03-24-2012 12:39 PM - edited 03-07-2019 05:45 AM
Hi everybody.
Please consider the following excerpt from my first book:
"When a PAgP mode of auto or desirable is configured on a port, you also have the option of configuring either of the following operational parameters:
Non-Silent—
An EtherChannel bundle does not form, until bidirectional connectivity has been confirmed. This parameter means that ports in the EtherChannel bundle must receive data, as well as be able to transmit data, in order for the bundle to form. Non-silent is default on Catalyst 5000 fiber-based FastEthernet and fiber-based gigabit Ethernet ports. This mode protects against unidirectional failures, where one of the transmit or receive links may fail. These failures are common on fiber-based connections and can cause bridging loops.
Silent—
An EtherChannel bundle forms, even if data has not been received from the remote device. This mode is the default mode of operation on all Catalyst 4000, 6000, and Cisco IOS-based switch ports, as well as on Catalyst 5000 copper ports. For unidirectional failures on Cisco IOS-based Catalyst ports, Catalyst 4000 ports, and Catalyst 6000 ports, other protocols, such as unidirectional link detection (UDLD), which detect these failures much more quickly than non-silent PAgP operation, are used to protect against unidirectional failures.
Cisco recommends that you do not modify the default silent or non-silent mode of operation.[Index Term: |EtherChannel|PAgP]"
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In nut shell, If i configure pagp auto silent mode, I am telling my switch just form etherchannel when prompted by other switch on the other end. Basically We are telling switch not to be bothered about verifying bi directional communication.
If we configure pagp auto non silent mode, we are telling the switch first verify if bi direction communication exists, if yes then form etherchannel when prompted by other switch on the other end.
Did i understand it correctly?
This is what my second book says:
" The key is in the phrase " If the other end is silent" The silent submode listens for any PAgP packets from the far end. If none is received, silent submode assumes that a channel should be built anyway"
If you expect a PAgP-capable switch to be on the far end,you should add the " Non-silent " key word to the desirable or auto mode. This requires each port to receive PAgP packets before adding them to a channel"
The above excerpt from the 2nd book contradicts what the first book says. 2nd book says a port configured with non silent option requires to receive PAgP packets before it could become part of ether channel. However my first book says it could be any traffic not necessarily PAgP packets a port configured with non silent mode should receive before it could participate in ether channel.
Now consider the following example
Let say we have two switches sw1--------------------sw2
Both switches are configured with pagp auto silent mode. How long does sw1 for instance wait for incoming pagp messages before giving up on forming etherchannel ?
Thanks and have a great weekend.
Solved! Go to Solution.
03-24-2012 12:58 PM
Hi Sarah,
Looking at the command reference guide, you can't configure both side as auto, because auto by default is silent mode. And if you put both sides in silent mode, than the Etherchannel does not form. If one side is auto the other side has to be desirable
Reference guide here:
Also, according to the config guide the hello packets are send every 30 seconds
Have a great weekend!
HTH
03-24-2012 01:41 PM
Hi Sarah,
I have never seen a server capable of running PAGP, because PAGP is a Cisco propriety protocol. On the other hand most servers are capable of running LACP which is standard base protocol. So, I would say no to both scenarios. Usually in a server to switch scenario, you run the server in passive mode and switch in active.
HTH
Reza
03-24-2012 02:44 PM
I would say yes, because if the silent side doesn't hear from the non-silent side within that time from (15-20 seconds from your book) than the silent side would bring down its interfaces. The same thing would apply the other way around.
HTH
Reza
03-24-2012 12:58 PM
Hi Sarah,
Looking at the command reference guide, you can't configure both side as auto, because auto by default is silent mode. And if you put both sides in silent mode, than the Etherchannel does not form. If one side is auto the other side has to be desirable
Reference guide here:
Also, according to the config guide the hello packets are send every 30 seconds
Have a great weekend!
HTH
03-24-2012 01:33 PM
HI Reza
"If you do not specify non-silent with the autoor desirable mode, silent is assumed. The silent mode is used when the switch is connected to a device that is not PAgP-capable and seldom, if ever, sends packets. A example of a silent partner is a file server or a packet analyzer that is not generating traffic. In this case, running PAgP on a physical port prevents that port from ever becoming operational. However, it allows PAgP to operate, to attach the port to a channel group, and to use the port for transmission. Both ends of the link cannot be set to silent. "
let say we have:
sw---------file server
( desirable silent)
Will ether channel form considering file server is not capable of PAgP ?
Sw---------------file server
( auto silent)
Will ether channel form considering file server is not capable of PAgp ?
Thanks and you havea great weekend as well.
03-24-2012 01:41 PM
Hi Sarah,
I have never seen a server capable of running PAGP, because PAGP is a Cisco propriety protocol. On the other hand most servers are capable of running LACP which is standard base protocol. So, I would say no to both scenarios. Usually in a server to switch scenario, you run the server in passive mode and switch in active.
