accessing a switch without IP address

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Feb 18th, 2008
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Im doing some discovery on my network and came across this..


the device I am telnetted into is a 3845

When I do a sho cdp neig...i get no IP information..Shouldn't I at least get

the mgt IP of vlan 1?


3845#sh cdp neighbors det

-------------------------

Device ID: CAT.SWITCh

Entry address(es):

Platform: cisco WS-C4006, Capabilities: Router Switch IGMP

Interface: GigabitEthernet0/0, Port ID (outgoing port): GigabitEthernet2/22

Holdtime : 168 sec


Version :

Cisco IOS Software, Catalyst 4000 L3 Switch Software (cat4000-I9S-M), Version 12

.2(25)EWA4, RELEASE SOFTWARE (fc1)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2005 by Cisco Systems, Inc.

Compiled Fri 23-Sep-05 13:31 by ssearch


advertisement version: 2

VTP Management Domain: ''''

Native VLAN: 119

Duplex: full


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Jon Marshall Mon, 02/18/2008 - 12:58
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Hi


You will only get an IP address if it has been configured. Are you sure it has been configured ?


Jon

nygenxny123 Mon, 02/18/2008 - 13:06
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that I can't verify...but what use would a switch be without an IP configuration?

Jon Marshall Mon, 02/18/2008 - 13:09
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Well it depends on whether it is a layer 2 or layer 3 switch :)


If it is layer 3 and it is meant to be doing inter-vlan routing i agree without IP config of some sort it's not a lot of use.


But if it is a layer 2 switch or a layer 3 switch acting as a layer 2 switch then the IP configuration is only needed for managing the switch itself. Without an IP config you can still have multiple L2 vlans and clients/servers/printers etc. connected into the ports and it will do it's job.


Jon

nygenxny123 Tue, 02/19/2008 - 06:04
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If it is acting as a layer 2 switch, wouldnt

the vlan interfaces have an IP address and thus be advertised via cdp ?

Hi There


No. A switch working at layer 2 would not require VLAN interfaces and thus would not have IP addresses assigned to these.


A switch working at layer 2 would have VLAN's configured and the relevant ports assigned to the desired VLAN.


Routing would happen by the switch being uplinked to a router via a trunk link, thus traffic for all VLAN's would traverse the trunk to the router. The router would then pass the traffic back down the trunk destined for the required VLAN, where the switch would switch it to the correct port on the VLAN or all ports on the VLAN, depending on whether there was an entry in the CAM table for the MAC of the destination host.


However a switch with no IP addresses configured would require local administration (via the console). For remote adminstration an IP addresses would be assigned to a VLAN interface and this would then be advertised via CDP.


HTH


Best Regards,


Michael

nygenxny123 Tue, 02/19/2008 - 13:35
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thx..


so if I configure only 1 vlan


Say vlan 12

and assign it an IP address of 192.168.2.2

is this what will be advertised via cdp to

the switch on the other end of my trunk?


so in theory I would have vlan 12 and vlan1

but not ip for vlan 1

Jon Marshall Tue, 02/19/2008 - 13:42
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Hi


Yes, 192.168.2.2 will be advertised and your switch IP would be on vlan 12 interface.


vlan 1 would still exist but it just wouldn't have a L3 interface with an IP address, assuming this is a layer 2 switch.


HTH


Jon

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