Question

How to NAT VLAN's within a Cisco 3750 Switch

Asked by: btec_bob

I have a 3750 stack with about 40 VLANS created. Each Client gets assigned a VLAN and has from 1 to 15 machines within that VLAN.  Each of the VLANS have DHCP enabled for the clients within that VLAN with their own subnet. I want to NAT all the traffic from each VLAN behind a specific IP address for that VLAN.

Example Client 12
Is allocated VLAN 12 with a network of 192.168.12.0/24 their gateway is 192.168.12.21

I want all traffic from that network/VLAN to be NAT behind 192.168.12.21

I tried to use "IP NAT inside" on the VLAN 12 itself but the command is not available. However it was available on a Cisco 6500.

How can I achieve this?


 

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Asked On
2009-09-15 at 08:35:38ID24733316
Tags

Cisco 3750 NAT VLAN IP

Topics

Outlook Groupware Software

,

Network Switches & Hubs

Participating Experts
2
Points
500
Comments
5

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Answers

 

by: that1guy15Posted on 2009-09-15 at 09:04:48ID: 25336397

Sorry, NAT is not supported in the 3750. You can create a routed port and send all traffic out it and then NAT at a connected router. If you must use the specific VLAN ips then you can also trunk from the 3750 to a router/firewall and nat from there but with 40 vlans you will need a good size trunk.

Most of your lower end multi-layer switches (3560, 3750) do not support nat.

Sorry

 

by: ikalmarPosted on 2009-09-15 at 09:11:34ID: 25336457

ip nat is not supported on 3750 also:(

 

by: btec_bobPosted on 2009-09-15 at 09:30:29ID: 25336672

Hi that1guy15

In relation to what you said about the 3750  and trunking. I do have an ASA5520 firewall that they all go through, currently I route that through subnet 192.168.1.0/24. I assume you are saying to simply make the connection a trunk and let the ASA do the NAT on the Trunk? Since I have very little inter VLAN traffic I assume the impact would be quite small. Is that correct?

 

by: that1guy15Posted on 2009-09-15 at 09:44:57ID: 25336818

" assume you are saying to simply make the connection a trunk and let the ASA do the NAT on the Trunk?"

That is correct. This will allow all those VLANs to exist on the ASA and you can then create NAT translations on each. Just keep in mind that this will put extra load on your ASA.

"Since I have very little inter VLAN traffic I assume the impact would be quite small. Is that correct?"

If you have little inter-vlan traffic then you might be fine.  just remember this can be a big internal bottle neck in your network especially if this is a 100Mb link.

 

by: btec_bobPosted on 2009-09-15 at 09:48:55ID: 31629000

Very clear and much appreciated

20120131-EE-VQP-002

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