Turning off Vlans

[Djarum]

I have a bunch of 2950t's. We recently changed over to an all linux live cd's for our work OS. If i plug into a switch port I cant get a dhcp ip..if i take any hub I have laying around and plug its uplink into the same cisco switch port I get a dhcp ip fine...I think its the vlaning on the switches that my linux pcs cant read...everthing is in the same vlan right now...vlan1..I need to remove all vlan tagging on the switches to test out my theory...Can someone help...I cant find a command to remove vlan tagging from the ciscoswitches...ios 12.1 I think, but don't hold me to it


Thanks in advance

 

[ADB100]

I doubt this is a VLAN tagging issue. By default the ports will be set to automatically negotiate trunking - this will only happen between Cisco switches as it relies on DTP (Dynamica Trunking Protocol). If you plug a host in the port will just connect as an access port without trunking. You can disable trunking by setting the port as an access only port:

interface FastEthernet0/1
switchport mode access

What I think is your problem is Spanning Tree. By default a 2950 runs PVST+ and each port will only start forwarding after about 45-seconds of going 'up'. You can shorten this time to be almost instant by enabling 'Spanning-Tree PortFast'. This can be done globally and it will affect all ports (so be careful) or on each interface.

!globally
spanning-tree portfast default

!interface
interface FastEthernet0/1
spanning-tree portfast

HTH

Andy

 

[vipergg]

You can also use the "switchport host" command on all your ports in addition to setting spanning tree portfast on , it turns off channeling and trunking on the ports and allows the port to come even a little faster than with portfast .

 

[Djarum]

Thanks guys,
I tried turning off spanning tree and it doesn't make a difference...

I think the problem is still nics in the linux boxes choke when they get the dhcp reply throuh the cicso switches that have some type of vlan tagging....

When i plug the uplink of a hub or another brand switch into the same port that the linux box was plugged and the linux box into that hub it works.I think that when the dhcp request comes back accross the hub it strips the vlan tag info off and presents the ip in "standard" format to the linux box.

I don't think the linux box is "smart" enough to parse the ip out of the dhcp info, but the hub does it for them...I have been using the workaround of having 3com hubs uplinked into my 2950t's but am running out of space on the hubs and have a ton of empty ports on my cisco's and need to get them to work on the 2950's



TIA
Chet

 

[Jsteve]

Maybe you can post you switch configs

 

[ADB100]

I am pretty certain your switch won't be sending Tagged traffic out of access ports. You would have to hard-code the ports as Trunks if this was the case. Normal config for an access port would look like:

interface FastEthernet0/1
switchport access vlan 10
switchport mode access
spanning-tree portfast

Post your switch config and maybe we can help more?

Andy

 

[Djarum]

I hope this is the config you are looking for....I'm a cisco neophyte but it doesn't look to descriptive to me....
Thanks for all your help,
If you need a different config, let me know FA0/10 is the switch port that I enabled portfast on...
Thanks again


Using 1691 out of 32768 bytes
!
version 12.1
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Switch
!
enable secret 5
enable password
!
ip subnet-zero
!
spanning-tree mode pvst
no spanning-tree optimize bpdu transmission
spanning-tree extend system-id
!
!
interface FastEthernet0/1
no ip address
!
interface FastEthernet0/2
no ip address
!
interface FastEthernet0/3
no ip address
!
interface FastEthernet0/4
no ip address
!
interface FastEthernet0/5
no ip address
!
interface FastEthernet0/6
no ip address
!
interface FastEthernet0/7
no ip address
!
interface FastEthernet0/8
no ip address
!
interface FastEthernet0/9
no ip address
!
interface FastEthernet0/10
no ip address
!
interface FastEthernet0/11
no ip address
!
interface FastEthernet0/12
no ip address
!
interface FastEthernet0/13
no ip address
!
interface FastEthernet0/14
no ip address
!
interface FastEthernet0/15
no ip address
!
interface FastEthernet0/16
no ip address
!
interface FastEthernet0/17
no ip address
!
interface FastEthernet0/18
no ip address
!
interface FastEthernet0/19
no ip address
!
interface FastEthernet0/20
no ip address
!
interface FastEthernet0/21
no ip address
!
interface FastEthernet0/22
no ip address
!
interface FastEthernet0/23
no ip address
!
interface FastEthernet0/24
no ip address
!
interface GigabitEthernet0/1
no ip address
!
interface GigabitEthernet0/2
no ip address
!
interface Vlan1
ip address 192.168.4.3 255.255.255.0
no ip route-cache
!
ip http server
!
!
line con 0
exec-timeout 0 0
line vty 0 4
password
login
line vty 5 15
password

 

[InDenial]

Looks to me like a negotiation issue... you are getting a false speed/duplex setting on the interfaces.

Check this by doing a show int FastEthernet0/x.

Check the settings on the nic on the linux box and compare it to what the interface negotiated. If the settings on the interface are not what you expect them to be. Set them hard.

The config you gave does not contain the configs you made to switchport FastEthernet0/10

InDenial

 

[Jsteve]

What interface is you dhcp server and lunix box are on? Can you post a "show interface f0/x" for both?

 

[Djarum]

sorry my config didn't show the running config for the port


this is one of the linux ports config
interface FastEthernet0/10
switchport mode access
no ip address
spanning-tree portfast
end

the dhcp "server" is actually a pix 506 handing off ip's and I dont knows which one it is (i'm at home now) but pretty sure it's

interface FastEthernet0/22
no ip address


Thanks again...

 

[vlf]

I'd put spanning-tree port fast on all of your end user connections. Sometimes, without it, the boot process just takes too long and processes like DHCP will timeout.

 

[BuckWeet]

If the DHCP request is coming in AFTER the ports go into forwarding state then it sounds to me like there is a buggy module in the kernel you're using..

what network cards are in the boxes you have..

Keep this in mind, if you think its a tagging issue, then the hub will either drop the frame.. or if it does goes through the hub, your PC will still get the tag'ed frame..

So this is an actual link negotiation issue, not a vlan tagging issue.

 

[vipergg]

You must use spanning tree portfast on all user ports that you want dhcp to work on , do not see that it is applied to any ports on the config you posted . Packets are only tagged if the link is a trunk and is carrying multiple vlans across 1 link . The tag gets stripped once it hits the end of the trunk . If your dhcp server is not on the same subnet then whatever is doing your routing must use helper addresses on the subnet you are trying to pull the addresses to in your case the 192.168.4.X subnet . To me it sounds like the spanning tree portfast issue seeing a hub works , change all your ports to use portfast execpt any links that may feed other switches or routers.