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Do VLANs have to be defined for trunking?
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I have a 3508 that the only ports in use are defined as trunks.

Do I have to explicitly create vlan entries in the vlan database in
order for the trunks to
pass tagged vlan packets?


joe mcguckin

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You may want to investigate:

Hope this helps.

Brad Reese
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Walter Roberson
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In article <(E-Mail Removed) .com>,
<(E-Mail Removed)> wrote:
:I have a 3508 that the only ports in use are defined as trunks.

o I have to explicitly create vlan entries in the vlan database in
rder for the trunks to
ass tagged vlan packets?

If you have the ports defined as trunks, then you must define
the VLANs that you want passed, as you don't usually want all
trunks to be able to pass all VLANs.

If, though, you have a situation in which all the ports are to be
considered equivilent for trunking purposes, and you just need the
switch to perform switching functions on the packets [e.g, consult the
MAC table to figure out where to pass them on to] then you may be able
to define the ports as access ports and just have the switch pass on
the tagged traffic as if it didn't know anything about tags.

You should check the allowed packet size for the interfaces. Generally
speaking, sometimes if trunking is not enabled, the maximum packet size
permitted on input is the usual ethernet maximum packet size, whereas
when trunking is enabled, slightly larger packets are allowed in order
to accomedate the VLAN tag (or possibly two layers of VLAN tag.) And on
some devices, the larger packets will be permitted through if trunking
is not enabled, but the packet will be counted in the interface
statistics as being a "baby giant" instead of a regular maximum-sized
packet as it would if the port were defined as a trunk.

If you do use this mechanism of defining the trunks as access
ports and just having the packets passed on as raw ethernet data,
then you run into a possible problem with per-VLAN spanning trees.
With the ports defined as access ports, the switch is going to
believe that -all- instances of the same MAC address should be delivered
the same way, which might not be correct: a particular MAC in
one VLAN could have a different path than the same MAC in a different
VLAN. This supposed that you have the possibility of having
the same MAC in different VLANs (e.g., you have policy-map VLAN
classification, or sone of your devices are doing protocol-based
classification, or you have QoS-related classification into
different VLANs), and supposed that there may be VLAN-dependant
topology loops further down.
"Mathematics? I speak it like a native." -- Spike Milligan
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