Question:
What will be
the Cisco 2951 implication of using a VLAN on MST instance 0.
I understand
it’s a bad practise to use MST instance 0 for a production VLAN but I cannot
rationalise and could not find an answer behind and the reasons behind this.
Can someone shade some light on this topic please ??
Answer:
I wouldn't say
that using instance 0 for production VLAN is generally a bad practice.
There are
issues when you have different MST regions or boundaries to STP bridges which
are under different administrative control, Instance 0 is always involved in
such cases. In MSTP, BPDUs are only transmitted in instance 0, the relevant
information of the other instances are contained in supplements called
M-records.
An example:
You have a
boundary to a RSTP bridge in VLAN 200 which is mapped to instance 2. A topology
change comming form that brigde will be forwarded inside your region in
instance 2 (M-record) and in instance 0 ("Main-BPDU"). Thus, you'll
see CAM-table flushing in VLANs mapped to instance 2 (like expected) but also
in VLANs mapped to instance 0 - and this is in most cases not desired.
There's a very recommandable blog Cisco 2951 router in ine.com:
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