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New Member

VLAN Basics Question

I apologize for the elementary nature of this question.

On a Cisco 7603 router having a 48-port 10/100Mbps module, I have created two separate layer 2 VLANs, and have set switchport mode access, and granted switchport access vlan 20x to one fastethernet port each. The ports and the VLANs are not shutdown.

How do I enable/configure communication between the VLAN's? I know that a router must be used, but as a router how do I configure the 7603 itself to facilitate communication between them? Do I have to incorporate an IP address on a port or VLAN of the 7603 and mls?

Bob Kirk

7 REPLIES
Cisco Employee

Re: VLAN Basics Question

You need configure layer 3 VLAN interfaces and configure IP address under the VLAN interfaces. The following page should help

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/12_1e/swconfig/l3_int.htm

New Member

Re: VLAN Basics Question

Thank you for the reply.

However I have on each of the two segments, many different IP networks (ie 192.168.x.x private networks) that will not be able to route beyond the layer 3 VLAN IP interface address if that address is on a different subnet than they reside on.

Is there any way to configure an alias for an IP address that I would assign to such a L3 Vlan? In other words, if I assign an IP of 192.168.1.1/24 for instance, can I also assign an alias to that interface on the 7603 of say, 192.168.2.1/24 as well so that the device can route traffic from hosts on both networks? Thank you,

Bob Kirk

New Member

Re: VLAN Basics Question

Let me clearify, Vlan is the virtual interface and might have only one IP address.

Your problem is simmilar to my. In my case i solve this problem by readressing all networks to /16 subnet mask istead of /24. Routing in my case is the OSPF.

PS may be my iplementation of the solution is not so good as it possibe, but i don't know how to solve this problem by another way.

New Member

Re: VLAN Basics Question

Thank you,

Please let me try and summarize;

1."Out of box", with every port configured only as a switchport with mode set to access for instance, and vlan 1 active, every port is a member of vlan1 and so the device functions like a true layer 2 switch.

2.When I create an additional vlan, and assign at least one port to it (but no ip), it is a layer 2 vlan that from the devices' perspective, has no connectivity to vlan 1 or any other vlan (l2 or l3) that may be created?

3.When I assign an IP address to that vlan, and turn on ip routing, the device acts as that "external" router which routes traffic between the new vlan and any other l3 vlans that may have been created.

Again, sorry to ask such a fundamental question here, but most pages I come across talk about vlans that span devices, not how a router might deal "internally" with l2 and l3 vlans exclisively on its own ports. Thanks again,

Bob Kirk

Purple

Re: VLAN Basics Question

Your understanding is correct , if you want to route you need to have a L2/L3 switch . Then to route you create what is called a SVI (switched virtual interface) " interface vlan XXX" and then assign an ip address to the interface with a address range that is big enough to cover all the devices you will have on that subnet .

New Member

Re: VLAN Basics Question

Hi Bob,

Answer to your previous question is Yes. You can put more than one address spaces on one interface and router will route for all networks (from that interface).

Example:

interface vlan 20

ip address 192.168.1.1 255.255.255.0

ip address 172.16.4.1 255.255.255.0 secondary

ip address 172.17.3.1 255.255.255.0 secondary

Although this is possible, it is not "how is should be". You should use this only when you transiting from old network to new network design. You should separate these networks (from example above) to 3 different vlans:

interface vlan 21

ip address 192.168.1.1 255.255.255.0

interface vlan 22

ip address 172.16.4.1 255.255.255.0

interface vlan 23

ip address 172.16.3.1 255.255.255.0

Hope this will help.

Best Regards,

Srdja

New Member

Re: VLAN Basics Question

Sincere thanks to all who have replied. This issue does in fact exist in the context of a transition from old to new network design. You've helped resolve my issue! Thanks again,

Bob Kirk

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