Question between Switches layer 2 and layer 3

Unanswered Question
May 20th, 2010

Hi everybody:

I'm having some kind of discussion with a consultant in the agency i provide IT services. He says that in a network that have layer 2 and layer 3 switches, it could cause collision and broadcast problems. In all years of experience i have in this area, including the CCNA, there's no way this issue happens. Of course, every port creates a collision domain, but in switching it only happens exclusively in every port for themselves. And all of the network i've been lately, their networks are configured this way, layer 2 switches for network components like PC, printers, ect... and layer 3 switches for Vlan's and routing, including interconnect segments in the network, so it seems that this guy says that all my network switches, should be layer 3, that cost a lot of money or use layer 2 switches and use routers to connect segments, and router are really slow making this decision, compared to a switch layer 3. I just want to know if this theory or fact, is possible or is viable. Please, if anybody can reply, the better, because this i have to present it as evidence to the administration, because i haven't found anything on the web agreeing with this theory or fact of his.

Thank You

gravis0516

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (4 ratings)
Loading.
Jon Marshall Thu, 05/20/2010 - 12:22

gravis0516 wrote:

Hi everybody:

I'm having some kind of discussion with a consultant in the agency i provide IT services. He says that in a network that have layer 2 and layer 3 switches, it could cause collision and broadcast problems. In all years of experience i have in this area, including the CCNA, there's no way this issue happens. I just want to know if this theory or fact, is possible or is viable. Please, if anybody can reply, the better, because this i have to present it as evidence to the administration, because i haven't found anything on the web agreeing with this theory or fact of his.

Thank You

gravis0516

That is quite simply wrong. The vast majority of LANs these days including many high performance Data Centres have a mixture of L2 and L3 switches and yes you could get broadcast and collision problems but that would be to do with the speed/duplex settings and nothing to do with whether the switch was L2 or L3.

I can only assume the consultant is on a commission for every L3 switch he/she sells

Jon

burleyman Thu, 05/20/2010 - 12:31

gravis

,

Jon is absolutely correct. If the consultant is saying that I would also be very leery of what he is telling you. He is either, like Jon said, on commission or not understanding what he is saying. We had someone in here like that once, needless to say we opted for someone else.

Mike

gravis0516 Thu, 05/20/2010 - 13:04

Hi:

I really appreciate the fast reply. Ja Ja Ja, the thing is that this comedian is a Unix consultant and he directly went to the administration, giving this argument about the layers in switches without any evidence whatsoever. So the administration comes to me, and ask if this is true. But they wanted to have facts written, not by mouth. So again, thank you for the reply, and if its no problem for your guys could you send me your position or role in your workplace, just to say to them that someone on Cisco with this "role" has given me a solid argument about this issue.

Thank You

gravis0516

Jon Marshall Thu, 05/20/2010 - 13:11

Gravis

Just to be clear neither Mike nor myself work for Cisco. But as you have visited the Cisco support community and been told it doesn't make any sense does that not help considering the consultant is a Unix engineer and not a Cisco engineer.

Not sure how my job title would help but the last position i held was as a Technical Infrastructure Architect for the network and yes, it was a bit of a mouthful

Jon

burleyman Thu, 05/20/2010 - 14:09

Gravis,

While I agree with Jon about the job title not meaning anything I have run across this type mentality of management in the past, thankfully I have not experienced that thinking where I work now. What I have been called is Infrastructure Engineer or Network Engineer, with Global IT operations.

Now as for some stuff or proof on this it is going to be tough to find something on this that will say Layer 2 and Layer 3 switch in the same network will not cause collision or broadcast problems just because they are layer 2 and layer 3 switch running on the same network. The reason you will not find this is because it definitely will not cause those problems just because they operate on different layers. My question to management on this is to have the consultant direct you to documentation that says there will be issues when you have layer 2 and layer 3 switches on the same network that cause collision and broadcast problems.

Mike

Leo Laohoo Thu, 05/20/2010 - 16:24

That's nothing.  There was a guy (with only two Cisco 2950 for his LAN) who posted an issue with two MS boxes with the same IP address and the MS helpdesk basically said that he should disable ARP cache.

I'm very, very lucky (and thankful) that Cisco wasn't made by or running Windows OS MS or I'd jump off the cliff!

Or how about a so-called CCIE who thinks that you can power up a rack of HP servers from PoE?

gravis0516 Thu, 05/20/2010 - 16:39

Hi:

I see this has brought some jokes on the cisco forum. The thing is that this guy went to the management without any evidence on this.

So this Post will be the perfect evidence for me to show the management that i'm not the only one in the world saying that this is wrong.

Thank you again for all the replies and the advices.

I can't believe that a CCIE said that, that's #$%$^ up. I can see that the title isn't everything.

Thank You

gravis0516

Leo Laohoo Thu, 05/20/2010 - 17:24

I've worked in an organization who treats network as an "illegitimate child".  So when the organization was rolling out a multi-million dollar project our recommendations and warnings were shoved into the "insanity" basket, aka it-will-never-happen-to-this-network.

So we all decided to stand back, fold our hands, smile, sip some coffee and waited for the fireworks to happen.  Note that the organization did not carry out a comprehensive review of the entire design.  The project was knocked up by a group of consultants who knew the right people in the organization.

During the middle of the implementation stage, project verifications kicked up a storm.  Basically it didn't work.  Of course the consultants banded together and started blaming networks for all their failure.  They put blame on the fact that networks did not warn management that the project will cause issues.

What the consultants (and their supporters within the organization) didn't count is that some of us kept the emails (including return receipts) and documents but management didn't.  At the end of the entire fiasco, management spent at least three times more to rectify the issue.  Sweeeeeeeeet!

Kids, this is what happens to you when you smoke confiscated weeds. 

gravis0516 Thu, 05/20/2010 - 18:36

Hi:

The same thing happens to us all the time, we give advice, they want another one, they took the other's company advice, we sat and laugh until everything explode, and they spend more in fixing this error instead of the first one. But that why you must love to work HAHAHA. In situations like this, we learn a lot, so at least we get a good outcome and a good laugh too. Well i think were getting off the subject, so again, I thank everybody for the help and the advice, so lets close this post, because there more people like me that need the same help and the same advice you gave to me.

Thank You

gravis0516

chris.rae07 Thu, 05/20/2010 - 19:26

Guys,

Can I make a point that all the Cisco switches come out with a Multilayer Image when purchased.

Either SMI - Standard Multilayer Image

          or

          EMI - Enhanced Multilayer Image

The difference if that the SMI supports static routing, HSRP etc

The EMI supports everything - Dynmaic routing such as OSPF, BGP, EIGRP etc

Both can support SVI's (VLAN Interfaces).

There is no performance difference.

Hope this helps

Chris

Jon Marshall Fri, 05/21/2010 - 00:53

chris.rae07 wrote:

Guys,

Can I make a point that all the Cisco switches come out with a Multilayer Image when purchased.

Either SMI - Standard Multilayer Image

          or

          EMI - Enhanced Multilayer Image

The difference if that the SMI supports static routing, HSRP etc

The EMI supports everything - Dynmaic routing such as OSPF, BGP, EIGRP etc

Both can support SVI's (VLAN Interfaces).

There is no performance difference.

Hope this helps

Chris

Chris

You can make the point by all means but it would be wrong

Not all Catalyst switches support routing protocols nor multiple SVIs ie. the 2950/2955/2960 switches for example are L2 only and as such you cannot run any routing protocol, static or dynamic and can only use one SVI at any one time for management of the switch.

Jon

Actions

This Discussion