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PortFast impact on DHCP/PXE clients

Posted on 2012-03-28
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Last Modified: 2012-04-05
I didn't where This question should go, it is a mixture of Portfast, PXE,Imaging software and DHCP:

First I need to know if the clients that are configured for PXE boot, whether they should have a static IP or Dynamic IP?
For instance if an image software is somewhere in  the network and we need to make clients boot in PXE mode, do we need first to prestage those clients in the Imaging software.
Regarding Imaging software and PXE, if a client doesn't have any OS at all, just bare metal, would this be able to boot in PXE and connect to the imaging software?

Knowing that most of the environment use DHCP to handout IPs at least for workstations.
Does that mean all access ports on the switch need to be configured with Portfast, if so why Cisco didn't enable portfast automatically right after you configure a port as an access port?

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Question by:jskfan
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by:Nayyar HH (CCIE RS)
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Re Portfast

Yes access ports connected to PC's should be configured with portfast this will enable them to transition to "forwarding" mode immediately on link up

Not all access ports can be enabled with portfast as this might cause a loop in the network - an example is an access port connected to another switch, this port should not be set to portfast mode.
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PXE relies on DHCP. So you need DHCP IP addresses with PXE clients
Now, these can be purely dynamic or static addresses (DHCP reservation) this has no impact on the client side of things.
Regarding your imaging software, it depends on the software itself. But PXE is MADE to boot computers without needing a local disk drive. So, usually, they can be PXE-booted even with no OS on the local HDD

Portfast: This is to work-around an issue with SpanningTree Protocol (STP). Standard spanning tree can take up to one minute to accept to transfer the packets that PXE sends. ANd most PXE implementations time-out after 30 seconds. Portfast is a way to reduce the time to a couple of seconds. It is REQUIRED not to use standard spanning tree when you have PXE client.
Why did not cisco enabled it by default? This is a question that can be aksed to Cisco, but knowing its answer won't solve your problems if you do have STP and you try to PXE-Boot...
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