DHCP - Server Port Based Address Allocation (WS-C2960X-24TS-LL)

[jeffjeffwong]

I'm trying to build a LAN with port-based fixed IP, handled by the switch's built-in DHCP server. It appears that WS-C2960X-24TS-LL would support this feature, but I have the following questions:

1) Do I need a router on top of the WS-C2960X-24TS-LL to achieve this, if it is a closed LAN without internet connectivity?

2) If I need L3 routing so that the LAN can connect to the internet, can I do it by adding a consumer-grade router to the WS-C2960X-24TS-LL? Or does it need to be some cisco router? 

[Seb Rupik]

Hi there,

1) for a single VLAN, you do not need a router. In fact the 2960X does support IP routing so it would be possible to configure multiple VLANs/ subnets on the switch and be able to route between them.

2) A ISP issued routers would be sufficient. It would probably not understand VLAN tags, so at the very least you would need to connect to it via an access port.

If you had previously opted for more than one VLAN/ subnet keep in mind that the 2960X does no perform NAT, this needs to be carried out on the router. There is a good chance that the router would not be configurable for multiple 'inside' subnets.

Also, if you have implemented multiple VLANs/ subnets on the 2960X you would need to configure static routes on the router instructing it how to reach those subnets on the switch.

In summary, if you have a single VLAN on the 2960X a normal ISP router would be fine. If you configure multiple VLANs, then your router would need to support VLAN tags OR  configurable NAT and static routing.

cheers,

Seb.

[jeffjeffwong]

Thanks for the info Seb. However please note the following:

1) I'd assume the "DHCP - Server Port Based Address Allocation" feature is something other than VLAN? I'm referring the the following feature:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960x/software/15-2_7_e/b_1527e_consolidated_2960x_cg/configuring_dhcp.html?bookSearch=true#d47754e4750a1635

2) I've been told that the WS-C2960X-24TS-LL (Lan Lite) do not support DHCP Server. However, from the Cisco Feature Search, it appears that both the Lan Base and Lan Lite version of the ios do support the "DHCP - Server Port Based Address Allocation" feature - see attached image. Which is true?

[Seb Rupik]

Hi there,

Are you sure it is the 'DHCP - Server Port Based Address Allocation' feature and not just 'DHCP Server' that you need on your network?

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/15-2_4_e/configurationguide/b_1524e_consolidated_2960p_2960c_cg/b_1524e_consolidated_2960p_2960c_cg_chapter_011001.html#task_54B910FB761949559A0CAEE03A4829D0

cheers,

Seb.

[jeffjeffwong]

Hi Seb,

I have multiple Ethernet devices (clients) connected to the switch. These devices are to be on the same LAN, and I would like to have the switch to act as a DHCP server, and hand out specific IPs to each of the devices according to the physical ports - therefore the "DHCP - Server Port Based Address Allocation"

1) I would like to know whether it is possible for the WS-C2960X-24TS-LL to do this (without any additional router / DHCP server), because I've been told Lan Lite does not support DHCP server feature. However, according to the Cisco Feature Search, C2960X with Lan Base software do support "DHCP - Server Port Based Address Allocation". 

2) In addition, I would like to know whether a typical router can be connected to the WS-C2960X-24TS-LL to give these devices (clients) internet connections. 

[Seb Rupik]

1) Certainly the Lan-lite version does support the 'DHCP - Server Port Based Address Allocation' feature and it is configured outside of the DHCP server feature. You will still need to provide a DHCP server somewhere on your network.

2) Yes, a router can be connected to the switch to provide an internet connection. My previous comments about 802.1q, NAT and static routing still stand.

cheers,

Seb.

[jeffjeffwong]

Hello Seb,

Thanks for the quick response. Now to make things clear, would an very basic ISP router (with the most basic DHCP feature), plus a  WS-C2960X-24TS-LL, achieve what I'm looking for, as follows?

1) assign port-specific IP to the devices connected to the WS-C2960X-24TS-LL (all on the same LAN);

2) provide internet connection to the LAN via the ISP router.

I haven't bought the WS-C2960X-24TS-LL switch yet and I don't want to get the wrong one. Currently deciding between the Lan Lite and Lan Base version. There's a big difference in price. 

Here is another question:

3) If get the WS-C2960X-24TS-L (Lan Base version), will I be able to create the LAN with the port-based address allocation feature, without any additional DHCP server / router? (with the WS-C2960X-24TS-L acting as a DHCP server)

Thanks. 

[Seb Rupik]

A basic ISP modem would be fine. The DHCP server running onboard would be unaware of the switch changing the client-id/ MAC address of the incoming DHCP request.

I would always chose a LAN Base model over a Lite, simply because of the bizarre restrictions imposed on the Lite model.

If you just need a DHCP server, why not connect an Raspberry Pi with ISC-dhcp-server running? ...but then you talk about wanting to use an internet connection, in which case you may as well use the modem provided by the ISP.

cheers,

Seb.

[jeffjeffwong]

There is something I still don't understand: If there is a DHCP server outside of the switch, how do the switch request for some absolute IPs from the dhcp server (isp router in our case) for the port-based address allocation? How does the switch ensure all the port-based IPs would not be assigned by the router to other devices connected to it?

I need a DHCP server that is capable to assign IPs based on the physical port. So after a device swap on a specific physical port, the replacement device (which will certainly have a different MAC address) would still get the same IP as the replaced device. 

[Seb Rupik · Accepted solution]

Forget what I said, just seen this:

"The DHCP server port-based address allocation feature is only supported on a Cisco IOS DHCP server and not a third-party server."

With this caveat you will need the LAN Base model.

[jeffjeffwong]

By the way Seb, I noticed the link that you provided is for the C2960 switches, not the 2960X, if it matters.
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960x/software/15-0_2_EX/security/configuration_guide/b_consolidated_152ex_2960-X_cg/b_consolidated_152ex_2960-X_cg_chapter_0101010.html?bookSearch=true#d122940e2046a1635