questions about flp(fast link pulse) and autonegotiation

Answered Question
Feb 4th, 2010

hi all,

i have some questions about flp and autonegotiation.

as i know, flp is used at autonegotiation.

A(roter fa0/0) --------------------------- B(switch fa0/0)

above connection,

the connection will be good if A is auto and B is auto.

=> both sides will be 100Mbps/full duplex.

=> both sides use FLP for autonegotiation.

if A is auto and B is 100/full manually

=> A will be 100/half, B will be 100/full

=> why A will be half duplex?

=> if B's fa0/0 have set up with 100Mbps/full duplex manually, does it send flp?

     if B sends flp, A will be receive the flp and set up its fa0/0 with 100/full.

      above my assumtion is wrong?

=> Does A's fa0/0 ignore the flp from B(100/half) because A setup auto for fa0/0?

     if yes, does B send the flp with 100Mbps = 1, half duplex = 0, autonegotiation = 0 ?

 

another question

both A and B connete with 100Mbps/full duplex well.

after connecting, A or B sends a periodic flp?  

i read the link text

http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00800a7af0.shtml

but i can't the operation exactly.

I have this problem too.
0 votes
Correct Answer by Giuseppe Larosa about 6 years 11 months ago

Hello Sanghee,

as far as I know FLP are used to signal 100 Mbps speed only but it is not completely correct they carry some information inside.

from second link you have provided see the comments in table 1 for case NIC 10 Mbps half switch auto:

>> Link is established, but switch does not see Fast Link Pulse                     (FLP) and defaults to 10 Mbps half-duplex.

so the problem becomes how can duplex sensed?

some search in the internet leads to this good entry in Wikipedia:

http://en.wikipedia.org/wiki/Autonegotiation

the FLP are modulated to carry very small information about capabilities of the two devices connected on the link and priority of each operational mode.

see also

http://etherealmind.com/ethernet-autonegotiation-works-why-how-standard-should-be-set/

I can say that when I started to work in networking we tried to use manual configuration as much as possible as explained in the above links this was caused by the fact that first implementations were somewhat proprietary.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Fri, 02/05/2010 - 02:09

Hello Sanghee,

as far as I know FLP are used to signal 100 Mbps speed only but it is not completely correct they carry some information inside.

from second link you have provided see the comments in table 1 for case NIC 10 Mbps half switch auto:

>> Link is established, but switch does not see Fast Link Pulse                     (FLP) and defaults to 10 Mbps half-duplex.

so the problem becomes how can duplex sensed?

some search in the internet leads to this good entry in Wikipedia:

http://en.wikipedia.org/wiki/Autonegotiation

the FLP are modulated to carry very small information about capabilities of the two devices connected on the link and priority of each operational mode.

see also

http://etherealmind.com/ethernet-autonegotiation-works-why-how-standard-should-be-set/

I can say that when I started to work in networking we tried to use manual configuration as much as possible as explained in the above links this was caused by the fact that first implementations were somewhat proprietary.

Hope to help

Giuseppe

Sanghee Han Mon, 02/15/2010 - 19:54

Thanks Giuseppe.

it's a good links to understand about flp and autonegotiation.

I'm return to my office today after the lunar New Year's Day.(it's a festival day of Korea)

now...I'm searching something to study.

and thanks for your answer(Gam Sa Hap Ni Da = Thanks^^)

Alan_Chemist Tue, 07/13/2010 - 16:01

Actually if you set one to Auto and the other manually to 100Mbps Full duplex you disable the auto-negotiation on the one set to 100Mbps Full duplex.

So in this case B's interface doesn't try to negotiate anything or respond to A's request.

A's interface tries to auto-negotiate but fails.  A, however, can detect the speed without need for the auto-negotiation mechanism because if can work this out based on the incoming electrical signal, so sets itself to 100Mbps to match B's.

On Cisco switches (all that I know) on 10/100Mbps ports if the duplex isn't set or can't be auto-negotiated they default to Half.  Hence your answer here.

On the GbE ports they act differently - they default to Full duplex.

Reference is CCIE Routing and Switching Certification Guide Fourth Edition, Cisco Press.

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