Do you need a Default gateway in VLANs?

[NZGhostChips]

Hi forum,

I'm working on a Packet Tracer 3.4.1.2 Skills Integration Lab as shown in the attachments below

When I was doing this exercise I noticed that a default-gateway is meant to be implemented. What is the purpose of assigning a default gateway in a network topology which has no routers?

Also, is there any use for a default gateway if VLAN's are being used like in this exercise, and if so could you explain what?

I noticed that the first network address for VLAN 88 is being used as the default-gateway for all 3 switches. What is the reason for this?

Sorry if these questions are silly, I'm just needing to clear up a few concepts.

Thanks guys :)

[Jon Marshall]

You have multiple vlans/IP subnets so you need default gateways.

Presumably at least one of the switches is a L3 switch ie. it is not just routers that you need to think about.

So PC3 is in a different IP vlan/IP subnet than PC5 and if it wants to send traffic to PC5 it will need to send it to it's default gateway because it cannot send it direct.

The default gateway will receive the packets, do a route lookup and forward them on to PC5.

The reason the switches have their own vlan is for management and again they have default gateways so you can connect to the switches from a remote IP subnet ie. any of your PCs.

One important point to note is that L2 switches only have one L3 SVIs ("int vlan <x>") and it's IP address should never be used as a default gateway for end clients.

L3 switch have multiple SVIs and these are used as the default gateways for clients in the vlan.

Jon

[NZGhostChips]

Hi Jon, thanks for your response.

No I don't believe they are L3 switches (I thought the icon in the diagram meant it is a L2 switch but I could be wrong).

What is the best way to check how many SVI's there are so I can confirm which switch it is? Why do you never use L2 switches SVIs as the default gateway? What is the consequence of doing so?

I'll post up the entire configuration that the exercise asks for if that will help.

Thanks again.

- GC

[Jon Marshall]

If none of them are L3 switches then you won't be able to communicate between vlans so PCs will only be able to talk to other PCs in the same vlan which I'm not sure is what your assignments is asking you for although I maybe misreading it.

A "sh ip int brief | include Vlan" should show you all the SVIs.

You never use SVIs on L2 switches as default gateways because they do not route traffic, it is as I say purely to connect to the actual switch itself.

Jon

[NZGhostChips]

Ah yes, that makes much more sense, thanks for the clarification.

By the way that command didn't work - it came up with a invalid input error.

To reiterate what you said, you only use the L2 SVI to connect remotely to that switch for management, not to direct the VLANs traffic to the L3 switch/router. Correct?

So in essence I would need to implement Inter-VLAN Routing such as Router-on-a-Stick in order to get those VLANs communicating with each other?

- GC

[Jon Marshall · Accepted solution]

Not sure why the command didn't work, it could be what you are using.

You can try "sh ip int br" instead which lists all interfaces.

Yes, on a L2 switch the SVI is used purely for management and never to route traffic for end clients.

If you want the vlans to be able to communicate with each other you need a L3 device to route between the vlans.

This is usually a L3 switch or you could use a router with subinterfaces.

It's not clear from your assignment whether the switch you are meant to configure should be acting a L3 switch.

Jon

[NZGhostChips]

Yeah the exercise isn't really about that, but rather to test basic switch configuration skills - my question was sort of a sidenote to that.

Perhaps the switch used in this exercise isn't the right kind for that command as you say.

I'll have a go at an inter-vlan routing exercise or two, and that should reinforce the concept.

Thanks again for your help Jon