Although EMISTP greatly enhances STP
capability, these features must deployed with care.
This section describes configuration issues that, if not followed, could
lead to an improper deployment of EMISTP. This section also provides the following
restrictive principles to abide by in network design:
Although a physical port can belong to multiple
STPDs, any VLAN on that port can be in only
one domain. Put another way, a VLAN cannot belong to two STPDs on the same physical
port.
Although a VLAN can span multiple domains, any LAN segment in
that VLAN must be in the same STPD. VLANs traverse STPDs only inside switches, not
across links. On a single switch, however, bridge ports for the same VLAN can be
assigned to different STPDs. This scenario is illustrated in VLANs Traverse Domains Inside Switches.
VLANs Traverse Domains Inside Switches
The VLAN partition feature is deployed under the premise that
the overall inter-domain topology for that VLAN is loop-free. Consider the case in
Looped VLAN Topology, VLAN red (the only VLAN in the
figure) spans STPDs 1, 2, and 3. Inside each domain, STP produces a loop-free
topology. However, VLAN red is still looped, because the three domains form a ring
among themselves.
Looped VLAN Topology
A necessary (but not sufficient) condition for a loop-free
inter-domain topology is that every two domains only meet at a single crossing
point.
Note
You can use MSTP to overcome the EMISTP
constraints described in this section.
