Comments on: VLAN Trunking using ISL http://bradhedlund.com/2007/11/26/vlan-trunking-using-isl/ Studies in Data Center Networking, Virtualization, Computing Wed, 16 May 2012 06:59:43 +0000 hourly 1 http://wordpress.org/?v=3.3.2 By: Greg Hinson http://bradhedlund.com/2007/11/26/vlan-trunking-using-isl/comment-page-1/#comment-25 Greg Hinson Fri, 25 Apr 2008 14:46:59 +0000 http://www.bradhedlund.com/articles/14/ethernet-trunking-using-isl/#comment-25 Thanks for the response Brad. I also found a reference in the exam certification guide, which references your last comment about the ISL range being changed to include 12 bits. Good stuff. Glad I finally got the right information regarding this. http://books.google.com/books?id=dKzpj4r7KCwC&pg=PA42&lpg=PA42&dq=extended+vlan+isl&source=web&ots=qMoWR7NT2P&sig=vZ_DPrgR-K0tvf8DU26DAYUKsOo&hl=en Thanks for the response Brad. I also found a reference in the exam certification guide, which references your last comment about the ISL range being changed to include 12 bits. Good stuff. Glad I finally got the right information regarding this.

http://books.google.com/books?id=dKzpj4r7KCwC&pg=PA42&lpg=PA42&dq=extended+vlan+isl&source=web&ots=qMoWR7NT2P&sig=vZ_DPrgR-K0tvf8DU26DAYUKsOo&hl=en

]]> By: Brad Hedlund http://bradhedlund.com/2007/11/26/vlan-trunking-using-isl/comment-page-1/#comment-23 Brad Hedlund Fri, 25 Apr 2008 03:19:19 +0000 http://www.bradhedlund.com/articles/14/ethernet-trunking-using-isl/#comment-23 Hi Greg, Good question. This is not a typo. ISL does in fact support extended range VLANs. This was not always the case as you indicate. Cisco changed ISL to recognize 12 bits of the 15 bit VLAN field in ISL. From the Catalyst 6500 CatOS 7.6 configuration guide on Cisco.com: "The valid range of user-configured Inter-Switch Link (ISL) VLANs is 1 to 1000 and 1025 to 4094" http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/7.x/configuration/guide/vlans.html#wp1088257 Cheers, Brad Hi Greg,
Good question. This is not a typo. ISL does in fact support extended range VLANs. This was not always the case as you indicate. Cisco changed ISL to recognize 12 bits of the 15 bit VLAN field in ISL.

From the Catalyst 6500 CatOS 7.6 configuration guide on Cisco.com:

“The valid range of user-configured Inter-Switch Link (ISL) VLANs is 1 to 1000 and 1025 to 4094″

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/7.x/configuration/guide/vlans.html#wp1088257

Cheers,
Brad

]]> By: Greg Hinson http://bradhedlund.com/2007/11/26/vlan-trunking-using-isl/comment-page-1/#comment-22 Greg Hinson Thu, 24 Apr 2008 18:14:16 +0000 http://www.bradhedlund.com/articles/14/ethernet-trunking-using-isl/#comment-22 I am reading your remark regarding the support for an extended vlan range support for ISL. Several other documents on Cisco's website indicate that although 15 bits comprise the ISL header, only the lower 10 bits are used. The total number of binary combinations with 10 bits is 1024, dissproving the existance of an extended ISL vlan range. Does this article imply that newer IOS allows a portion of the additional remaining 5 bits to be used for the upper range, or is this a misprint on ISL? Comments? I am reading your remark regarding the support for an extended vlan range support for ISL. Several other documents on Cisco’s website indicate that although 15 bits comprise the ISL header, only the lower 10 bits are used. The total number of binary combinations with 10 bits is 1024, dissproving the existance of an extended ISL vlan range.

Does this article imply that newer IOS allows a portion of the additional remaining 5 bits to be used for the upper range, or is this a misprint on ISL? Comments?

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