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New Member

Two trunk links to each switch in Cisco ???

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!!

9 REPLIES
VIP Super Bronze

Two trunk links to each switch in Cisco ???

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

New Member

Two trunk links to each switch in Cisco ???

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?

VIP Super Bronze

Two trunk links to each switch in Cisco ???

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

New Member

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

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.

Hall of Fame Super Gold

Two trunk links to each switch in Cisco ???

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.

VIP Super Bronze

Two trunk links to each switch in Cisco ???

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

New Member

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

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

switchport trunk native vlan 99

switchport mode trunk

!

interface FastEthernet0/2

switchport trunk native vlan 99

switchport mode trunk

!

interface FastEthernet0/3

switchport trunk native vlan 99

switchport mode trunk

!

interface FastEthernet0/4

switchport trunk native vlan 99

switchport mode trunk

!

interface FastEthernet0/5

switchport trunk native vlan 99

switchport mode trunk

Theres no  mention of etherchannel there either.

Hall of Fame Super Gold

Two trunk links to each switch in Cisco ???

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.

Must be an old book.

New Member

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

its not a book, its the current online study for CCNA , maybe they are just there for illustration purposes to get you into the idea of there being 2 links (that later should be configured as etherchannel once you reach the CCNP level). Maybe I am looking too much into this...was something I spotted in the lab work & wanted to run past the folk on this forum.

Thanks for your help with this much appreciated.

Cheers

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