Hello Protocol
The TRILL Hello Protocol is similar to ISIS LAN Hello Protocol with the
following differences. Hello packets are sent at the same interval as ISIS Hello packets and are
formatted similarly, with the following exceptions:
- TRILL Hello packets are not padded
- TRILL elects only one Designated Node per LAN
The TRILL Hello packet header is format is shown in the following figure:
TRILL Hello Packet Header
The TRILL Hello includes the following encoded information carried in the
Multi-Topology Aware Port Capabilities TLV (MT-PORT-CAP) and Special VLAN and Flags sub-TLV
(VLAN-FLAG):
- VLAN ID of Designated VLAN
- Copy of VLAN ID used to send TRILL Hello
- 16-bit unique port ID of sending RBridge
- Sending RBridge‘s nickname
- Bypass pseudonode flag (BY)
- VLAN mapping indicator flag (VM)
- Access port indicator flag; no TRILL data traffic (AC)
- Appointed forwarder indicator flag (AF)
- Sending port is a trunk port; no end-station access (TR)
TRILL Hello Port Capabilities TLV
RBridges that detect no other RBridges on a port must assume the role of the Designated RBridge
and be the appointed forwarder for all Forwarding VLANs. If an RBridge detects that the elected
Designated RBridge is no longer active, the remaining RBridges must re-elect a new Designated
RBridge or if no other RBridges are present, the last remaining RBridge must assume the role of
the Designated RBridge.
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