If an L1 Border RBridge Nickname is configured at an RBridge and that RBridge has both L1 and L2 adjacencies, the multilevel feature as specified in this document is turned on for that RBridge and normally uses an L2 nickname in both L1 and L2 although, as provided below, such an RBridge may have to fall back to multilevel unique nickname behavior [RFC 8397
], in which case it uses this L1 nickname. In contrast, unique nickname multilevel as specified in [RFC 8397
] is enabled by the presence of L1 and L2 adjacencies without an L1 Border RBridge Nickname being configured. RBridges supporting only unique nickname multilevel do not support the configuration of an L2 Border RBridge Nickname. RBridges supporting only the single-level TRILL base protocol specified in [RFC 6325
] do not support L2 adjacencies.
RBridges that support and are configured to use single nickname multilevel as specified in this document MUST
support unique nickname multilevel [RFC 8397
]. If there are multiple border RBridges between an L1 area and L2, and one or more of them only support or are only configured for unique nickname multilevel [RFC 8397
], any of these border RBridges that are configured to use single nickname multilevel MUST
fall back to behaving as a unique nickname border RBridge for that L1 area. Because overlapping sets of RBridges may be the border RBridges for different L1 areas, an RBridge supporting single nickname MUST
be able to simultaneously support single nickname for some of its L1 areas and unique nickname for others. For example, RB1 and RB2 might be border RBridges for L1 area A1 using single nickname while RB2 and RB3 are border RBridges for area A2. If RB3 only supports unique nicknames, then RB2 must fall back to unique nickname for area A2 but continue to support single nickname for area A1. Operators SHOULD
be notified when this fallback occurs. The presence of border RBridges using unique nickname multilevel can be detected because they advertise in L1 the blocks of nicknames available within that L1 area.
In both the unique nickname approach specified in [RFC 8397
] and the single nickname aggregated approach specified in this document, an RBridge that has L1 and L2 adjacencies uses the same nickname in L1 and L2. If an RBridge is configured with an L1 Border RBridge Nickname for any a Level 1 area, it uses this nickname across the Level 2 area. This L1 Border RBridge Nickname cannot be used in any other Level 1 area except other Level 1 areas for which the same RBridge is a border RBridge with this L1 Border RBridge Nickname configured.
In addition to the manageability considerations specified above, the manageability specifications in [RFC 6325
] still apply.
Border RBridges replace ingress and/or egress nickname when a TRILL Data packet traverses a TRILL L2 area. A TRILL Operations, Administration, and Maintenance (OAM) message will be forwarded through the multilevel single nickname TRILL campus using a MAC address belonging to the destination RBridge [RFC 7455