etherchannel config

Answered Question
Dec 21st, 2007
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Anyone know what the difference between using desirable or desirable non-silent is ? When using non-silent was does that actually do versus using just desirable? Have not been able to find the answer anywhere in cisco docs . I know the recomendation is to use non-silent between cisco devices ..

Correct Answer by Edison Ortiz about 9 years 4 months ago

Silent/Non-Silent Mode


When you deal with fiber connections, there is a possibility that, even if a receive (Rx) transceiver dies, the transmit (Tx) transceiver on the other end is still up. In a similar scenario, packets can get black holed.


It is important for the switch that transmits to remove this port from the EtherChannel bundle. In order to do so on the Catalyst 5500/5000, you set PAgP in non-silent mode. Non-silent mode means that, if the Rx does not receive traffic, the port is not put into the channel. However, use of non-silent mode is not enough because this detection happens only when the channel is formed.


In order to prevent the black holing of traffic when the channel is already formed, this occurs:


1.


PAgP detects that the Rx port does not receive any traffic.

2.


PAgP resets the Tx transceiver of the port that does not receive traffic. PAgP resets it for 1.6 seconds so that the switch on the other end also resets the port.

3.


The faulty port does not join the channel anymore because no traffic is received on that port.


On the Catalyst 5500/5000, set non-silent mode on fiber strands and silent mode on copper strands. This is both the default and recommended setting because, on fiber connections on the Catalyst 5500/5000, the negotiation is usually not available, so there is no way to detect the problem at a physical layer.


Default PAgP Settings on the Catalyst 4500/4000 and 5500/5000


By default, PAgP is auto for a plug-and-play implementation. Disable PAgP manually from the ports where there is no need to have it.


By default, the silent mode is on. Non-silent is acceptable as well. However, because a port can be connected to a device that does not send traffic (for example, a sniffer), it is more general to have silent enabled.

Recommendations


*


Use the non-silent keyword when you connect to a device that transmits bridge protocol data units (BPDUs) or other traffic. Use this keyword with the auto or desirable mode. PAgP non-silent adds an extra level of link state detection because it listens for BPDUs or other traffic in order to determine if the link functions properly. This adds a form of UniDirectional Link Detection (UDLD) capability that is not available when you use the default silent PAgP mode.

*


Use the silent keyword when you connect to a silent partner (which is a device that does not generate BPDUs or other traffic). An example of a silent partner is a traffic generator that does not transmit packets. Use the silent keyword with auto or desirable mode. If you do not specify silent or non-silent, silent is assumed.

*


The silent mode does not disable the PAgP ability to detect unidirectional links. However, when you configure a channel, non-silent prevents a unidirectional port from even joining the link.

*


A PAgP configuration (the set port channel {desirable | auto} command) is safer than a non-PAgP configuration (the set port channel on command). A PAgP configuration provides protection for unidirectional links and also avoids misconfigurations that can arise when there are ports channeling on one side of the link and not on the other side.

*


Refer to Understanding and Configuring the Unidirectional Link Detection Protocol Feature for more information on UDLD.


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Jon Marshall Fri, 12/21/2007 - 12:11
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Glen


If you are connecting to a device that is not PagP-capable then you would use the silent keyword eg.


If you connect to a file server for example using etherchannel. The file server does not participate in PagP. If you have the non-silent keyword then the channel will not form because the switch will not receive any PagP packets from the server.


Setting it to silent will allow the ports to become operational.


HTH


Jon

glen.grant Fri, 12/21/2007 - 15:35
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Jon I thought if you were going to a server you have to use like channel-group 1 mode on because it did not use pagp packets . Can you clarify , anyway thanks for the info .

Correct Answer
Edison Ortiz Fri, 12/21/2007 - 16:50
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Silent/Non-Silent Mode


When you deal with fiber connections, there is a possibility that, even if a receive (Rx) transceiver dies, the transmit (Tx) transceiver on the other end is still up. In a similar scenario, packets can get black holed.


It is important for the switch that transmits to remove this port from the EtherChannel bundle. In order to do so on the Catalyst 5500/5000, you set PAgP in non-silent mode. Non-silent mode means that, if the Rx does not receive traffic, the port is not put into the channel. However, use of non-silent mode is not enough because this detection happens only when the channel is formed.


