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Cisco 3750 spanning tree portfast

Posted on 2011-02-25
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Last Modified: 2012-05-11
Let’s say I am configuring my fast Ethernet ports on a 3750

Some ports have the following configuration (Lets call this config1):
      interface FastEthernet1/1
      switchport access vlan 100
      spanning-tree portfast

Other ports have the following configuration (Lets call this config2)
interface FastEthernet1/6
      switchport access vlan 100

The following are my spanning tree commands:

spanning-tree mode rapid-pvst
spanning-tree etherchannel guard misconfig
spanning-tree extend system-id
spanning-tree uplinkfast
spanning-tree backbonefast


Questions:
1.      Does this mean that I can only plug Ethernet connections into config1?
2.      What happens if I plug an Ethernet cable into a port with config2?
3.      Can I just disable a port completely
4.      What if I wanted to connect a switch into a fast Ethernet port instead of a fiber port? Would I have to make that port a trunk port? If so will all inter VLAN routing work.


The reason for the question is that I have read articles that have told me not to plug cables into particular ports that do not have spanning tree portfast for fear of flooding, broadcast storms etc..
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Question by:adimit
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Don Johnston earned 200 total points
ID: 34983501
1. The 3750 is an Ethernet switch. You can't connect anything but ethernet to it.
2. After 30 seconds, the port will move to forwarding state.
3. Yes. In interface config mode issue the command "shutdown".
4. You wouldn't have to. But you could. If the port needed to carry more than one VLAN, it would have to be a trunk.
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by:MrRude
MrRude earned 100 total points
ID: 34983541
I don't understand question 1. it is an ethernet port so yes only ethernet will fit
2. It will take a little longer to come up to forwarding state
3. yes - shutdown command
4. Trinking port with spanning tree portfast disabled
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Author Comment

by:adimit
ID: 34983723
the reason for ?1 and 2 is that I have been told that I should not plug cables into ports not configured for spanning tree (see my last 2 lines in the original question). Is that true?
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by:Don Johnston
Don Johnston earned 200 total points
ID: 34983740
Spanning-tree can't really be disabled. It's on by default and unless you turn on portfast and BPDU-filter, you're fine.
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by:mikebernhardt
mikebernhardt earned 100 total points
ID: 34984644
"spanning-tree portfast" doesn't turn on spanning tree, which is already on by default.

"spanning-tree portfast" bypasses the normal spanning tree listen-learn-forward mechanism so that hosts can come on line as soon as they are plugged into the port. It's good to use it on host ports. It should NEVER be used on trunk or uplink ports.

You can configure the fast ethernet port exactly the same you configure the fiber port with trunking and multiple vlans and it will work fine.
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by:jjmartineziii
jjmartineziii earned 100 total points
ID: 34984677
you should only enable portfast on access ports. this allows the port to "come up" faster than without it. Without it, a port can take up to 60 secs to go through the STP process
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Author Comment

by:adimit
ID: 35037620
Hello

I beleive the following summarizes what I was told. Let me know if I am wrong:

1)      If the port is not connected to any device –OR– its purpose in unknown –OR– the port is connected to another switch then “spanning-tree portfast” should NOT be set.
2)      If the port is connected to a server or a non-switch device, then it is safe to enable “spanning-tree portfast”
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Expert Comment

by:mikebernhardt
ID: 35038244
That's correct. Although if the port is not connected to any device then it really doesn't matter how the port is set. If I know that the purpose of the switch is primarily to support hosts/servers then I might turn it on on all user ports, connected or not.

There is a feature called "bpdu guard" which will shut down the port if it sees BPDUs entering a port that had portfast enabled. I think whether it's enabled by default depends on the model or version of software, but you can turn it on if it isn't already. That way you can turn on portfast everywhere (except uplinks) and if someone plugs in a little switch under their desk, the port will shut down until you turn off portfast.
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