Problems with pagp silent-mode

[yifansun_cisco]

I have some problems with pagp!

In the Official Certification book,there is a graph:

"Even if the two interface are using PAgP auto mode,the link will still eventually

come up,although not as a channel." how can i understand this!

I have a test that set the switch port with the auto mode, the etherchannel becomes up!

why does the book say that? what does it mean?can U give an explanation? thanks!

[yifansun_cisco]

configuration:

SW1:

show run int fa0/1

!

interface FastEthernet0/1

channel-protocol pagp

channel-group 1 mode auto

SW2:

show run int fa0/2

interface FastEthernet0/2

channel-protocol pagp

channel-group 1 mode auto

the phenomenon :

SW1

SW1#show etherchannel su

Flags:  D - down        P - in port-channel

        I - stand-alone s - suspended

        H - Hot-standby (LACP only)

        R - Layer3      S - Layer2

        U - in use      f - failed to allocate aggregator

        u - unsuitable for bundling

        w - waiting to be aggregated

        d - default port

Number of channel-groups in use: 1

Number of aggregators:           1

Group  Port-channel  Protocol    Ports

------+-------------+-----------+----------------------------------------------

1      Po1(SU)           PAgP   Fa0/1(P)

SW2:

SW2#show etherchannel su

Flags:  D - down        P - in port-channel

        I - stand-alone s - suspended

        H - Hot-standby (LACP only)

        R - Layer3      S - Layer2

        U - in use      f - failed to allocate aggregator

        u - unsuitable for bundling

        w - waiting to be aggregated

        d - default port

Number of channel-groups in use: 1

Number of aggregators:           1

Group  Port-channel  Protocol    Ports

------+-------------+-----------+----------------------------------------------

1      Po1(SU)           PAgP   Fa0/2(P)

PC1 could ping PC2!

Can you explain this ? thanks!

the topology is below:

[George Joseph · Accepted solution]

I should 1st apoligize to Jawad Mukhtar - coz i've removed my initial reply to your post since my post had a typo which also removed his correct answer as he replied to my comments (i didn't notice that)

PS: Port-channel configuration is normally used when multiple interfaces are connected b/w switches. Ether channel takes advantage of the existing interfaces by bundling interfaces to allow incremental additaions to the available bandwidth.

Below is the aswer provided by Jawad

You can use following combination to create etherchannel using PAgP.

desirable on Switch-1   -  desirable on Switch-2

auto on Switch-1         -  desirable on Switch-2

desirable on Switch-1   -  auto on Switch-2

[jawad-mukhtar · Accepted solution]

NP George it doesnt matter.  Thanks and keep smiling..

[yifansun_cisco]

thanks for answers!

But i still have some question!

I update the question. can you help me with that question ?

make a progress everyday!

[yifansun_cisco]

yes,i know what you describe. forgive me...

The question is the etherchannel becomes up  if the two switches begin with the mode auto.

(the phenomenon is above)

I read that

"Even if the two interface are using PAgP auto mode,the link will still eventually

come up,although not as a channel " from the CCNP-SWITCH Official Certification Guide.

I don't understand this!

can you explain this sentence to me according to my test above so that i can understand it deeply!

Many thanks!!

make a progress everyday!

[yifansun_cisco]

Thanks , George! can you explain explicitly to me?

I update the question!

make a progress everyday!

[Rolf Fischer · Accepted solution]

Hi,

I assume your're testing with the Packet Tracer?

Although this is a very good software for testing and cert-preparing, it is still a simulation and not a real-world lab and in this particular case it seems not to act like real switches do.

Configuring link aggregation gives you a couple of choices:

- Negotiation vs. On-Mode

- when Negotiation: PAgP vs. LACP

- when PAgP: Mode auto vs. desirable; optional: Silent-Mode

- when LACP: Mode passive vs. active

...

PAgP mode auto doesn't send any protocol-data in order to establish the channel, it just waits for receiving such data from a neighbor (which has to be in desirable mode) to become bundeled.

The non-silent option adds the requrirement of receiving data from the neighbor device in order to avoid blackholing when fiber links become undidirectional. (From my point of view, UDLD is a better choice, I can't find any advantage in using the non-silent option.)

I think what your book says is that ports in an auto-auto setup might result in an up/up state but won't be bundeld in a channel (because no negotiation takes place). STP will block one of the ports in order to avoid a bridging loop.

Hope that helps

Rolf

[yifansun_cisco]

thanks for your answer!

make a progress everyday!