In order to prevent the black holing of traffic when the channel is already formed, this occurs:


1.


PAgP detects that the Rx port does not receive any traffic.

2.


PAgP resets the Tx transceiver of the port that does not receive traffic. PAgP resets it for 1.6 seconds so that the switch on the other end also resets the port.

3.


The faulty port does not join the channel anymore because no traffic is received on that port.


On the Catalyst 5500/5000, set non-silent mode on fiber strands and silent mode on copper strands. This is both the default and recommended setting because, on fiber connections on the Catalyst 5500/5000, the negotiation is usually not available, so there is no way to detect the problem at a physical layer.


Default PAgP Settings on the Catalyst 4500/4000 and 5500/5000


By default, PAgP is auto for a plug-and-play implementation. Disable PAgP manually from the ports where there is no need to have it.


By default, the silent mode is on. Non-silent is acceptable as well. However, because a port can be connected to a device that does not send traffic (for example, a sniffer), it is more general to have silent enabled.

Recommendations


*


Use the non-silent keyword when you connect to a device that transmits bridge protocol data units (BPDUs) or other traffic. Use this keyword with the auto or desirable mode. PAgP non-silent adds an extra level of link state detection because it listens for BPDUs or other traffic in order to determine if the link functions properly. This adds a form of UniDirectional Link Detection (UDLD) capability that is not available when you use the default silent PAgP mode.

*


Use the silent keyword when you connect to a silent partner (which is a device that does not generate BPDUs or other traffic). An example of a silent partner is a traffic generator that does not transmit packets. Use the silent keyword with auto or desirable mode. If you do not specify silent or non-silent, silent is assumed.

*


The silent mode does not disable the PAgP ability to detect unidirectional links. However, when you configure a channel, non-silent prevents a unidirectional port from even joining the link.

*


A PAgP configuration (the set port channel {desirable | auto} command) is safer than a non-PAgP configuration (the set port channel on command). A PAgP configuration provides protection for unidirectional links and also avoids misconfigurations that can arise when there are ports channeling on one side of the link and not on the other side.

*


Refer to Understanding and Configuring the Unidirectional Link Detection Protocol Feature for more information on UDLD.


glen.grant Fri, 12/21/2007 - 17:29
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Edison , great explanation .Better than any cisco doc I have ever read . Confirms my decision to always use desirable non-silent on etherchannels . Also answered my questions. Thanks to all and have a merry xmas....

Edison Ortiz Fri, 12/21/2007 - 18:45
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Hi Glen,


Actually, I don't want to take credit for it. This was taken from a document I saved on my archives, too bad I couldn't find something similar in a public URL.


Merry Christmas to you and yours :)

bbentley05 Tue, 01/22/2008 - 14:55
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Edison,


Thanks for the in-depth information. I've actually had the "black hole" fiber etherchannel occur - not fun.


I'd like to ask a follow up question...How can you avoid this in a cross-stack etherchannel? Currently we have 3750 stacks through out our campus running cross-stack etherchannels in the "on" mode. If I config them to use LACP "active" can I protect the network from this fiber "black hole" issue?

Edison Ortiz Tue, 01/22/2008 - 16:27
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Yes, LACP will protect you since it has some intelligence built into the protocol.


HTH,


__


Edison.

bbentley05 Tue, 01/22/2008 - 17:08
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Thanks for the quick response. Will enabling LACP introduce any new issues with Rapid-PVST convergence? I understand that PAgP adds 18 seconds. Can I expect this same behavior?

Edison Ortiz Tue, 01/22/2008 - 19:26
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I haven't seen a document where it states that PAgP adds 18 seconds convergence delay, can you share such document ?


Thanks

bbentley05 Tue, 01/22/2008 - 20:03
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Edison,


It was printed in CCIE Professional Development - Cisco LAN Switching by Cisco Press. Chapter 7 "Advanced Spanning Tree" pages 72-74. I've only posted an excerpt due to copyright. Sorry for the formatting.


-Brad



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