routing public & private subnets through the same router

[ccip4911z]

So, im pretty new to cisco world. Im not sure if this is even possible. Basically, i want to have public and private subnets on the same router/switch. I would like to use my 3750E stack for intervlan routing, but if this setup will only work with my 2821 as the L3 device, thats fine.

My current setup (working)

AT&T Modem -----> Cisco 2821 (L3) -----> 3750 stack (L2)

GE\0 of 2821: DHCP from AT&T (103.43.241.X)

GE\1 of 2821: VLAN sub-interfaces (10.1.x.x,10.2.x.x,10.3.x.x,10.4.x.x and public static block from AT&T 45.29.x.x)

With this config, the public ips are routed thru the router, just like the private subnets. The private subnets are routed out thru AT&T's DHCP ip (NAT). I can use ACLs/QOS or what ever i please to affect both private and public subents.

Now with the new setup, we are switching to Spectrum for our ISP. Im being told that Spectrum routes there public subnets different then the way AT&T does it. AT&T routes there public subnets to the modem's IP (which allows you to hand the entire subnet to your own router) and Spectrum routes there public ips thru RIPv2 to the modem (which makes you keep the public subnets gateway on the Spectrum modem and doesnt allow you to move it to your own router).

Ive tried to get my current setup to work this way, but im unable to get the 2821 to play nice with the spectrum modem.

Is there anything you guys can suggest to get my current setup to work?

Brian

[Georg Pauwen]

Hello,

to be clear, in your current setup, you have the DHCP from AT&T (103.43.241.X) and the public static block from AT&T 45.29.x.x). Do you have these two different address spaces from Spectrum as well ? Either way, can you post the current config you have (the one which is NOT working) ?

[ccip4911z]

I'll grab the config when I get back. But no, I don't have the 2 address spaces with spectrum. If I bridge the spectrum modem to my 2821, I get the DHCP, but I lose the static IP block since it uses rip on the modem to route the public block.

[paul driver]

Hello

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@ccip4911z wrote:

So, im pretty new to cisco world. Im not sure if this is even possible. Basically, i want to have public and private subnets on the same router/switch. I would like to use my 3750E stack for intervlan routing, but if this setup will only work with my 2821 as the L3 device, thats fine.

My current setup (working)

AT&T Modem -----> Cisco 2821 (L3) -----> 3750 stack (L2)

GE\0 of 2821: DHCP from AT&T (103.43.241.X)

GE\1 of 2821: VLAN sub-interfaces (10.1.x.x,10.2.x.x,10.3.x.x,10.4.x.x and public static block from AT&T 45.29.x.x)

Spectrum routes there public ips thru RIPv2 to the modem (which makes you keep the public subnets gateway on the Spectrum modem and doesnt allow you to move it to your own router).

Ive tried to get my current setup to work this way, but im unable to get the 2821 to play nice with the spectrum modem.

Is there anything you guys can suggest to get my current setup to work?

Brian

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Either way the main focus will be on the cisco rtr -If your ISP perform NAT or not you can still do it on the cisco. rtr

So -
Cisco RTR
Cisco Wan interface = public ip or ip received from ISP
Cisco Lan interface - create sub-interfaces for each lan subnet

default route pointing towards wan interface and isp next-hop ip
enable nat for lan subnets
possibly apply some basic context based access control for the rtr, (if the isp isnt providing any security)

Cisco 3750
Disable ip routing
Apply Default-gateway of defined mgt subnet
Create trunk-port for cisco rtr lan connection
create L2 vlan for lan clients

res
Paul