EMISTP Deployment Constraints
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.
- 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.
- 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.
NoteYou can use MSTP to overcome the EMISTP constraints described in this section.