IPv4 Address Configuration on Cisco IOS Catalyst Switch

(Rene Molenaar) #1

This topic is to discuss the following lesson:

(vadim z) #2

Its been awhile and I just wanted to refresh memory on how initially configure the switch. Particularly L2 switch. Most are L3 these days but nevertheless. This was the only article related I was able to find and I feel a couple of things are missing, particularly those that I had questions about. Thats:
default gateway probably should be included, showing also difference for L3 config as is in the lesson, but also for L2 which is different because its, well, just l2 switch;

so on L2 switch how do we assign management IP address? Can we even create a SVI on pure L2 switch? What that ip then will be assigned to?

on L3 switch while using SVI for management access is well known its not always desirable or even the best. So what are other options (assuming there is no dedicated management port)? Assign IP to physical port? Use loopback?

What about this ‘autoconfiguration with DHCP’ option for initial switch configuration CISCO keeps mentioning but never really explains? I tried but it never worked - the only way I could use DHCP is by assigning to one physical interface ‘ip address dhcp’ option. But thats not the ‘autoconfiguration’ of the switch, just one interface. I hoped ‘autoconfiguration’ will show me how the management IP is configured but since it never worked…

Sorry with all these questions but I thought it is all related to what seems should be a simple thing the lesson is focusing on - initial configuraiton and management access. And just as I started to think about it I realized I cant definitively answer these questions.

(Lazaros Agapides) #3

Hello again Vadim, nice to see you here!

No need to apologize about asking questions, that’s what we’re here for, to answer them as best we can!

The only way you can access the switch using SSH or Telnet is via an IP address that has been assigned to an SVI. An L2 switch does indeed have SVIs and you can actually configure more than one. If you want to have access to them you either need to connect a PC to a port on the same VLAN as the SVI, or put the PC on another subnet and configure the default gateway for the switch so it can reach other networks.

If you have a pure L2 switch, you cannot assign IP addresses the ports nor can you create loopbacks. Other than console/aux access, the only way to obtain a CLI on the switch is via an SVI. If you want the redundancy that management interfaces or loopbacks provide, you will just have to buy a higher end L3 switch.

This feature is actually quite innovative. It is an option that allows you to obtain a configuration file using the DHCP protocol. DHCP is a protocol that provides much more information than just an IP address, subnet mask, gateway, and DNS server. It has many options that can be added to provide additional information to the requesting host. The “autoconfiguration with DHCP” feature takes advantage of DHCP in this way.

If it is enabled, the IOS device asks for an IP address using DHCP. The DHCP server will respond with the IP address, subnet mask, gateway and DNS server, but will also provide a url from which the device can download it’s configuration file. This will replace the running config and will allow the device to be automatically configured. THis is similar to the way that IP phones will obtain configuration information from the DHCP option 150 which provides the address of a TFTP server from where configs can be downloaded.

Now for this to work, the DHCP server must be configured with the appropriate option and a server must also be configured that will contain the appropriate configs.

I hope this has been helpful!

Laz