Re: interface Vlan vs L3 port switch

Interesting questions.

No STP for L3 ports ... STP is for the L2 links.

Not sure I see the point of having a trunk link between two switches as well
as a L3 port on each connecting each to each other. Maybe there are other
devices on the network which are broadcast based and cannot be routed?
Routed traffic across one link and broadcast on the other? Perhaps this is
to extend a particular vlan?

Maybe this design will allow for rspan across the switches for management
purposes? Separate L3 and L2-only networks ... Humm ...

Not sure I know why, but I think your questions are good amigo. I would
keep asking these questions to your colleague and even propose a lab test.

If you learn more of why this design is proposed, this may help to
understand if it is a sound design.

I do not see any gotchas ... but perhaps the team can think of some. As far
as other concerns and chances for error, I would ask you to consider the
"usual safe guards" ... disable negotiations, bpduguard, root, port
security, etc ... etc ..

HTH,

Andrew

On Wed, May 18, 2011 at 10:50 AM, Juan <fferrer10_at_gmail.com> wrote:

> Thanks Andrew,
>
> The topology is something like this:
>
>
> SW1
> SW2
>
> L3port*SW1* L2port*SW1*(Vlan2) <------Dot1q trunk------> L2port*SW2*(Vlan2)
> L3port*SW2*
> |
> |
> | |
>
> |_________|
> |_____________|
> Physical
> cable1 Physical
> cable2
>
>
>
>
> , and obviously the Layer3 ports of the 2 switches are on the same ip
> network. As you are directly interconecting 2 ports of each switch,
> is there a possible risk of having a loop, maybe at booting, when the
> switches are turnning on? And what happens with the STP, may it block some
> port or cause the systems to continously complain about vlan mismatch?
>
> I never used a scenario like this, nor in lab environments... to me it
> seems a "bad" solution, but it would be helpfull if you can point the
> problems this topology could cause, if you clearly see anyone.
>
> Regards
>
> Juan
> 2011/5/18 ALL From_NJ <all.from.nj_at_gmail.com>
>
>> Do I understand the topology like this:
>>
>> L3port-SW1 <-> L2-SW2 <-> L3port-SW3
>>
>> Having any L3 device connect to your L2 switch is fine.
>>
>> Having two L3 devices connecting to your L2 switch and configuring these
>> ports within the same VLAN means that they can talk just fine ;-)
>>
>> I would suggest to use portfast on the interfaces connecting to the L3
>> devices. You may also consider bpdufilter as well as a default for when
>> using portfast.
>>
>> Is it a good design in your network? Humm ... you would know best.
>>
>> HTH,
>>
>> Andrew
>>
>> .
>>
>> On Wed, May 18, 2011 at 2:42 AM, <fferrer10_at_gmail.com> wrote:
>>
>>> Hi Guys,
>>>
>>> I have some doubts about a config a colleague of me is doing using L3
>>> Catalyst
>>> switches: in order to have IP communication on 2 linked switches, he is
>>> using a
>>> L3 port on each one, and conecting this port (with a cable) to a L2 port
>>> of a
>>> common vlan (in each switch)... As the vlan crosses the switches and has
>>> L2
>>> ports on each one, he can connect another L3 port, with the same ip
>>> network, to
>>> a port of the second switch and have the 2 switches speaking IP in this
>>> vlan
>>>
>>> My first impression was "this can be or could be a potential loop, why
>>> not
>>> using the Vlan interface, that is done for this cases?"... Anyway, since
>>> he
>>> told me that is plannig to run BGP and he does not like a vlan interface
>>> as
>>> bgp speaker, i am wondering if there are reasons to totally reject this
>>> strange/curious design.
>>>
>>> Anyone can provide help to see the possible problems of this scenario on
>>> a
>>> production environment, please?
>>>
>>> TIA and best regards.
>>>
>>>
>>> Blogs and organic groups at http://www.ccie.net
>>>
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>>>
>>>
>>>
>>>
>>>
>>>
>>
>>
>> --
>> Andrew Lee Lissitz
>> all.from.nj_at_gmail.com
>>
>
>
>
> --
>

-- 
Andrew Lee Lissitz
all.from.nj_at_gmail.com
Blogs and organic groups at http://www.ccie.net