optical ports and "line protocol"

m4rtin · June 2012

I made a following connection between two SFP's:

As you can see, Tx of SFP in Gi1/0/1 is connected to Rx of SFP in Gi1/0/2. SFP in Gi1/0/2 receives light:

                                           Optical   Optical
           Temperature  Voltage  Current   Tx Power  Rx Power
Port       (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
---------  -----------  -------  --------  --------  --------
Gi1/0/2      47.7       3.21       9.9      -7.4      -7.3

Why isn't SFP in port Gi1/0/2 up(line protocol is down)?

dead_p00l · June 2012

From the image and unless i'm missing something you only have light going in one direction, tx from 1/0/1 to rx on 1/0/2 on a pair of standard LX optics. you should also have a fiber for tx from 1/0/2 to rx on 1/0/1.

Coolhandluke · June 2012

Yeah they should be in pairs. one for TX one for RX

m4rtin · June 2012

dead_p00l wrote: »

From the image and unless i'm missing something you only have light going in one direction, tx from 1/0/1 to rx on 1/0/2 on a pair of standard LX optics. you should also have a fiber for tx from 1/0/2 to rx on 1/0/1.

Yes, light is going only in one direction. Only Tx of Gi1/0/1 is connected to Rx of Gi1/0/2:

Optical   Optical
           Temperature  Voltage  Current   Tx Power  Rx Power
Port       (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
---------  -----------  -------  --------  --------  --------
Gi1/0/1      47.2       3.31       9.2      -7.1     -40.0   
Gi1/0/2      47.7       3.21       9.9      -7.5      -7.2

How does Gi1/0/2 know that it's Tx isn't connected? How does the "line protocol" detect that there is no connection between Gi1/0/1 Rx and Gi1/0/2 Tx?

PS it's a theoretical question not a practical one

dead_p00l · June 2012

Im not 100% certain about this but if i had to guess I would think that there is some type of messaege sent back over the transmit side to the receiving optic letting it know that it has both forward and return light. Hopefully someone else will know for certain and will be able to chime in.

jason_lunde · June 2012

udld enabled?

m4rtin · June 2012

jason_lunde wrote: »

udld enabled?

By default those two optical ports have following UDLD settings:

switch#sh int status | i Gi1/0/
Gi1/0/1                      notconnect   1            auto   auto 1000BaseLX SFP
Gi1/0/2                      notconnect   1            auto   auto 1000BaseLX SFP
switch#sh udld Gi1/0/1         

Interface Gi1/0/1
---
Port enable administrative configuration setting: Follows device default
Port enable operational state: Disabled
Current bidirectional state: Unknown
switch#sh udld Gi1/0/2

Interface Gi1/0/2
---
Port enable administrative configuration setting: Follows device default
Port enable operational state: Disabled
Current bidirectional state: Unknown
switch#

Even if I disable UDLD, ports do not come up when only Tx of Gi1/0/1 is connected to Rx of the Gi1/0/2:

switch#sh udld Gi1/0/1         

Interface Gi1/0/1
---
Port enable administrative configuration setting: Disabled
Port enable operational state: Disabled
Current bidirectional state: Unknown
switch#sh udld Gi1/0/2

Interface Gi1/0/2
---
Port enable administrative configuration setting: Disabled
Port enable operational state: Disabled
Current bidirectional state: Unknown
switch#sh int status | i Gi1/0/
Gi1/0/1                      notconnect   1            auto   auto 1000BaseLX SFP
Gi1/0/2                      notconnect   1            auto   auto 1000BaseLX SFP
switch#

Any other ideas?

Forsaken_GA · June 2012

Are you trying to make the ports come up? If so, I can probably help.... we actually use unidirectional fiber links on purpose for VOD

if so, issue the 'unidirectional send-only' command under the interface

m4rtin · June 2012

Forsaken_GA wrote: »

Are you trying to make the ports come up? If so, I can probably help.... we actually use unidirectional fiber links on purpose for VOD

if so, issue the 'unidirectional send-only' command under the interface

I would like to understand why Gi1/0/2 is not up(line protocol is down) while Tx of Gi1/0/1 is connected to Rx of Gi1/0/2.
Looks like my switch does not support this command:

switch#
switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
switch(config)#int Gi1/0/1
switch(config-if)#uni?
% Unrecognized command
switch(config-if)#^Z 
switch#
23:40:07: %SYS-5-CONFIG_I: Configured from console by console
switch#

Even many UDLD related tutorials describe the situation I have:

..but for some reason my Gi1/0/2 is down

Any other suggestions?

Forsaken_GA · June 2012

Do you have keepalives turned on?

m4rtin · June 2012

Forsaken_GA wrote: »

Do you have keepalives turned on?

