EtherChannel Silent Mode & Non-Silent

[beerilavi]

Hello,

Once again I come asking your assistance after being confused in my reading through the CCNP SWITCH cert guide. First off, the basis of my first question is: You should add the non-silent parameter if you expect a PAgP-capable switch to be located on the far end. If PAgP is not heard on the active port, the port remains up, but PAgP reports to STP that the port is down. So, what does this mean for the port-channel? Does it mean that if say I had 4 physical links in the channel then now it becomes 3 because this link wasn't added to the channel? I'm not sure of the interaction with STP, other than that STP treats the entire channel as one link.

Question two is  about silent mode: even if two interfaces are configured with PAgP auto mode, the link comes up, although not as a channel. I'm assuming the default silent mode is in use here, but really, what kind of a link are we talking about here if it's not an EtherChannel? The book says data can still pass over it, so it sounds like any switch to switch link, and STP eventually transitions through to the forwarding state.

Bottom line, I'm confused, and any feedback or clarification would be most welcome.

Beeri

[Madhukrishnan Gopinathan Nair]

Your question is not clear. 

But i would think pagp auto on both sides will not help to bundle the intefaces in to channel.

The link may be up, but not part of channel and normal stp rules apply. 

Hope this helps.

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Thanks,

Madhu

[beerilavi]

Hi,

I don't see how I could make my question any clearer. Regarding the second question, if 2 or more interfaces are configured interconnecting 2 switches, using PAgP auto mode then that is not a normal connection between switches. If an EtherChannel is formed, STP enables all links.

[Marek Janecek]

Hi !

Question 1:

Does it mean that if say I had 4 physical links in the channel then now it becomes 3 because this link wasn't added to the channel?

-Yes, operational will be 1 port-channel consisting of 3 links and 1 normal links where PAgP wasn't negotiated

I'm not sure of the interaction with STP, other than that STP treats the entire channel as one link.

-As stated above STP will see 2 links for which standard STP rules will apply (first link is portchannel and second is that normal link where port-channel wasn't negotiated)

Question 2:

Correct, links where port-channel wasn't negotiated are treated as normal individual links.

[Peter Paluch]

Beeri,

Quite honestly, I am confused as well despite being with Cisco technologies for over 10 years by now.

The "non-silent" explanation is extremely vague at best. There are some materials, including the following document:

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html

suggesting that the "silent" and "non-silent" keywords were there to provide some resiliency against uni-directional link conditions if UDLD is not available for whatever reason. However, the descriptions do not really make sense to me. For the "desirable" mode, regardless of the chosen mode on the command line, PAgP always starts in non-silent mode - this can be proved by debugging the PAgP operations. I am not certain if the choice of "non-silent" or "silent" has any bearing in PAgP "desirable" mode.

In addition, I was never able to see how the choice of "silent"/"non-silent" affected the "auto" mode. In the "auto" mode, the switch never starts the negotiation itself - rather, it always waits for the "desirable" end to send PAgP messages. Hence, the first establishment of the EtherChannel must be accomplished by explicit signalling. I have never been able to force the "auto" ports to become bundled, regardless of the "silent"/"non-silent" setting. Whether this setting has an effect AFTER the EtherChannel has been established, I am not sure - I was not able to fully comprehend the scarce information provided by Cisco. Surely enough, there were never scenarios presented in which it would be shown unambiguously how this setting modifies the bundle creation or behavior. I even went to check the U.S. patent where PAgP is described (you can search for it using patents.google.com, looking for Port Aggregation Protocol) but that patent does not cover the "silent"/"non-silent" terminology at all, and instead, only suggests some optional states in the finite state machine controlling the EtherChannel creation that could suggest the "silent"/"non-silent" operation, but the patent fails to describe them precisely as well, perhaps because of their optionality.

So to sum it up - I am as confused as you are about the purpose of the "silent" and "non-silent" in the PAgP, compounded by the fact that it appears that these settings only apply to the "auto" mode. Perhaps internal Cisco employees well-versed in reading C code could look up the specific source code and find out what these settings really do, but regarding the documentation I have been able to dig out so far, it has always failed to provide a comprehensive explanation.

I share your frustration with this topic.

Best regards,
Peter