One-Arm MLAGs

In some wiring closet applications, bridge port extenders (BPEs) are located in the closet, and the controlling bridge (CB) is located more centrally, and there are only two links wired from the CB to the closet. An important capability of Extended Edge Switching rings is to provide redundancy among BPEs that have only two trusted ports. In this configuration the CB neighbor has only one link to one of the cascades of the ring. To overcome this limitation, you can use a one-arm MLAG to achieve CB redundancy. One-Arm MLAG shows a ring formed using CBs connected to a BPE ring using two one-armed MLAGs. One-armed MLAG uses virtual MLAG ports where a physical port connection is not present. The virtual port is always down, so MLAG blocking rules are not applied.
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One-Arm MLAG

CB-1 and CB-2 are both aware of both MLAGs.

On CB-1, Cascade 1 is configured with port 2 as the native cascade port, and similarly, on CB-2, Cascade 2 is configured with port 2 as the native cascade port.

On CB-1, Cascade 2 is configured to use MLAG 11. On CB-2, Cascade 1 is configured to use MLAG 10. Such configurations are retained by the local CB even if the specified MLAG does not exist when the configuration is first processed.

When the first MLAG comes up, CB-1 and CB-2 elect a master that is the controller of all CSP sessions with BPEs that reside at any tier other than tier 1 (the BPEs directly connected to the CB). The same mastership applies to both MLAGs. Because the MLAGs are one-armed, the master sends messages to such BPEs through the CB that is directly connected to tier 1 of the related cascade. You can determine which CB is the master using the show vpex ports {ports_list} command.

Each CB establishes and controls the CSP session with the tier 1 BPE that is locally connected. The master CB establishes and controls all CSP sessions with BPE that are not at tier 1.