Network booting

[Csaba Vasarhelyi]

Dear Community,

First of all, I'd like to apologize if this is a dumb topic but I am very curious about this.

I wanted to redeploy clients through network. When the clients wanted to boot from network, they got a "PXE-E51: No DHCP or proxyDHCP offers were received" error. I immediately thought that there's something wrong with the switch configuration (our department moved from our previous location where it worked perfectly). So I checked the switch config.

It was like this:

interface GigabitEthernet0/21
 switchport access vlan 500
 

"Shame on me" - I thought. So I just added a line

interface GigabitEthernet0/21
 switchport access vlan 500
 switchport mode access
 

Still no success. So what should I do? I just opened a search engine and started to search for my problem! There was an ip-helper topic which discussed this, tried to set it up but no, no success again.

So I tried with this one:

interface GigabitEthernet0/21
 switchport access vlan 500
 switchport mode access
 spanning-tree portfast

Yeah, it solved the problem, but I am not exactly sure why it forwards DHCP offers to clients with this settings? Could somebody explain it to me? I tried to find the answer in my CCNA notes but could not find any related information.

[Jon Marshall · Accepted solution]

When you enable portfast on a switch port that port can begin forwarding traffic as soon as the client is up and running.

Without portfast the port has to go through the STP listening and learning stages and while it is doing this no traffic can be forwarded.

What is probably happening is that without portfast enabled the client tries to send it's request but the port is not forwarding. By the time the port has started forwarding the client has in effect given up because it's previous requests didn't get a response.

Jon

[Csaba Vasarhelyi]

Ouch this hurts right now. Seems I have to study more this SPT thing. :) Thank you very much!