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Old 05-23-2007, 03:59 AM   #1
Registered: Nov 2004
Posts: 63

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Question Pxe server issue ,why do i need to enable port-fast

I have a pxe based linux kick start install.

I have DHCP/tfpt/nfs server running on my linux box

Everything is working fine as long my client ports on the switch are enabled with spanning-tree port-fast. but as soon as i disable this option the PXE boot fails to get ip address from the dhcp.

Problem is my setup i am not suppose to use port-fast as my ports may be trunk ports.

Can someone tell me Why do i need to enable port-fast to make sure that pxe based linux install works fine , is there an alternative for that ???
Old 05-23-2007, 12:15 PM   #2
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Registered: Dec 2004
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When you enable the "spanning-tree port-fast" on your port it effectively cuts out about 30 seconds of delay due to the switches listening and learning states. Enabling spanning-tree port-fast sets the port to forward immediately and bypasses the listening and learning states. I imagine your dhcp request is timing out when "spanning-tree port-fast" is not enabled. I don't know how your switch is setup but we normally only used trunked ports as inter-connects between other switches and routers. I would request a non-trunked port and that should resolve your issue.
Old 05-23-2007, 01:07 PM   #3
Registered: Nov 2004
Posts: 63

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Question My all ports are in access ports

Here is my setup , may be you can helpme?

My DHCP is in Access mode ,(switch port mode access connected to switch)

I have my two clients which will request for the DHCP IP for pxe installation.

Both the clients connected currently connected to switch in access mode and port-fast is enabled on the switch. Currently this in access mode and if i remove/disable port-fast, clients are not able to obtain ip from the DHCP.

Current client setups,Assuming that my clients are in VLAN 200

switchport mode access
switchport access vlan 200
spanning-tree port-fast.

Current DHCP setup
switchport mode access
switchport access vlan 200

Just to be more clear Will it work if i move my clients ports on the switch to trunk ports?? for example if i change the configuration for the clients to the following on the switch.

switchport mode trunk
switch trunk allowed vlan 200

Do I need to apply the same settings to DHCP also??

Last edited by alix123; 05-23-2007 at 01:09 PM.
Old 05-23-2007, 04:28 PM   #4
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Registered: May 2007
Location: Brisbane, QLD, AU
Distribution: Gentoo, LFS, Slackware, (K)ubuntu
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If you were to change the ports your clients were connected to from port-fast to not port-fast, your clients probably wouldn't be able to get an IP address, since the port wouldn't come up fast enough. Port-fast skips a number of the tests that a port does when it comes up (detects a link), so that it's ready in say 3-5 seconds instead of 30-60 seconds. So when PXE sends out a request for an IP address, the switch port is still down because it's still running it's tests.

And I'm a bit shaky on Cisco (which I'm assuming you're using, since those look like Cisco commands; we use HP for switches and Cisco for routers), but it's my understanding that one of the biggest differences between an "access" port and a "trunk" port is that access ports are expecting network packets that are untagged from a VLAN point of view and therefore can only handle one VLAN per port, while trunk ports expect their packets to be tagged and therefore can handle multiple VLANs per port, but the device sending the packets to that port, e.g. your server, needs to be able to tag their own packets.

In summary: it's probably best to leave your PC (and server) ports with port-fast and access mode. Ports that make up links between switches should probably not be port-fast (because this would defeat the purpose of using spanning-tree), and should be trunk mode.
Old 04-03-2014, 07:30 AM   #5
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Registered: Mar 2014
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Exclamation client not sending DHCPREQUEST after receiving DHCPOFFER

I am facing similar problem, just want to knon if it is due to the same reason?

PS: using cisco switch N5k and uefi pxe server as rhel6.5

I have set the vlan mode to access for both UEFI pxe server and client side, I could ping from pxe server to gateway and legacy pxe boot completed through VIC card.

Then, I tried uefi pxe boot through the same network card(VIC) .

Pxe client broadcasts DHCPDISCOVER and server is reverting with DHCPOFFER, but this keeps on happening, client is not sending DHCPREQUEST further.

Is it because of the port-fast disabled?
but I am not seeing this issue while legacy pxe boot.

Last edited by geekmaxwell; 04-03-2014 at 08:11 AM.


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