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TOPIC: VLAN setup and VLAN routing basics

VLAN setup and VLAN routing basics 11 years 2 months ago #24969

I'm confused about VLAN routing. I'm using a 3Com 3870 (L3) switch to subnet my LAN from one /24 to 4 /24s. I've created the the 3 additional VLANs with their member ports (Membership is "untagged") . I've created 4 IP Interfaces using one VLAN per interface configured with the 4 /24 networks.
What I'm shooting for is:
VLANs 2,3, & 4 to see VLAN 1, but not each other.
Servers, DHCP, DNS etc. will all be on VLAN1. (VLAN 1 preexists as the Management VLAN with all ports as "untagged" members except those defined for VLANs 2,3,4)

This will be my core switch/router when the implementation is done.

The Router interface shows 4 valid routes that correspond to each subnet defined by the IP interfaces. What additional (static) route entries do I have to create to get the aforementioned configuration? I've tried several "stabs" at creating routes but the switch rejects them. "Failed to set route", which I take to mean I've entered some senseless data.

Input fields are:
Destination: host or network to route to
Subnet Mask:
Default Gateway: defined as the "next hop"
Metric: ?

First ; do I have my VLANs/IP Interfaces set up properly? - I think I do.
Foremost, what should my routing table look like?

I feel like I'm missing something simple. Would setting the IP interface for VLAN1 to a /22 help?

I've read all the VLAN/VLAN routing articles on this site and more...and It's just not sinking in.

Re: VLAN setup and VLAN routing basics 11 years 2 months ago #24970

If the router is showing the routes what more do you need?

Once packets get to the router the router knows where to send them because it has the routes.

The restriction of vlan access? That would be done with an access list typically, and not with the route.

Make sure your default gateways are set correctly on each subnet. Maybe thats throwing it off...

Re: VLAN setup and VLAN routing basics 11 years 2 months ago #24974

Thanks for the reply Todd,

All of the documentation I've read emphasizes that, by default, VLANs do no see each other. And IP routing must be enabled to facilitate this.

The router table shows 4 entries that have the status of 'local'.
Destination SubMask Metric Gateway Status State /24 1 - - Local Valid /24 1 - - Local Valid /24 1 - - Local Valid /24 1 - - Local Valid

It looks as if the router knows all of the networks but VLAN1 cannot ping anything on VLAN2 vice versa, etc, etc. ad infinitum.

As far as default gateways go, you can see from above that the routing table does not specify a default for any of the local route statements. These route statements are linked to each IP interface - so if I take an IP interface down - unplug the ethernet jack form the VLAN port, the route statement goes away.
Other than static route entries there is no where I would specify a default gateway for each subnet. ...and I'm not even sure what they would be? I can config a single default gateway for the router - but in a test environment I'm not sure what that would be either!

Do I create another VLAN and config the IP Interface as a /22 and make that Interface the default gateway? Can I just use the default VLANs IP interface as the default gateway?

I'm migrating from a single broadcast domain - so I'm used to flat topologies.

Re: VLAN setup and VLAN routing basics 11 years 2 months ago #25011

Ok i was a little off on what you wanted to know, this might help out more.

Each network is going to need a default gateway setup, in simple terms the default gateway is the router's interface on a specific network segment... yours might be...
This is an IP address on each network segment (vlan) that devices send traffic bound for another network (vlan). SO...

If pc01 (ip address wants to talk to pc02 (ip address it sees that ip address is not local so it send it to the default gateway (the router) the router recieves the packets and forwards it out to the correct network (vlan).

I don't know how to configure that on a 3Com switch... Maybe someone else can help out???

don't know with different ip 11 years 2 months ago #25154

i've tried vlan using packet tracer..

when using the same ip on router..i can ping it..
but i cannot configure for the different ip
let say R1 and R2 with and
i want R3 with the 192.168.30.X

here is the packet tracer that i practise..

<a href="</a> ;



Re: VLAN setup and VLAN routing basics 11 years 1 month ago #25184

The router needs to have multi - sub - interfaces configured one for each network you want to route to and from.

THIS IS NOT EXACT SYNTAX... I can't remember it right now, and i'm at work and can't dig it up...

int fa0/0

*don't put and ip address*

int fa0/0.1
encapsulation dot1q
ip address

int fa0/0.2
encapsulation dot1q
ip address

On the switch the you need to configure it as a trunk port.

I think I'm forgetting a step, but i'm at work and can't bang through it... Hopefully this gets you going.
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