VLAN Trunking and Routing Qs

Discussion in 'Cisco' started by Bob Simon, Nov 18, 2008.

  1. Bob Simon

    Bob Simon Guest

    If one has several switches connected via VTP trunks to the central
    switch, does ONLY the central switch need to be configured with the ip
    routing statement and SVIs?

    Is there any reason to change the far end of the trunks from defaults
    (switchport mode dynamic auto and switchport trunk encapsulation
    negotiate) and lock them down to match the trunk config of the central
    switch (switchport mode trunk and switchport trunk encapsulation
    dot1q)?

    Finally, I see that with dot1q there is only one instance of STP
    supported per VLAN. Could someone please explain to me the
    significance of this? When does one need multiple STP instances per
    VLAN?
     
    Bob Simon, Nov 18, 2008
    #1
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  2. Bob Simon

    Trendkill Guest

    On Nov 18, 9:13 am, Bob Simon <> wrote:
    > If one has several switches connected via VTP trunks to the central
    > switch, does ONLY the central switch need to be configured with the ip
    > routing statement and SVIs?
    >
    > Is there any reason to change the far end of the trunks from defaults
    > (switchport mode dynamic auto and switchport trunk encapsulation
    > negotiate) and lock them down to match the trunk config of the central
    > switch (switchport mode trunk and switchport trunk encapsulation
    > dot1q)?
    >
    > Finally, I see that with dot1q there is only one instance of STP
    > supported per VLAN.  Could someone please explain to me the
    > significance of this?  When does one need multiple STP instances per
    > VLAN?


    Answer to the first question is yes. Although for redundancy
    purposes, it is best to have two central switches and HSRP configured
    to mitigate outages during trunk, module, or switch failures. Yes, in
    my opinion it is wise to match your trunk statements and ensure you
    have consistent configs. If for no other reason, its easier to
    recognize the configuration and if/when new resources come in, they
    don't have to ask themselves what the original designer was thinking.

    What do you mean one instance of stp per vlan? Spanning tree runs on
    a per vlan basis (one root, blocks where needed, etc). I am not aware
    of a reason why you would have more than one instance per vlan.
     
    Trendkill, Nov 18, 2008
    #2
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  3. Bob Simon

    Bob Simon Guest

    On Tue, 18 Nov 2008 06:29:46 -0800 (PST), Trendkill
    <> wrote:

    >On Nov 18, 9:13 am, Bob Simon <> wrote:
    >> If one has several switches connected via VTP trunks to the central
    >> switch, does ONLY the central switch need to be configured with the ip
    >> routing statement and SVIs?
    >>
    >> Is there any reason to change the far end of the trunks from defaults
    >> (switchport mode dynamic auto and switchport trunk encapsulation
    >> negotiate) and lock them down to match the trunk config of the central
    >> switch (switchport mode trunk and switchport trunk encapsulation
    >> dot1q)?
    >>
    >> Finally, I see that with dot1q there is only one instance of STP
    >> supported per VLAN.  Could someone please explain to me the
    >> significance of this?  When does one need multiple STP instances per
    >> VLAN?

    >
    >Answer to the first question is yes. Although for redundancy
    >purposes, it is best to have two central switches and HSRP configured
    >to mitigate outages during trunk, module, or switch failures. Yes, in
    >my opinion it is wise to match your trunk statements and ensure you
    >have consistent configs. If for no other reason, its easier to
    >recognize the configuration and if/when new resources come in, they
    >don't have to ask themselves what the original designer was thinking.
    >
    >What do you mean one instance of stp per vlan? Spanning tree runs on
    >a per vlan basis (one root, blocks where needed, etc). I am not aware
    >of a reason why you would have more than one instance per vlan.


    Trendkill,
    I may have misunderstood the 3750 configuration guide section on
    configuring vlan trunks. What I took to be a limitation may, in fact,
    be normal behavior. Here's the exact wording:

    IEEE 802.1Q Configuration Considerations

    The IEEE 802.1Q trunks impose these limitations on the trunking
    strategy for a network:

    In a network of Cisco switches connected through IEEE 802.1Q trunks,
    the switches maintain one spanning-tree instance for each VLAN allowed
    on the trunks. Non-Cisco devices might support one spanning-tree
    instance for all VLANs.
     
    Bob Simon, Nov 18, 2008
    #3
  4. Bob Simon

    Trendkill Guest

    On Nov 18, 10:39 am, Bob Simon <> wrote:
    > On Tue, 18 Nov 2008 06:29:46 -0800 (PST), Trendkill
    >
    >
    >
    > <> wrote:
    > >On Nov 18, 9:13 am, Bob Simon <> wrote:
    > >> If one has several switches connected via VTP trunks to the central
    > >> switch, does ONLY the central switch need to be configured with the ip
    > >> routing statement and SVIs?

    >
    > >> Is there any reason to change the far end of the trunks from defaults
    > >> (switchport mode dynamic auto and switchport trunk encapsulation
    > >> negotiate) and lock them down to match the trunk config of the central
    > >> switch (switchport mode trunk and switchport trunk encapsulation
    > >> dot1q)?

    >
    > >> Finally, I see that with dot1q there is only one instance of STP
    > >> supported per VLAN.  Could someone please explain to me the
    > >> significance of this?  When does one need multiple STP instances per
    > >> VLAN?

