DHCP Vlan

[orionpex]

Hi all,


I have an issue, and need some assistance with solving it. I am attempting to use a Vlan as for public DHCP via my ISP.

I have multiple IP's from ISP and would like use that VLAN as sort of a dump switch per say. For better understanding here is my setup.


I created Vlan6 on my 3550.
I added fastE ports 0/44 through 0/48 to that Vlan.
I then plugged port fastE 0/48 to the my cable modem.
I then plugged four devices that I wish to pull public DHCP IP's to fastE's 0/44 through 0/47.


I configured vlan6 as "ip address DHCP"


Now I'm not sure if there are any other configs that I would need to add.

Please assist.

 

[imbadatthis]

do this :

int vlan 6
shut down

do not assume that its shut down until the switch tells you it is

then do a 'no shut' that should generate a proper dhcp request for your vlan interface.

C3550(config-if)#
*Apr 16 04:30:23.348: %LINK-5-CHANGED: Interface Vlan23, changed state to administratively down
*Apr 16 04:30:24.348: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan23, changed state to down
C3550(config-if)#no shut
C3550(config-if)#
*Apr 16 04:30:29.512: %LINK-3-UPDOWN: Interface Vlan23, changed state to up
*Apr 16 04:30:30.512: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan23, changed state to up
C3550(config-if)#
*Apr 16 04:30:30.588: %DHCP-6-ADDRESS_ASSIGN: Interface Vlan23 assigned DHCP address 10.254.23.3, mask 255.255.255.0, hostname c3550

Current configuration : 56 bytes
!
interface Vlan23
 ip address dhcp hostname c3550
end
Interface              IP-Address      OK? Method Status                Protocol
Vlan1                  unassigned      YES NVRAM  up                    up
Vlan6                  unassigned      YES unset  down                  down
Vlan23                 10.254.23.3     YES DHCP   up                    up

We must go always forward, not backward
always up, not down and always twirling twirling towards infinity.

 

[imbadatthis]

orionpex, i know it seems like a minor thing... buuuuut you keep adding the IP dhcp client command BEFORE YOU bring the vlan up.

do this :

int vlan 6
shut down

do not assume that its shut down until the switch tells you it is

then do a 'no shut' that should generate a proper dhcp request for your vlan interface.

C3550(config-if)#
*Apr 16 04:30:23.348: %LINK-5-CHANGED: Interface Vlan23, changed state to administratively down
*Apr 16 04:30:24.348: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan23, changed state to down
C3550(config-if)#no shut
C3550(config-if)#
*Apr 16 04:30:29.512: %LINK-3-UPDOWN: Interface Vlan23, changed state to up
*Apr 16 04:30:30.512: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan23, changed state to up
C3550(config-if)#
*Apr 16 04:30:30.588: %DHCP-6-ADDRESS_ASSIGN: Interface Vlan23 assigned DHCP address 10.254.23.3, mask 255.255.255.0, hostname c3550

Current configuration : 56 bytes
!
interface Vlan23
 ip address dhcp hostname c3550
end
Interface              IP-Address      OK? Method Status                Protocol
Vlan1                  unassigned      YES NVRAM  up                    up
Vlan6                  unassigned      YES unset  down                  down
Vlan23                 10.254.23.3     YES DHCP   up                    up

We must go always forward, not backward
always up, not down and always twirling twirling towards infinity.

 

[orionpex]

I tried the shut and no shut, however I still get the same result.
I also rebuilt it all with the vlan being activated first prior to adding the the client command to the vlan. I am thinking that there is a another issue with the DHCP on the device.

######################

here is an output i got 15 min after shutting down the vlan

3550.sw(config)#
3550.sw(config)#                                             
1d04h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0xB602B0E1
1d04h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0xB602B0E1                 
1d05h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x77D493DC                       
1d07h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x3C891ADC
$                                                                                                                
1d19h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x55DA5A1E
1d19h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x55D         
1d21h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0xA8DF4640
1d21h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0xA8DF4640
1d22h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x4434CFF2
1d23h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x50DFB5E
1d23h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x811CE91A
2d00h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0xD590C68B
3550.sw(config)#
3550.sw(config)#
3550.sw(config)#
2d03h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x7B920838
2d03h: DHCP: Received a BOOTREP pkt Not for us..:  xid: 0x7B920838 
3550.sw(config)#
3550.sw(config)# 
3550.sw(config)#

Below here is the output right after I no shut the vlan.

