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Reading this quote,

The 802.1Q standard defines one unique Spanning Tree instance to be used by all VLANs in the network.

STP runs on the Native VLAN so that it can communicate with both 802.1Q and non-802.1Q compatible switches.

This questions is based on 802.1D standard, How would be the behavior between two switches with ISL encapsulation, once which ISL doesn't support Native VLAN?

Beside, using ISL instead of 802.1Q, that won't disrespect the first statement?

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  • I'm not sure I understand the question. By default, Cisco switches use PVST, and that runs STP instances per VLAN, not one for all VLANs like CST.
    – Ron Maupin ♦
    Dec 2 '15 at 19:02
  • Yeah, my question I guess, It's about CST, running a single instance that
    – TMoraes
    Dec 2 '15 at 20:10
  • Cisco switches use more PVST+ than PVST. Was ISL discontinued/removed from Cisco Switches?
    – TMoraes
    Dec 2 '15 at 20:14
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    I used PVST as the generic for PVST/PVST+. ISL still exists on older Cisco switches, and I believe some of the more recent switches can be set to ISL encapsulation to interoperate with them, but some of the low-end switches don't support ISL. I'm still unsure of what you mean in your original question.
    – Ron Maupin ♦
    Dec 2 '15 at 20:21
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    I believe part of why even Cisco ditched ISL is no support for a Native VLAN (aka, receiving an untagged frame when trunking was enabled). As such, if a CST/STP/PVST switch sent a BPDU with no VLAN Tag (dot1q or ISL) the ISL only switch would simply drop the frame.
    – Eddie
    Dec 2 '15 at 23:18
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The first statement deals with 802.1Q, not ISL which is a different protocol, so the first statement has no relationship to ISL. Two switches connected with ISL will not have a Native VLAN because they don't use 802.1Q.

802.1Q has the concept of a native (untagged VLAN) because it inserts a VLAN tag inside the layer-2 frame for tagged VLANs, but just leaves the layer-2 frame alone for untagged (native) VLANs.

ISL encapsulates a layer-2 frame inside an ISL frame for all VLANs, so there is not an untagged (native) VLAN. Each VLAN uses ISL frame encapsulation and runs a separate STP (802.1d) instance.

Maybe you are confusing 802.1Q (tagging) with 802.1d (STP)?

This link has a chart comparing 802.1Q, PVST, and PVST+ which may help clarify this for you.

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6
  • I know, They are differents protocols, 802.1Q support Native Vlans, unlike of ISL, which doesn't support it. My question is about 802.1d but not between two switches running 802.1Q encap, which is well defined on quote above, but between two switches using ISL encap. As STP runs on the Native VLAN how STP runs on ISL? It doesn't have Native VLAN. Got it?
    – TMoraes
    Dec 2 '15 at 21:06
  • You are confusing 802.1Q, which is where that statement comes from, with ISL, which is a completely separate protocol from 802.1Q. ISL does not follow 802.1Q rules. It does use 802.1d STP, just not the 802.1Q extension. Originally for 802.1d, there were no VLANs, so the VLAN concept doesn't apply to 802.1d. 802.1Q came later to allow VLANs for 802.1d. In the meantime, Cisco came up with their own VLAN concept with ISL. In essence, ISL and 802.1Q were in competition to add VLANs to 802.1d.
    – Ron Maupin ♦
    Dec 2 '15 at 21:12
  • What was the year which standardized the 802.1Q?
    – TMoraes
    Dec 2 '15 at 21:20
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    Apparently, 802.1d was standardized by the IEEE in 1990 (Radia Perlman developed STP well before that), and 802.1Q was standardized in 1998. I know Cisco had ISL before 1998, but I'm not sure when it was released.
    – Ron Maupin ♦
    Dec 2 '15 at 21:34
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    Well, 802.1Q is actually more than just a tagging method; it is an extension of 802.1d, incorporating the STP algorithm into the new standard, and adding VLANs to it. ISL, with PVST, just kept the original 802.1d, but encapsulated it in an outer frame. 802.1Q doesn't encapsulate, so the non-tagged BPDUs can't be differentiated between VLANs, which is why there is one 802.1d STP instance for all VLANs. ISL encapsulates everything, so the BPDUs are separated by VLAN.
    – Ron Maupin ♦
    Dec 2 '15 at 21:48

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