802.1s Multiple Spanning Tree Protocol (MSTP)
an instance by using a port trunk. The following example shows how you can use a port trunk with 802.1Q (tagged) VLANs and MSTP without unnecessarily blocking any links or losing any bandwidth.
Problem: |
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| Solution: | |||||||||||||||||||||
An MST instance with two |
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| Configure one trunked | |||||||||||||||||||||
separate |
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| link for the two VLAN | |||||||||||||||||||||
links blocks a VLAN link. |
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| memberships. | |||||||||||||||||||||
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Nodes 1 and 2 cannot communicate because MSTP is blocking the link.
Nodes 1 and 2 can communicate because the MST instance sees the trunk as a single link and 802.1Q (tagged) VLANs enable the use of one (trunked) link for both VLANs.
| Figure |
| Connectivity within the Same MST Instance |
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N o t e | All switches in a region should be configured with the VLANs used in that |
| region, and all ports linking MSTP switches together should be members of |
| all VLANs in the region. Otherwise, the path to the root for a given VLAN will |
| be broken if MSTP selects a spanning tree through a link that does not include |
| that VLAN. |
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Terminology
Bridge: See “MSTP Bridge”.
Common and Internal Spanning Tree (CIST): Comprises all LANs, STP, and RSTP bridges and MSTP regions in a network. The CIST automatically determines the MST regions in a network and defines the root bridge (switch)