3550.sw(config)#     
2d03h: DHCP: DHCP client process started: 10
2d03h: RAC: Starting DHCP discover on Vlan6
2d03h: DHCP: Try 1 to acquire address for Vlan6
2d03h: DHCP: allocate request
2d03h: DHCP: new entry. add to queue
2d03h: DHCP: SDiscover attempt # 1 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
2d03h: %LINK-3-UPDOWN: Interface Vlan6, changed state to up
2d03h: DHCP: SDiscover attempt # 2 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
2d03h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan6, changed state to up
2d03h: DHCP: SDiscover attempt # 3 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0%Unknown DHCP problem.. No allocation possible
2d03h: DHCP: Waiting for 5 seconds on interface Vlan6
2d03h: DHCP: Try 2 to acquire address for Vlan6
2d03h: DHCP: allocate request
2d03h: DHCP: zapping entry in DHC_PURGING state for Vl6
2d03h: DHCP: new entry. add to queue
2d03h: DHCP: SDiscover attempt # 1 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
3550.sw(config)#
2d03h: DHCP: SDiscover attempt # 2 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
3550.sw(config)#
3550.sw(config)#
2d03h: DHCP: SDiscover attempt # 3 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
3550.sw(config)#
3550.sw(config)#
3550.sw(config)#
3550.sw(config)#
3550.sw(config)#
3550.sw(config)#%Unknown DHCP problem.. No allocation possible
2d03h: DHCP: Waiting for 10 seconds on interface Vlan6
2d03h: DHCP: deleting entry 25599CC 0.0.0.0 from list
2d03h: DHCP: Try 3 to acquire address for Vlan6
2d03h: DHCP: allocate request
2d03h: DHCP: zapping entry in DHC_PURGING state for Vl6
2d03h: DHCP: new entry. add to queue
2d03h: DHCP: SDiscover attempt # 1 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
2d03h: DHCP: SDiscover attempt # 2 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
2d03h: DHCP: SDiscover attempt # 3 for entry:
2d03h: DHCP: SDiscover: sending 279 byte length DHCP packet
2d03h: DHCP: SDiscover 279 bytes 
2d03h:             B'cast on Vlan6 interface from 0.0.0.0
2d03h: DHCP: deleting entry 2E9C574 0.0.0.0 from list%Unknown DHCP problem.. No allocation possible
2d03h: DHCP: Waiting for 15 seconds on interface Vlan6

 

[Quadratic]

Is your service provider doing static DHCP for your public address block? Even off of a modem, more often than not they'll ask you to hard-code the static IPs on your end. Your modem would still be able to pass DHCP discovers/requests to the upstream CMTS, and even lease IPs, but it wouldn't be the IPs you need. Make 100% certain if your ISP is routing a block to you (most likely) or doing static DHCP.


CCNP, CCDP, CCIP

 

[imbadatthis]

Quadratic, he says he can get IP address through DHCP if he doesn't put both interfaces on vlan 6 which is the confusing part for me.


We must go always forward, not backward
always up, not down and always twirling twirling towards infinity.

 

[orionpex]

Correction guys, what I stated is that I can not pull an IP address on the vlan interface as imbadatthis did on his output. It does not work whether I use 1 or all the ports.

fast0/48 <<<- I plugged either into the wireless or the cable modem for testing
fast0/46 <<<- Connection from my firewall
fast0/47 <<<- Connection from my Server
fast0/45 <<- Connection from my TermServer

all of them are assigned to vlan6.
current issue I can not pull an IP address to the vlan interface no matter the setup.


Additionally, I can pull an IP through the Vlan via fastE 0/46 onto my firewalls WAN port. However this only occurs when I plug the 3550 into my wireless and use it.So when I swap the the connection from my wireless directly to the cable modem, I get nothing.

Nevertheless, no matter which format I use, that being wireless or cable modem; I still receive the errors show in the debug output.

 

[Quadratic]

Orionpex: Can you confirm 100% what method your service provider expects you to use to connect to their network? Can you confirm if they intend for you to use DHCP, or to hard-code that address yourself? Do whatever method they say is intended, and attempt a laptop test or similar to confirm if you can get any level of connectivity to the intended default gateway, even with the simplest design.


imbadatthis: It's possible for a "static IP customer" to connect to the cable modem and use DHCP to lease a different, dynamic IP.

From the provider's perspective, *usually* you'd set up a static IP customer by assigning the static's default gateway as a secondary IP on the CMTS's cable interface, allowing the customer (or technically anyone connecting on that Cable interface) to use it. Since the modem is effectively just acting as an L2 bridge for the customer equipment, and multiple customers are connecting on a shared broadband segment, technically there's no hard association between a customer's cable modem and their static IP, nor will a modem reject a DHCP attempt to lease a dynamic IP. There are other ways to do it, including static DHCP based on the customer MAC address, but that is more overhead and more rare for a provider to impliment, thus the secondary IP method seems to be the most popular.

Bottom line is that the perceived association between a modem and the static IP assigned to a business *usually* doesn't exist, and is a bit of a misconception.


CCNP, CCDP, CCIP

 

[orionpex]

I bypassed the 3550 and just put a repeater in between the cable modem and equipment which allowed me to pull the addresses on to my equipment. Ultimately the issue seems to be with my 3550, either configuration issue or possible code issue, nevertheless something I need to resolve

 

[VinceWhirlwind]

I'd be putting "portfast" on the switchports, myself.

Another thing is - are you getting a link light on fa0/48 when you connect your switch to the cable modem?