Trunking Question

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May 8th, 2010
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im wondering if there is anything to be gained by having 2 trunks running across my distribution layer switches, where each will allow only the vlans for each switch block....


so switch block 1 which includes access layer switches 1 and 2 will support vlans 2-15 and they use one trunk....


switch block 2 which includes access layer switches 3 and 4 will support vlans 16-30 and they will use a separate trunk....


I think there may be some value in doing this....


Thank you

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Jon Marshall Sat, 05/08/2010 - 02:30
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Joe


Well you get dedicated bandwidth per group of vlans and you can limit the influence of STP issues on each trunk link. However if they are single trunk links then this is not preferable to an etherchannel. Bear in mind if you allow a set of vlans on one trunk link if that trunk link fails then the vlans cannot use the other link.


And even if you allowed the vlans on the other link but made sure they were blocked by mamipulating STP costs you still have the failover time of STP/RSTP compared to a failure in a single link in the etherchannel.


There are times when a dedicated link is useful for example with the FWSM and having a dedicated link for stateful failover replication but overall i'm not sure what the benefits are. Even if you made the 2 links etherchannel trunks it just seems to be adding unnecessary complexity where you could simply scale one etherchannel trunk link correctly.


Jon

ex-engineer Sat, 05/08/2010 - 02:34
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Jon i would use etherchannels.....2 channels carrying a separate set of vlans...


the only reason why i would think of doing that is to minimize the failure domains for layer 2.....


any value you think?ever seen it done?

Jon Marshall Sat, 05/08/2010 - 02:37
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ex-engineer wrote:


Jon i would use etherchannels.....2 channels carrying a separate set of vlans...


the only reason why i would think of doing that is to minimize the failure domains for layer 2.....


any value you think?ever seen it done?


Not really because even if you do limit the impact of failure often an STP loop in any of the vlans can take out the switches anyway because of the CPU load. I haven't seen this setup before and like i say i can't see a major benefit.


Other way to look at it is if 2 is better than 1 link then why not use 3 links or 4 links etc..  In other words i'm struggling to see a major advantage unless you had a specific vlan or number of vlans that were very high bandwidth and you wanted to segregate this traffic.


Jon

Leo Laohoo Sat, 05/08/2010 - 02:38
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You want to use Etherchannel, ok this I understand.  But each groups of VLANs use a specific trunk link?  This I do not understand the value.  (I don't think you can configure Etherchannel like this either.)

Jon Marshall Sat, 05/08/2010 - 02:42
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leolaohoo wrote:


You want to use Etherchannel, ok this I understand.  But each groups of VLANs use a specific trunk link?  This I do not understand the value.  (I don't think you can configure Etherchannel like this either.)


Leo


You can configure it by simply using the "switchport trunk allowed vlan ..." command but like you i don't see the benefit.


Jon

Leo Laohoo Sat, 05/08/2010 - 02:49
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Learn something new everyday.  Thanks Jon! 

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