[c-nsp] Lot of input errors on a NPE-G1 interface

Matlock, Kenneth L MatlockK at exempla.org
Wed May 23 16:04:58 EDT 2012


Think of it this way. What does Portfast do? It allows you to skip from
blocking to forwarding, bypassing listening and learning states.

If you have portfast enabled to a L2 device, and that L2 device also has
another connection to the same L2 you've created a bridging loop until
the first BPDU comes in and the switch disables one of the ports
involved. 

Now, configuring portfast on a port going to and end host or router is
fairly benign since there's no other link to the same L2 bridged through
the device. You'd have to plug 2 ports from the end device (or router)
into the same L2, and explicitly configure them to be bridged in order
to create a bridging loop.  But normally you're only going to plug a
single cable from an end device into the L2 so you know bridging loops
won't get created, so it's 'safe' to skip listening/learning steps and
go right to forwarding, to decrease the time it takes for the device to
be 'live'. 

Make sense?

Ken Matlock
Network Analyst
303-467-4671
matlockk at exempla.org




-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Matlock, Kenneth
L
Sent: Wednesday, May 23, 2012 1:23 PM
To: Chris Gotstein; Gert Doering
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Lot of input errors on a NPE-G1 interface

" hubs, concentrators,  switches, bridges, etc... to this interface when
portfast is enabled, can cause temporary bridging loops."

Those are all Layer 2 devices. A router is a layer 3 device (unless you
explicitly turn bridging on).

Ken Matlock
Network Analyst
303-467-4671
matlockk at exempla.org




-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Chris Gotstein
Sent: Wednesday, May 23, 2012 1:19 PM
To: Gert Doering
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Lot of input errors on a NPE-G1 interface

%Warning: portfast should only be enabled on ports connected to a single
host. Connecting hubs, concentrators,  switches, bridges, etc... to this
interface  when portfast is enabled, can cause temporary bridging loops.

My understanding of this was a router would be included as well since
it's used to connect multiple hosts.  I only enable portfast on ports
connected to end-user devices.  I don't see a good reason to enable it
on ports connected to routers, but that's just how it was explained to
me, I very well could be wrong.

On 5/23/2012 1:29 PM, Gert Doering wrote:
> Hi,
>
> On Wed, May 23, 2012 at 01:15:16PM -0500, Chris Gotstein wrote:
>> It's probably not going to address the overrun issue, but from a best

>> practices stand point, it should not be enabled on interfaces that 
>> connected to other connected devices, ie a router or switch.
>
> Uh, so it should only be turned on for switchports that connect to...
> "no device"?
>
> Anyway: right for "to switch", dead wrong for "to router".  It should 
> be turned on for any connection that is known to not go to a switch or

> hub, and doubly so if rapid-pvstp is used (due to TCNs being sent on a

> link flap otherwise, possibly causing stalls elsewhere).
>
> As far as a switch is concerned, a "router" is the same thing as "a 
> host" - it doesn't forward layer2 things, so can't cause a routing
loop.
>
> gert
>

--
---- ---- ---- ----
Chris Gotstein, Network Engineer, U.P. Logon/Computer Connection U.P.
http://uplogon.com | +1 906 774 4847 | chris at uplogon.com
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