InterVLAN routing

Unanswered Question
Apr 10th, 2008

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a008019e74e.shtml#maintask1

I was reading over the howto at the above link, and I got hung up on step 6. Are they talking about a default gateway/gateway of last resort here? They use the term "default router", so I didn't want to assume that they meant default gateway. Just an FYI, I am learning about inter-VLAN routing on layer 3 capable switches, and I've actually got it working using up to step 5 in the above howto in a lab environment, but when I take away the default gateway address on my host machines, they can't see each other any longer. I pretty much deduced that this is because I haven't set up a default gateway on the switch itself, which leads back to why I'm asking you this question in the first place... Is Step 6 exactly what I need to do, and if so, if I'm using a routerless setup (just layer 3 routing on the switch (a 3550)) what IP should be used as this address?

Attached is a pretty diagram of what I have so far. Again, the hosts can ping each other when they have a default gateway programmed into them (just like the diagram says), but as soon as I remove that default gateway, they get all screwy.

I have this problem too.
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Jon Marshall Thu, 04/10/2008 - 05:42

Michael

Step 6 is if you are routing between vlans on your L3 switch and then have an external router to route to other subnets.

As you are only using a 3550 switch on it's you do not need to do step 6.

The default-gateway on the hosts will always be needed to allow hosts in different vlans to communicate with each other.

Jon

michaeljsmalley Thu, 04/10/2008 - 05:56

Jon,

So if I have hosts that are going to be pulling addressed from DHCP servers on both VLANS (which were previously isolated... one DHCP server on each VLAN), then I should be good to go, as the default gateway should be auto assigned by the respective DHCP server depending on the VLAN the PC is connected to? Also, my major problem at this time is trying to connect two switches on one VLAN. Both switches have two VLANs, but I only want routing to be done on one of the two. In other words, switch A will have both VLAN 100 and 900 configured, with routing between them, and switch B will have VLAN 100 and 900 too, but no routing. If I wanted to connect these two switches on only one VLAN (which I tried unsuccessfully using a crossover cable, what are the specific commands required on the connecting interfaces? Here's a rough visual:

SWITCH A[Port on 100]<------>[Port on 100] SWITCH B

Say I also had ports on 900 on switch A. I want to route across to 100 within switch A, and send it out to 100 on switch B.

Jon Marshall Thu, 04/10/2008 - 05:58

Yes, although you do not need a DHCP server on each vlan. You could use the same DHCP server for both vlans and for the vlan that doesn't have the DHCP server in it

int vlan

ip helper-address

Jon

Jason Fraioli Thu, 04/10/2008 - 06:56

Michael,

If you remove your host computers gateway, you will not be able to communicate outside of the vlan. If you want to communicate outside of your vlan, your host computer needs to know where to go (default gateway) to get to other vlans. Your access layer switches do not need default gateways.

If you are following the article in the link you posted, then your gateway address for your host computers will be;

10.1.2.1 for vlan 2

10.1.3.1 for vlan 3

10.1.10.1 for vlan 10

Now the 3550's default route will be pointed to that router, they call it by name as "Default Router", but in reality, it is just a router. The 3550 has a default route pointing to the "Default Router".

Hope that makes sense.

michaeljsmalley Thu, 04/10/2008 - 07:22

Thank you for your assistance. I don't want to say that the issue is completely clear to me yet, but it's getting there, and largely because of the your assistance. I'll post if I run into any other issues related to this. Again, thanks for the help, I'll be sure to pay it forward.

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