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Fibre Channel (SAN)

Posts: 1
Registered: ‎12-29-2011

how do I add more isl ports to an existing trunk group

i have 2 switches in a fabric with 2 isl's in a truck group. i have added new isl port to the switches but they have joined a new truck group. i would like to have all three ports in the same truck group.


not sure how to do this.

New Contributor
Posts: 2
Registered: ‎12-07-2011

Re: how do I add more isl ports to an existing trunk group

A new ISL connection between the same pair of switches should automatically add to the same trunk as long as all the requirments are met.


The most common issue is that you must have all the ports in the trunk in the same port group. A port group is a group of eight ports based on the user port number ie 0-7 or 8-15. This has to be the same for the switches on both sides of the trunk. If either of them have one of the ISL connections in a seperate port group it will create a new seperate trunk.


Occasional Contributor
Posts: 13
Registered: ‎06-14-2004

Re: how do I add more isl ports to an existing trunk group

The main criteria to maeet for Trunkable ISLS are

  1. the physicla FC ports must be in a common physical trunkable port group on both SAN Switches or directors
    e.g. ports 0, 3, and 7 in trunk port group 0-7 on switch 1 and  ports 64, 65, 71 in trunk port group 64-71 on switch 2
  2. all the physical FC ports used for the ISLs must have trunking enabled
  3. the individual ISL link speeds must be the same; currently all 1G, all 2G, all 4G, all 8G, or all 16G.
  4. the actual "delay" distance on all the ISLs to be trunked together must match to within 500 metres.
    So a 13.1 km ISL and a 13.6km ISL MAY form a trunked ISL..  While a 13.1 km and a 13.7 km ISL will never form a trunked ISL.
    Different DWDMs add different "delay" distances to FC links. 
    The perceived DWDM delay is also affected by the  FC speed set; we had a Cisco DWDM that added 10km of delay distance at 4G and 2km at 2G

  5. other ISL port configuration related values like link vc init mode must also match.

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