Diff. with ISL and 1.q

chinamanchinaman Posts: 167Inactive Imported Users
I read the ICND book regarding the ISL and 1.q tagging, in that book it mentioned that the key diff, between ISL and 1.q is the native Vlan.

Okay I know that 1.q has no vlan header and recalculation of FCS.
ISL encapsulate the original frame in 26 header and 4 byte fcs.

both isl and 1.q support 12 bit vlan ID and both support span tree per vlan.

1900 SW is an ISL and 2950 only suports 1.q,

Can anyone help me regading the native vlan ? It is not mentioned on the sybex book and not also on the CBt videos.

Thanks..

Comments

  • EdTheLadEdTheLad Posts: 2,112Member ■■■■□□□□□□
    chinaman wrote:
    I read the ICND book regarding the ISL and 1.q tagging, in that book it mentioned that the key diff, between ISL and 1.q is the native Vlan.

    Can anyone help me regading the native vlan ? It is not mentioned on the sybex book and not also on the CBt videos.

    Thanks..

    The native vlan is the vlan that is assigned to all ports by default in the switch.This happens to be vlan 1,all ports by default are members of vlan 1,since all ports are access ports by default this vlan id will be stripped from the frames when they exit the switch so you dont really need to think about this tagging.
    So it looks like your books are saying 1.q supports native vlan while ISL doesnt.
    Networking, sometimes i love it, mostly i hate it.Its all about the $$$$
  • chinamanchinaman Posts: 167Inactive Imported Users
    thanks for the info, I understand that all ports on the switch 2950 are part of vlan 1 by default, how about 1900 is it also by default native vlan 1?
  • bmaurobmauro Posts: 307Member
    A 1900 switch can only run ISL trunking - not dot1q, and ISL does not have a native vlan.

    So I'm not sure what you're asking. A 1900 switch will not have a native vlan since it runs ISL - which means that all frames will be encapsulated in ISL

    Whereas a 2950 switch running dot1q will not add a dot1q header on its native vlan fames - since its understood that a frame lacking dot1q header means it is a member of the native vlan.

    Hope that helps.
  • mikej412mikej412 Posts: 10,090Member
    chinaman wrote:
    the key diff, between ISL and 1.q is the native Vlan.
    ISL tags all the vlan traffic, including the native vlan. So all traffic going through an ISL trunk is tagged.

    802.1q tags everything except the native vlan traffic. Untagged traffic going through an 802.1q trunk is the native vlan traffic.
    :mike: Cisco Certifications -- Collect the Entire Set!
  • bmaurobmauro Posts: 307Member
    Mike - got a quick question on what you posted.

    For dot1q - the native vlan is "vlan 1" and if a frames destination is vlan 1 dot1q will not add it's header and a trunk port will see this lack of a header to mean its a member of the native vlan (vlan 1)

    For ISL - it does not understand "native vlan" and encapsulates all frames. So for ISL it just tags the frame as Vlan 1, and really does not have a native vlan.

    Just want to make sure I got this.
  • mikej412mikej412 Posts: 10,090Member
    bmauro wrote:
    For ISL - it does not understand "native vlan" and encapsulates all frames. So for ISL it just tags the frame as Vlan 1, and really does not have a native vlan.

    Yeah, that's it! It does get confusing.....

    VLAN 1 is named "default"

    Everything starts out in VLAN 1 unless configured otherwise -- ISL or dot1Q.

    The 802.1q default native vlan is VLAN 1.....

    Native VLAN is an 802.1q concept -- 802.1q doesn't tag traffic in the native vlan.

    So yeah, it doesn't make sense to say native vlan with ISL...... In the simple default case, ISL tags everything (including the default VLAN 1).

    802.1q tags every thing except the native vlan (which defaults to VLAN 1 which is named default)...

    In real life with 802.1q you usually pick a VLAN other than the default VLAN1 to be the 802.1q Native VLAN.
    :mike: Cisco Certifications -- Collect the Entire Set!
Sign In or Register to comment.