[c-nsp] Switch Port Question

[Aaron Riemer]

Hi Lincoln,

>yes,
>swltch 2 would consider the interface to be operationally down (loss of
light).
>switch 1 is still receiving light. 

If the interface is considered to be operationally down why does it still
transmit light out its TX path?? This is what is confusing me.

Thanks,

Aaron.


-----Original Message-----
From: Lincoln Dale [mailto:ltd at cisco.com] 
Sent: Tuesday, 25 January 2011 10:52 AM
To: Aaron Riemer
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Switch Port Question


On 25/01/2011, at 12:48 PM, Aaron Riemer wrote:
> This is a really basic question and I should know the answer. I am just
> pondering over loop guard and UDLD and take this for example:

note that at a fundamental level, mechanisms in Spanning Tree are always
going to be different to those that work on physical ports.

for example, if you had a port-channel (LAG bundle) of 4 physical member
interfaces and ran STP over that, how many of those interfaces in the bundle
would be moving BPDUs?
answer: 1.

this should show quite clearly why mechanisms such as UDLD are important.

> Two switches are connected via a pair of fibre. Now let's say the TX path
> from switch1 (RX on switch2) fails or is disconnected but the other fibre
is
> still operational. I would have thought that logically Switch 2 should see
> that it is no longer receiving "light" on its RX path and bring the port
> down? 

yes,
swltch 2 would consider the interface to be operationally down (loss of
light).
switch 1 is still receiving light.

> 
> 
> 
> Obviously this is not the case.

it should be.


> Therefore my question is what determines if
> a port is physically "UP"? What physical checks are done by the switch
port
> and how does it determine if media is attached?

the PHY/transceiver tells the switch that it has link.


cheers,

lincoln.