Switch L3, Switch L2 and VLAN

[Pioneer]

Good Morning Guys!


I'm here with a question that can even be quite easy but I can not understand..

Suppose I have a switch L3 connected to multiple switch's L2! Each of these switch's L2 is connected to a set of terminals that are assigned to specific VLANs (which are of no interest to the case!).

My first question, although not the main, is whether the connections between switch's L2 and switch L3 must be TRUNK port's L3? If I want to exchange traffic between the VLANs in switch's L2 i have to be sure that i have TRUNK port's connecting the two switch's (L2 nad L3), right?

The other issue and my main question is, for example, if a packet arrives to the switch L3 from another door (not the doors connected to the switch's L2), and that packet doesn't have any vlan assigned what does the switch L3 do with the packet??

Thank you in advance for your attention.

PS: Sorry for any mistakes made in English, but I'm Portuguese!

 

[Emulex]

well you must consider that even the lowliest of L2 services are really static routing L3-lite these days (talking about business class). so they have a default (GVRP off, VLAN1 ports 1-48).

Many low end modern switches can do trunking (no protocol) but the question is can your nice fancy l3 switch?

And keep in mind the term trunking does have a specific cisco/non-cisco meaning classicly versus procurve.

So maybe you can tell us which hardware pieces so we know their capabilites? Cisco has proprietary ways to deal with things that many competitors have more "industry standard" methods and IT is not the same way to solve the same problem.

 

[Pioneer]

Hi Emulex. First of all, thank you very much for your reply. I'm searching like crazy for an answer ans i can't get to one.

I'm going to try to be a little more specific.

This is a project (first part of the project is to make the design of the network, and the second part is to make restrictions, like ACL's) that i have to do, where i have two buildings in one city with local connectivity between them and then i have one building in another city that has connectivity with the other two over the internet.
Each of the buildings are divided into floors.

First, i have already divided the network and so i have a different subnet on each of the floors. Then, i have some switch's L2 on each of the floors connecting the end hosts. Then on each floor i have switch's L3 connecting the switch's L2 (with some redundancy).

Now the issue is that i have to make use of the VLAN's functionality so traffic doesn't get all mixed up.

My real question is that if a packet reaches a switch L3 of a floor of one of the buildings, let's say from another building, then that packet won't have a VLAN assigned to it. How would the L3 switch know where to sent it?


Sorry i can't be any more specific, but i really can't say if i'm going to use CISCO or not because it's just a project. There will be no real implementation.

But if someone can answer to the question, with CISCO or not, i would be really thankfull.

Thank's one more time.

 

[jlazzaro]

My first question, although not the main, is whether the connections between switch's L2 and switch L3 must be TRUNK port's L3? If I want to exchange traffic between the VLANs in switch's L2 i have to be sure that i have TRUNK port's connecting the two switch's (L2 nad L3), right?

If the L2 switches are only serving a single access VLAN then there is no requirement for trunking. If the L2 switches will serve multiple access VLANs, you must trunk up to the L3 so dot1q can differentiate between the various VLANs. I would recommend trunking in the first scenario regardless of its requirement, as it will make your life easier when future requirements arise.

The other issue and my main question is, for example, if a packet arrives to the switch L3 from another door (not the doors connected to the switch's L2), and that packet doesn't have any vlan assigned what does the switch L3 do with the packet??

Packets (layer 3) don't have VLANs assigned to them...only frames (layer 2) carried over a trunk will have a VLAN tag (minus your native VLAN). Additionally, EVERY switchport is assigned to a VLAN. If it is not explicitly configured, it will be operating in the default VLAN 1. What would happen to that traffic is dependent on how VLAN 1 (shouldn't be) configured on your network.

 

[drebo]

Uhg, the plural of "switch" is "switches", not "switch's".

 

[Pioneer]

Packets (layer 3) don't have VLANs assigned to them...only frames (layer 2) carried over a trunk will have a VLAN tag (minus your native VLAN). Additionally, EVERY switchport is assigned to a VLAN. If it is not explicitly configured, it will be operating in the default VLAN 1. What would happen to that traffic is dependent on how VLAN 1 (shouldn't be) configured on your network.

Hi, thanks for your reply.

Let's assume that i have the same topology from the beginning. Switches L2, with trunk port to switch L3. Connected to the switch L2 are the end hosts.

So, if i have two buildings (A and B, with same topology, and connected to each other) and i want to ping from one vlan in building B to the other vlan in building A, would it be possible? What would the switch L3 do with the icmp echo request from the vlan on building B?

Uhg, the plural of "switch" is "switches", not "switch's".

Hi.

Thanks for the correction!

Always learning.



Thanks again for all of your help...

 

[jlazzaro]

So, if i have two buildings (A and B, with same topology, and connected to each other) and i want to ping from one vlan in building B to the other vlan in building A, would it be possible?

of course

What would the switch L3 do with the icmp echo request from the vlan on building B?

My real question is that if a packet reaches a switch L3 of a floor of one of the buildings, let's say from another building, then that packet won't have a VLAN assigned to it. How would the L3 switch know where to sent it?

you need to differentiate in your mind VLANs & Ethernet (layer 2) versus IP (layer 3). each layer 3 switch has both VLANs (for broadcast separation) and corresponding IP interfaces (for routing). these IP interfaces are typically the default gateway of each host in a specific network. layer 3 switches know where to send destination IP traffic based on their IP routing tables.

 

[Pioneer]

of course




you need to differentiate in your mind VLANs & Ethernet (layer 2) versus IP (layer 3). each layer 3 switch has both VLANs (for broadcast separation) and corresponding IP interfaces (for routing). these IP interfaces are typically the default gateway of each host in a specific network. layer 3 switches know where to send destination IP traffic based on their IP routing tables.

Ohhh, i think i got it now...

So, it doesn't really matter wich VLAN's are assigned to the Switch L3 when a ip packet arrives from the outside, right? Because the only thing that the switch L3 will do is to forward the packet to the right interface so it can arrive to it's destiny.... It won't even look for VLAN's since it's a IP packet.

Did i understood correctly?

Man, i really appreciate your help. Many thanks.

 

[jlazzaro]

Ohhh, i think i got it now...

So, it doesn't really matter wich VLAN's are assigned to the Switch L3 when a ip packet arrives from the outside, right? Because the only thing that the switch L3 will do is to forward the packet to the right interface so it can arrive to it's destiny.... It won't even look for VLAN's since it's a IP packet.

Did i understood correctly?

Man, i really appreciate your help. Many thanks.

Yes, you have the right idea. OSI model is the basis for all of this, learn it well

 

[Pioneer]

Yes, you have the right idea. OSI model is the basis for all of this, learn it well

Believe me, i will.

Thank you very much jlazzaro.