management?
Is a native vlan needed? If so, what traffic (other than management
traffic) will it carry?
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