HTH
Reza
03-24-2012 01:51 PM
Hi Reza thanks for your help. Below is the excerpt from my book regarding PAgP negotiation delay.
PAgP Negotiation Delays
If you configure a PAgP mode of auto or desirable, when a port first initializes due to a physical link being detected, the Layer 2 line protocol of the port does not come up until PAgP negotiates a bundle or until PAgP negotiation times out. If a bundle is successfully negotiated, this port coming up normally occurs within a matter of seconds, but if a PAgP negotiation timeout occurs, a port does not come up at Layer 2 for approximately 15-20 seconds. In other words, if a non-EtherChannel capable device connects to a switch port that has a PAgP mode of auto or desirable configured, it does get a Layer 2 connection for approximately 15-20 seconds. If workstations are connected, this situation can cause unacceptable delays when a workstation starts up, especially if the workstations are fast computers that boot up quickly. The workstation might not be able to obtain an IP address via Dynamic Host Configuration Protocol (DHCP), causing logon failures to the network among other problems.
Based on the above, there is certian time for PAgP negotiation to occur, If no PAgP negotiation occurs with in that time, PagP negotiation times out. My question is : will this happen regardless of mode i.e silent or non-silent mode?
thanks
03-24-2012 02:44 PM
I would say yes, because if the silent side doesn't hear from the non-silent side within that time from (15-20 seconds from your book) than the silent side would bring down its interfaces. The same thing would apply the other way around.
HTH
Reza
11-20-2012 04:09 AM
TIP
In practice, you might notice a delay from the time the links in a channel group are
connected until the time the channel is formed and data can pass over it. You will
encounter this if both switches are using the default PAgP auto mode and silent submode.
Each interface waits to be asked to form a channel, and each interface waits and listens
before accepting silent channel partners. The silent submode amounts to approximately a
15-second delay.
Even if the two interfaces are using PAgP auto mode, the link will still eventually come up,
although not as a channel. You might notice that the total delay before data can pass over
the link is actually approximately 45 or 50 seconds. The first 15 seconds are the result of
PAgP silent mode waiting to hear inbound PAgP messages, and the final 30 seconds are the
result of the STP moving through the listening and learning stages.
by Dave Hucaby
03-24-2012 04:06 PM
Hi Reza.
You have a valid point. But before LACP came along, how did we form etherchannel between cisco switch capable of pagp and a server?
I am very confused how different books even cisco links say same things so differently.
For example,
" By default PagP operates in silent submode with the desirable and auto modes and allows ports to be added to an etherchannnel even if the other end of the link is silent and never transmits PAgP packets"
The key is in the line phrase" If the other end is silent" .The silent submode listens for any pAgp packets from the far end,looking to negotiate a channel. If none is received,silent submode assumes that a channel should be build nyway so no more PAgp packets are expected from the far end"
Based on the above we shoud expect a port configured with auto silent connected to server which does not participate in pagp, to form etherchannel.
My book rambles on:
" This allows a switch to form an etherchannel with a device such as file server that does not participate in pagp."
Now my big confusion is what to believe.
My other books and your link suggest that the only difference between silent mode and non silent mode is in silent mode a port does not verify bi- direction connectivity before participating in etherchannel while in non silent mode a port first verify bi-directional connectivity before participating in etherchannel formation.
The big question is what function exactly silent mode/non-silent mode perform? Do they verify bi-directional connectivity or do they allow a cisco catalyst switch configured with PAgp to form etherchannel with nodes( e.g file server etc) which do not participate in PAgp.
I apologize for the long winded mails I have been shooting since morning.
Thanks and have a great weekend.
03-25-2012 10:53 AM
I found the answer
Basically, in silent mode, a switch's port will not verify bi directional connectivity. Pagp in silent mode ( auto or desirable) is waiting for pagp negotiation to occur. If no negotiation occurs in 15 to 20 seconds, Pagp simply quits and does not inform the STP about it.And it shoud not. Because not receiving PAgp messages does not necessarily mean uni directional link exist. It could well be possible the other end is not configured with Pagp or configured with mode " on" . So PAgp is not sure whether the link is uni directional or the other end is not configured properly.In such case other protocol such as udld could be used in conjunction with Pagp silent mode.
In non-silent mode, a switch port first verifies the bi -directional connectivity. It does so by observing RX and TX pins of the the port. If the traffic is being sent or received, the port concludes a bi- directional connectivity exists . PAgp then bundles the port . However, if no traffic is received on RX,then PAGp will wait for 15 to 20 seconds before informing stp about the port as being down. The important thing to note is PAgp does inform STP in non-silent mode as compared to Silent mode where it silently simply quits after around 20 seconds.
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Now the question I have is how does PAgp decide which ports to bundle? . For example if we have PAgP running on 10 ports. and we know max 8 ports can be bundled. In such case how does PAgp decides which ports to bundle?
Thanks and have a great weekend.
I
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