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Community Member

whats the point of PagP?

Can someone tell me what the point of running LACP or PagP between two switches/routers for ether-channel is? My feeling is just set up the links on each side and set the mode to "on" and be done with it. It seems like running a protocol to do this same thing just causes unnecessary traffic between the links.

I use layer 3 port-channels (ether-channels) and have never enabled PagP or LACP.

Thx

- EM

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Purple

Re: whats the point of PagP?

   It's really a function of preference . PAGP and LACP will check to verify the channel - trunk parameters match on each and won't bring the channel up if they are incorrect .  As another poster said there is specific warnings about the  "forced on" mode.  Myself we usually do use either PAGP or LACP unless  restricted by say a server nic that does not support  either protocol.  I have not seen where it causes any kind of delay bringing up the channel , it comes up almost immediately. The traffic generated by these is minimal .   If you want  to use on thats' fine too.  You will find even varying reccomendations in cisco's best practice docs , some say to use PAGP or LACP and other say to use  "on"  so  take your pick .

Serious traffic problems can  result from mixing manual mode with PAgP or LACP modes, or with a port  with no EtherChannel configured. For example, if a port configured in on mode is connected to another port configured in desirable mode, or to a port not configured for  EtherChannel, a bridge loop is created and a broadcast storm can occur.  If one end uses the on mode, the other end must  also.

5 REPLIES

Re: whats the point of PagP?

Configuring the channel mode to "on" can cause loops in your network if the cabling is mixed up or your channel is misconfigure. Using PagP or Lacp will catch some of there errors.

Re: whats the point of PagP?

The first things that comes in my mind is that mode on involve building of the etherchannels without any negotiation. Mode pagp and lacp involve negotiation. One is Cisco proprietary and one is industry standard.

Depending from case to case you might one that only one interface of your end peers to start negotiation for building a channel (LACP or PAGP in conjunction with EEM for example) or you want this channel to be always on (mode on).

If it's not very clear my answer please let me know.

Re: whats the point of PagP?

Just a comment on this..... I understand your points on LACP and PAgP, however I rather set it up to on (without any negotiation).

If you're doing EtherChannels you still have to check that it can be done, and check all the characteristics on the ports anyway. I just prefer to set it to on as I always done it because honestly don't see any benefits from these protocols (i think they just add extra negotiation and delay)

Pretty much as setting the trunk to on as opposed to using DTP to negotiate trunking capabilities.

Federico.

Purple

Re: whats the point of PagP?

   It's really a function of preference . PAGP and LACP will check to verify the channel - trunk parameters match on each and won't bring the channel up if they are incorrect .  As another poster said there is specific warnings about the  "forced on" mode.  Myself we usually do use either PAGP or LACP unless  restricted by say a server nic that does not support  either protocol.  I have not seen where it causes any kind of delay bringing up the channel , it comes up almost immediately. The traffic generated by these is minimal .   If you want  to use on thats' fine too.  You will find even varying reccomendations in cisco's best practice docs , some say to use PAGP or LACP and other say to use  "on"  so  take your pick .

Serious traffic problems can  result from mixing manual mode with PAgP or LACP modes, or with a port  with no EtherChannel configured. For example, if a port configured in on mode is connected to another port configured in desirable mode, or to a port not configured for  EtherChannel, a bridge loop is created and a broadcast storm can occur.  If one end uses the on mode, the other end must  also.

Community Member

Re: whats the point of PagP?

Thanks for the responses everyone. I think it is a matter of preference, depending on the stability of your network and type

of switches/routers you're using, and ultimately what you're trying to accomplish.

Thx,

- EM

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