Re: Two trunk links to each switch in Cisco ???

bazzamacca1980 · ‎12-27-2011

Hi,

I am studying CCNA 3 Lan Switching.

I have a question about the multiple trunk links that show up in my study notes.

Throughout the notes it explains about the trunk link betwenn switches, which i understand that once STP kicks in one of these links will be blocked.

When I go to the packet tracer sample exercises the switches now have 2 trunk links between each switch.

Can someone explain why the jump to 2 links??

Would STP then block 3 of the trunk links and leave one open?

I have attached an image of what I mean.

Thanks guys!!

Reza Sharifi · ‎12-27-2011

Hi,

Usually when there are 2 links, you create an Etherchannel (bundle the links together) to get higher bandwidth. You can put up to 8 physical links in an Etherchannel.

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_configuration_example09186a0080094bc5.shtml

HTH

bazzamacca1980 · ‎12-27-2011

Hi,

I was thinking that, but there is no mention of creating the 2 links to etherchannel in the packet tracer or even how to configure it.

Could there be any other purpose for the 2 links or am I just misinterpreting it?

Reza Sharifi · ‎12-27-2011

I think, it is Etherchannel. It is pretty common for access layer switches to uplink to the Distro and core via Etherchannels.

What book is that?

HTH

bazzamacca1980 · ‎12-27-2011

Hi,

Its the Cisco Networking academy online study material, Exploration 3.

I will re-read my labs, to see if there is mention of etherchannel in the one with multiple trunks in that particular lab.

What would happen if you configured 2 trunk links on the same switch with the same VLAN's does it automatically become an etherchannel? Just thinking its a bit strange if it is etherchannel as there is no real reference to configure it in the notes.

I found a mention of it in the notes

hopefully its ok to post it here...

"

Redundant links are configured in the form of an EtherChannel that protects against a single link failure. Cisco EtherChannel technology enables you to aggregate multiple physical links into one logical link. This can provide up to 80 Gb/s of aggregate bandwidth for with 10 Gigabit EtherChannel. "

But then it goes on to say that etherchannel is a CCNP topic... so it doesnt give any config examples or anything like that.

Its really puzzling me now... why would there be etherchannel in the labs if there is no guidance on configuring it?

If its of any use the 4 trunk links coming from the 1st switch in the lab are all set to be native.

Leo Laohoo · ‎12-27-2011

What would happen if you configured 2 trunk links on the same switch with the same VLAN's does it automatically become an etherchannel?

No it won't. If you have two (or more) trunk links coming-and-going to a pair of switches it's good practice to put them in etherchannel.

Reza Sharifi · ‎12-27-2011

No, that will not happen. You have to configure the Etherchannel or Portchannel manually, then add the physical links to it, then use LACP, PAGP, or On on both sides of the link. It is not automated.

Have a look at this config guide to get more details on how to configure a Portchannel

http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_8_ea1/configuration/guide/swethchl.html

HTH

bazzamacca1980 · ‎12-27-2011

If its of any use the 4 trunk links coming from the 1st switch in the lab are all set to be native.

I have no problem with the configuration of these links, I was just looking for a reason why there would be 2 from a single switch going to another single switch. I understand the redundancy issue & why you need to have multiple paths etc, its just the 2 trunk link thing thats throwing me. I know this will annoy me if I cant understand why they are there.

here is the config of the links.

interface FastEthernet0/1