Which Layer 3 switch needs the Sub-Interfaces on it?

[JoshGarmonCiscoCCNA]

Packet Tracer file

I was able to open it and see the topology...hope you can to.

Switch A - Vlan 1 Root Primary Vlan 2 Root Secondary

Switch B - Vlan 2 Root Primary Vlan 1 Root Secondary

I'm thinking I only need to configure one of them but I'm not sure.

[Jon Marshall · Accepted solution]

I would say neither, you need to use SVIs instead.

With L3 switches you don't often need or use subinterfaces and in fact looking at the topology it would make no sense to use them.

The thing about subinterfaces, just like a physical L3 port, is the vlan/IP subnet terminates on that interface.

So the only logical place you could configure them is on the ports connecting the top switches to the access switch.

But then the interconnect between the top switches is pointless because no traffic in those vlans would ever go across it.

Whereas with SVIs ("int vlan <x>") these are virtual interfaces which means the physical ports are not L3 and are therefore not terminating the vlan.

So you could configure an SVI for each vlan on both top switches and run HSRP/VRRP/GLBP between them.

Now the etherchannel link does provide redundancy and you also have redundancy if one of the top switches fails.

I appreciate the diagram says subinterfaces but it makes no sense to me looking at the topology.

Jon

[JoshGarmonCiscoCCNA]

Hi Jon, thanks for clarifying.

You're probably right, I keep thinking in terms of a Router which is I brought up Sub-Ints.

[Masoud Pourshabanian]

Hello,

Your question was about sub interface or Spaning tree? What did you mean by "I'm thinking I only need to configure one of them but I'm not sure" ?

[Jon Marshall]

The question was about using subinterfaces and where to configure them.

Although on rereading the question I'm not so sure myself now :-)

Jon

[Masoud Pourshabanian]

Hi Jon,

Thanks for your reply. I have a question. Who gives rate to question and who gives rate to the answers?

[Jon Marshall]

The question can't be rated, it just shows the total points and number of ratings given to posts within the thread at the top within the actual question.

The author of the thread can rate any post (except their own) and can also mark any post as correct.

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Just for completeness if you are a VIP you can also mark any post as being VIP endorsed although that is not used as often as other ratings.

Jon

[Masoud Pourshabanian]

Thanks for your complete answer.

Masoud

[JoshGarmonCiscoCCNA]

No you're correct.

I wanted to know how to get both PCs to talk to each other in this particular situation as they are in different Vlans.

[Masoud Pourshabanian]

Hello,

Your commands indicate that you are configuring PVST( STP per Vlan).

You need to configure it in both switches. I will tell you why.

The default bridge priority is 32768

In Switch 1 : Vlan 1 primary changes the bridge priority to  24586 for Vlan 1.

In Switch 1:  Vlan 2 secondary changes the bridge priority to 28762 for Vlan 2.

If you do not configure Switch 2

Switch 2 : Vlan 1 will still have the default priority which is 32768

Switch2  : Vlan 2 will still have the default priority which is 32768

As result Switch 1 will become root bridge for both vlan1 and 2 because it has lower priority for both Vlans.

The purpose of PVST is sharing traffic between two switches by choosing one switch as root bridge for some vlans and choosing the other as root bridge for other vlans.

Hope it helps,

Masoud