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Replace Cisco Router 2621 with Catalyst 3750 layer-3

I have a situation below:

A Cisco Catalyst 3560 called SW1 running only in Layer-2.  Port f0/1 of SW1 is connected to
interface F0/0 of Cisco router 2621.  The IP address of the F0/0 on Cisco 2621 is 192.168.1.254/24.
Interface F0/0 os the Cisco 2621 router also has secondary IP address of 10.0.1.254/24. 
Everything on the Catalyst is setup as default in VLAN1.  IP address of VLAN 1 in SW1 is 192.168.1.1/24. 
There are other machines connected to other ports of the SW1.

A Cisco Catalyst 3560 called SW2 running only in Layer-2.  Port F0/1 of SW2 is connected to
interface F0/1 of Cisco router 2621.  The IP address of the F0/1 on Cisco 2621 is 192.168.2.254/24.
Interface F0/1 of the Cisco 2621 router also has secondary IP address of 10.0.2.254/24. 
Everything on the Catalyst is setup as default in VLAN1.  IP address of VLAN 1 in SW2 is 192.168.2.1/24. 
There are other machines connected to other ports of the SW2.

Machines connected to SW1 can communicate with machines connected to SW2 without any issues because
the Cisco 2621 router functions as a routing device.

Now the customer wants to replace the Cisco 2621 with a Catalyst 3750 running IOS version c3750-ipservicesk9-mz.122-53.SE2.bin
strictly as a layer-3 only without making any changes on the other two Catalyst 3560's. In other words, they want the
Catalyst 3750 to function just like the Cisco 2621.  By the way, there is a very simple OSPF configuration on the Cisco 2621
router


Here is what I propose to do on the Catalyst 3750:

- disable VLAN 1
- enable port f0/1 on the Catalyst as routed port (no switchport),
- asssign IP address of 192.168.1.254/24 and secondary IP address of 10.0.1.254/24 to port f0/1 of the Catalyst 3750,
- enable port f0/2 on the Catalyst as routed port (no switchport),
- asssign IP address of 192.168.2.254/24 and secondary IP address of 10.0.2.254/24 to port f0/2 of the Catalyst 3750,
- enable routting (ip routing and ip subnet-zero) on the Catalyst 3750,
- enable OSPF on the Catalyst 3750:

interface f0/1
  no switchport
  ip address 192.168.1.254 255.255.255.0
  ip address 10.0.1.254 255.255.255.0 secondary
  no shut

interface f0/2
  no switchport
  ip address 192.168.2.254 255.255.255.0
  ip address 10.0.2.254 255.255.255.0 secondary
  no shut

  router ospf 100
   no auto-summary
   network 192.168.1.0 0.0.0.255 area 0
   network 192.168.2.0 0.0.0.255 area 0
   network 10.0.1.0 0.0.0.255 area 0
   network 10.0.2.0 0.0.0.255 area 0

After this, machines connected to SW1 should be able to communicate with machines connected to SW2, correct?

One more thing, how does one go about completely disabling layer-2 to a Cisco Catalyst 3750 and only use it for Layer-3 function?

Many thanks in advance.

2 REPLIES
Cisco Employee

Re: Replace Cisco Router 2621 with Catalyst 3750 layer-3

Hello,

Your approach is correct. As long as you configure those two ports as L3

ports, things should work fine. As far as your second question regarding

disabling L2 is concerned, if you do not enable any vlan (do not create any

other L3 virtual interfaces, nobody will be able to communicate through that

switch. If you further want, you can configure "no Switchport" on all

interfaces forcing all interfaces to be L3.

Hope this helps.

Regards,

NT

Hall of Fame Super Gold

Re: Replace Cisco Router 2621 with Catalyst 3750 layer-3

You do not need the 3750.

Just enable IP routing in the 3560, as they are good layer 3 switch like the 3750.

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