Cisco 3750 Series Switch

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Apr 15th, 2009
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I read in the documentation that the Cisco 3750 series supports DHCP auto-IP assignment without any initial configuration. Does this mean that if I hook into a port on the switch using CAT-5 (i.e. DHCP server hooked to port 1) and turn on the switch out of the box (with no configuration) that it will automatically send DHCPDISCOVER requests to obtain its IP, or do I still need to hook into the switch via the console and configure an IP first? If this is the case (it pings the DHCP server), you should be able to communicate with the switch via Ethernet to configure it after it has automatically obtained its IP from the DHCP server, without ever having to use the console cable, correct?

Also, when hooking the DHCP server up to port 1, will you need to use a patch cable or crossover cable to have the switch auto-discover it's IP via DHCP (if this is even possible)?

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Giuseppe Larosa Wed, 04/15/2009 - 05:23
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Hello Justin,

use the following reference

you can connect a straight cable to another switch or router connected to your network infrastructure.

A router can relay DHCP messages using

int fas x/y

ip helper-address dhcp-

on the L3 interface facing the new C3750

Hope to help


j_karimi1 Wed, 04/15/2009 - 05:32
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Hi Giuseppe,

So does this mean that you cannot use a DHCP server on a local PC to configure the switch IP address?

Also, in order to use another router or switch as a DHCP server, you would still need to configure it, correct?

In any event, it seems as though there is no getting around using the console cable at SOME point unless you already had an existing configured router/switch, correct?

Giuseppe Larosa Wed, 04/15/2009 - 05:41
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Hello Justin,

if you are in a lab setup it is faster to use the console cable connected to the serial port of your PC.

After that you can assign ip addresses to Vlan interfaces.

Of course you can also run a dhcpd service on your PC but this doesn't make the difference.

table 3-1 of the document says that default configuration is no ip address configured but the switch is ready to look for an ip addres using DHCP.

So having a DHCP server can be a way to skip the connecting to the console step but in a lab environment it makes little difference.

Hope to help


hobbe Wed, 04/15/2009 - 06:11
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If I am not mistaken.

the switch will lock into a dhcp scope and take an address from it if there is one available, that much is certain.

From there I think that it will display the HTTP server and you can login via the http server. from there you could either change the addresses to a static or write a script that does it for you.

enable telnet and so on.

personally I would use the blue cable any day over the graphical.

when i upgrade i do not even download it, just the binary.



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