Hello-I am just finishing off chapter on Trunking and wondered what the difference was between the 2 encapsulations-in the book I am reading it says that dot1q uses frame tagging-whereas ISL encapsulates the frame,putting a header in to ID the frame, My question is whether there is an advantage of using one over the other? At my college the only switches we have are 2950 ,which dont support ISL anyway. TIA
The short answer is that ISL is a pre-standard cisco proprietary protocol whereas 802.1q is an IEEE standard. Cisco now reccomends moving away from ISL to 802.1q. So, study ISL for the exams, if you must, but use .1q on the job.
Dot1q is the way to go, as it is an industry standard. ISL is not a standard, and it is dead in the market.
This is what ISL looks like:
The original ethernet frame (including the original FCS check) is encapsulated with a 26 byte header, and a new 4 byte FCS as a trailer. The VLAN identification from which the original frame is associated with is found within 10 bits of the 26 byte header, so ISL had the ability to identify vlans 1 thru 1023.
Dot1q does not use an encapsulation technique. Dot1q uses "tagging," where a
4 byte field is inserted into the original frame immediately after the MAC addresses before traversing the trunk. Because the frame has now been modified by inserting additional bits, the FCS must be re-computed and re-written. Dot1q has a 12 bit field for VLAN identification, allowing for additional VLAN numbering (1-4095).
Cisco is still using ISL "for internal purposes". For example, if you have Catalyst 6500 Supervisor 2 in "Native Mode" (i.e. you have Sup CatOS module and IOS MSFC module), the trunking protocol between Sup and MSFC is ISL. Sure, both modules are proprietary Cisco hardware, but somehow Cisco prefers ISL versus 802.1q in this scenario.
Since an example could be better understood... You have two switches (A and B ) with these VLAN configured ( 1,2,3 on either Switch ) and native VLAN set by default on each switch ( number 1 ) When the switch A need to send frames to switch B owned by VLAN id 2 and 3 it tags them and send them on the trunk. Instead traffic origination from VLAN 1 will not be tagged and will be sent as is over the trunk.
Now you configure switch B trunk to consider "native" traffic for VLAN 3:
When switch A sends untagged traffic for VLAN 1 ( which it considers native ) over the trunk, switch B considers this untagged traffic as native but it will tag it as a member of VLAN 3 ( which switchB considers native ). The same thing happens when switch A sends traffic tagged as VLAN 3. So switch B considers native either the ingress trunk traffic which is untagged or tagged with VLAN id 3. When switch B sends VLAN id 3 traffic over the trunk, since it considers it native, it sends it untagged.
I know, I made a mess and maybe I'll not be understood but I hope you'll get it anyway.
QinQ ( 802.1q tunneling ) is prevalently used when deploying MPLS VPLS. Simply put a frame is double tagged ( tagged twice ). eg. Dest MAC | Source MAC | EtherType | tag | EtherType | tag | EtherType | Data | FCS
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For switches running 802.1Q as the trunking mechanism, the native VLAN of each port on the trunk must match. By default all COS ports are in VLAN 1; and the native VLAN on the IOS devices is also configured for VLAN 1, so the native VLAN does match. If you choose to change the native VLAN, use the set vlan command for COS switches or the switchport trunk native vlan command for IOS switches to specify the native VLAN. Remember that the native VLAN must match on both sides of the trunk link for 802.1Q; otherwise the link will not work. If there is a native VLAN mismatch, Spanning Tree Protocol (STP) places the port in a port VLAN ID (PVID) inconsistent state and will not forward on the link.
My question is this, In what scenario would you need to change the native VLAN?
*ISL is a Cisco proprietary protocol* for the interconnection of multiple switches and maintenance of VLAN information as traffic goes between switches. In ISL, the original frame is encapsulated and an additional header is added before the frame is carried over a trunk link. First ISL came that works only for cisco devices later *. ISL is supported in Cisco 1900 series switches only. But 802.1q is su pported in cisco 2900 and above series switches.
In 802.1Q(IEEE standard) the trunking device inserts a 4-byte tag into the original frame and recomputes the frame check sequence (FCS) before the device sends the frame over the trunk link.switch
_*ISL support only cisco switches but 802.1q support cisco and non cisco switches both.*___