No, keepalives are not turned on:

switch#sh run int Gi1/0/1            
Building configuration...

Current configuration : 38 bytes
!
interface GigabitEthernet1/0/1
end

switch#sh run int Gi1/0/2            
Building configuration...

Current configuration : 38 bytes
!
interface GigabitEthernet1/0/2
end

switch#sh interfaces Gi1/0/1 | i Keep
  Keepalive not set
switch#sh interfaces Gi1/0/2 | i Keep
  Keepalive not set
switch#

If I try the very same setup on the Cisco WS-C3560-24TS, the ports do not come up as well:

WS-C3560-24TS#sh run int Gi0/1
Building configuration...

Current configuration : 36 bytes
!
interface GigabitEthernet0/1
end

WS-C3560-24TS#sh run int Gi0/2
Building configuration...

Current configuration : 36 bytes
!
interface GigabitEthernet0/2
end

WS-C3560-24TS#sh int transce
If device is externally calibrated, only calibrated values are printed.
++ : high alarm, +  : high warning, -  : low warning, -- : low alarm.
NA or N/A: not applicable, Tx: transmit, Rx: receive.
mA: milliamperes, dBm: decibels (milliwatts).

                                           Optical   Optical
           Temperature  Voltage  Current   Tx Power  Rx Power
Port       (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
---------  -----------  -------  --------  --------  --------
Gi0/1        36.1       3.26      10.0      -7.4      N/A    
Gi0/2        35.5       3.27       8.7      -7.3      -8.0   


WS-C3560-24TS#sh int Gi0/2 status

Port      Name               Status       Vlan       Duplex  Speed Type
Gi0/2                        notconnect   1            auto   auto 1000BaseLX SFP
WS-C3560-24TS#

However, if I connect those very same SFP's to Cisco 4506 SUP(WS-X4516-10GE) ports Gi1/5 and Gi1/6, one of the ports does come up! One difference I see is that my Cisco 4506 does not support DDM for SFP ports on SUP while both WS-C3560-24TS and Cisco 3750 series do.

Any other ideas what might cause such behavior?

Forsaken_GA · June 2012

newp, not a clue. Sounds like the access switch is enforcing a valid rx and tx on the pairs for it to consider the port up, and that's not something I have a problem with. Your curiosity on this matter is greater than my own, if you find an answer, please share!

m4rtin · June 2012

Forsaken_GA wrote: »

newp, not a clue. Sounds like the access switch is enforcing a valid rx and tx on the pairs for it to consider the port up, and that's not something I have a problem with. Your curiosity on this matter is greater than my own, if you find an answer, please share!

Looks like it depends on "speed nonegotiate"

ME-C3750-24TE#sh int transce
Transceiver monitoring is disabled for all interfaces.

If device is externally calibrated, only calibrated values are printed.
++ : high alarm, +  : high warning, -  : low warning, -- : low alarm.
NA or N/A: not applicable, Tx: transmit, Rx: receive.
mA: milliamperes, dBm: decibels (milliwatts).

                                           Optical   Optical
           Temperature  Voltage  Current   Tx Power  Rx Power
Port       (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
---------  -----------  -------  --------  --------  --------
Gi1/0/2      47.1       3.24       9.9      -7.3      -7.4   


ME-C3750-24TE#sh int Gi1/0/2 status

Port      Name               Status       Vlan       Duplex  Speed Type
Gi1/0/2   test               notconnect   6            auto   auto 1000BaseLX SFP
ME-C3750-24TE#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
ME-C3750-24TE(config)#int Gi1/0/2
ME-C3750-24TE(config-if)#speed nonegotiate
ME-C3750-24TE(config-if)#end
ME-C3750-24TE#sh int Gi1/0/2 status
1w0d: %SYS-5-CONFIG_I: Configured from console by console
1w0d: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/2, changed state to up
ME-C3750-24TE#sh int Gi1/0/2 status

Port      Name               Status       Vlan       Duplex  Speed Type
Gi1/0/2   test               connected    6            full   1000 1000BaseLX SFP
ME-C3750-24TE#
1w0d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/2, changed state to up
ME-C3750-24TE#

I really don't see a point for speed nonegotiate in case of 1000BASE-LX/LH port. Or am I missing something?

Forsaken_GA · June 2012

m4rtin wrote: »

Looks like it depends on "speed nonegotiate"

Ah, that makes sense, if the port is set to negotiate, and it can't because of no valid rx/tx pair, then yeah, I can see why it would stay down.

I really don't see a point for speed nonegotiate in case of 1000BASE-LX/LH port. Or am I missing something?

There exists some gear that simply doesn't do negotiation. We have several pieces of equipment in our backoffice infrastructure like this, and we have to hardcode speed and duplex in order to get a link.

optical ports and "line protocol"

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