Help with future Network, how do you inter-vlan router properly?

Unanswered Question
Feb 19th, 2010

Hi there,

Apologies if this seems kind of simple but I just can't get my head around it. The attatched network diagram is what my network looks like now but future connections will extend this at least 12 systems which will be connected in a daisy chain format, although not ideal but this is due to locations and expense of the Radio WPtP.

All devices that have an ip address on the network (30-40 per site) are currently all on the subnet in a class B format so that everyone can see everyone, there are 8 xp based machines at each site.

each site has 3 Cisco 3750G 24 ethernet port switchs, 2 hanging off the Main switch which also has the 2 Wireless PtP ethernet devices attached.

So what I have been trying to do unsuccesfully is to give each site a subnet and then allowing routing between each subnet, do to this network only having the Layer 3 switches I believe the way this should be done is put all the devices in a VLAN and therefore eseentially have a vlan per site. So my thought was to keep the two switches that go in the main switch as Layer 2 and then make the Main switch a Layer 3 allowing inter-vlan routing? Does this sound correct or have I totally got the wrong end of the stick?

I have read case studies that do creat 2+vlans per switch and then the top switch allows devices that are attached to the same vlan via another switch to talk.

If you think that this is a complecated process I would be more than happy to read up on it but I have been struggling to find configuration and explanations on the internet.

Thank you in Advance



I have this problem too.
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francisco_1 Fri, 02/19/2010 - 02:27

Local site.

You could do it that way by having the top switch (Distribution switch)  routes between vlans locally in a site. So as you mentioned have the 2 switches below (Access switches) connect to the top switch via layer 2 (Either a trunk or access port depending if you have a single vlan of multiple vlan on the access switch) and enable ip routing on the top switch and create SVI for the vlans. Make sure the vlans on the access switches exist on the distribution switch. Also it's recommended not to span vlans between access switches. So each access switch will have it own vlan number and address space.

Or you could enable routing on the access switches as well. By doing it this way vlans dont need to exist on the distribution switch. All the distribution switch will do is simple routes packets for your access layer switches.

Between sites.

Best design is to keep broadcasts within a site by enabling ip routing on the top switches between your sites. So for routing you could use IGP protocols such as EIGRP or OSPF to route packets between your sites.

You are better off going through the Cisco Design Guides.

You will find best practice Guides from the link below

nathansouthgate Sat, 02/20/2010 - 07:18

Thank you for your reply. So with every site, does every switch need to have all the vlans (all sites) on them or is it just the Distribution switch's that connect to the other sites distribution Switch's? So I got the routing routing with two vlans on a single site, because I am just using Layer 3 Switches to connect the sites up is there any need to trunk the outgoing ports (distribution switch to distribution switch)? I was planning on using ospf for this, does this sound advisable?

Kind Regards and thanks in Advance



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