    >
    > >Answer to the first question is yes.  Although for redundancy
    > >purposes, it is best to have two central switches and HSRP configured
    > >to mitigate outages during trunk, module, or switch failures.  Yes, in
    > >my opinion it is wise to match your trunk statements and ensure you
    > >have consistent configs.  If for no other reason, its easier to
    > >recognize the configuration and if/when new resources come in, they
    > >don't have to ask themselves what the original designer was thinking.

    >
    > >What do you mean one instance of stp per vlan?  Spanning tree runs on
    > >a per vlan basis (one root, blocks where needed, etc).  I am not aware
    > >of a reason why you would have more than one instance per vlan.

    >
    > Trendkill,
    > I may have misunderstood the 3750 configuration guide section on
    > configuring vlan trunks.  What I took to be a limitation may, in fact,
    > be normal behavior.  Here's the exact wording:
    >
    > IEEE 802.1Q Configuration Considerations
    >
    > The IEEE 802.1Q trunks impose these limitations on the trunking
    > strategy for a network:
    >
    > In a network of Cisco switches connected through IEEE 802.1Q trunks,
    > the switches maintain one spanning-tree instance for each VLAN allowed
    > on the trunks. Non-Cisco devices might support one spanning-tree
    > instance for all VLANs.


    On a per switch instance, I suppose that could work, although not sure
    how that works if you have different trunks with different vlans/
    pruning. Either way, haven't done a lot of non-cisco, so one instance
    per vlan is status quo.
     
    Trendkill, Nov 18, 2008
    #4
  5. Bob Simon

    Stephen Guest

    On Tue, 18 Nov 2008 06:29:46 -0800 (PST), Trendkill
    <> wrote:

    >On Nov 18, 9:13 am, Bob Simon <> wrote:
    >> If one has several switches connected via VTP trunks to the central
    >> switch, does ONLY the central switch need to be configured with the ip
    >> routing statement and SVIs?
    >>
    >> Is there any reason to change the far end of the trunks from defaults
    >> (switchport mode dynamic auto and switchport trunk encapsulation
    >> negotiate) and lock them down to match the trunk config of the central
    >> switch (switchport mode trunk and switchport trunk encapsulation
    >> dot1q)?
    >>
    >> Finally, I see that with dot1q there is only one instance of STP
    >> supported per VLAN.  Could someone please explain to me the
    >> significance of this?  When does one need multiple STP instances per
    >> VLAN?

    >
    >Answer to the first question is yes. Although for redundancy
    >purposes, it is best to have two central switches and HSRP configured
    >to mitigate outages during trunk, module, or switch failures. Yes, in
    >my opinion it is wise to match your trunk statements and ensure you
    >have consistent configs. If for no other reason, its easier to
    >recognize the configuration and if/when new resources come in, they
    >don't have to ask themselves what the original designer was thinking.
    >
    >What do you mean one instance of stp per vlan? Spanning tree runs on
    >a per vlan basis (one root, blocks where needed, etc).


    cisco specific.

    original 802.1q was "standard" with 1 spanning tree.

    other switch manuacturers just like cisco found this painful in
    practice (i worked on Nortel gear for a while) and would implement
    more than 1, up to 1 per vlan.

    this made for even more problems when you hooked the different systems
    together, so eventually we got a multiple spanning tree standard.
    now all we have to do is get all manufacturers to implement it (pretty
    much done) and then use it by default (which as usual cisco do not
    do).

    802.1 <something> specifies MST which lets you have 1 spanning tree
    per set of vlans - and you get to choose which vlans are in which.
    Logical limit of this is one per vlan.

    note you can still get caught. typically you can choose x vlans from
    the total 4k or so - often x is 64 or 256.

    but you may have a lower (much lower) limit on the number of spanning
    trees, and may need to derate that number if you tweak the timers....

    I am not aware
    >of a reason why you would have more than one instance per vlan.

    --
    Regards

    - replace xyz with ntl
     
    Stephen, Nov 19, 2008
    #5
  6. Bob Simon

    Stephen Guest

    On Tue, 18 Nov 2008 08:13:32 -0600, Bob Simon <>
    wrote:

    >If one has several switches connected via VTP trunks to the central
    >switch, does ONLY the central switch need to be configured with the ip
    >routing statement and SVIs?
    >
    >Is there any reason to change the far end of the trunks from defaults
    >(switchport mode dynamic auto and switchport trunk encapsulation
    >negotiate) and lock them down to match the trunk config of the central
    >switch (switchport mode trunk and switchport trunk encapsulation
    >dot1q)?
    >
    >Finally, I see that with dot1q there is only one instance of STP
    >supported per VLAN. Could someone please explain to me the
    >significance of this? When does one need multiple STP instances per
    >VLAN?


    you only need multiple instances if your VLAN carries more than 1
    topology - Xylan tin used to have policy based vlans where the
    topology varied by protocol.

    it managed to generate some whole new classes of network problem that
    no one had seen before......

    1 other instance was where you had token ring bridging where spanning
    tree might be used within transparent switches, but a separate
    spanning tree controlled the "single route explorer" topology for
    source routing. Again this has died with along with the Token Rings
    that spawned it.

    So generally - 1 spanning tree at most per vlan.
    --
    Regards

    - replace xyz with ntl
     
    Stephen, Nov 20, 2008
    #6
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