TRILL Control Plane Protocol

TRILL RBridge network topologies are constructed and managed using ISIS. ISIS was chosen because it does not require IP and is easily extended using new TLVs for carrying TRILL-specific data elements. There are a few TRILL specific protocol additions not covered by ISIS:

TRILL Hello Protocol
MTU Size Probe
Ethernet Station Address Distribution Information (ESADI) Protocol

Although ISIS is used to distribute RBridge and TRILL bridge link information, TRILL‘s use of ISIS is distinctly separate from L3 ISIS. TRILL ISIS control protocol packets use a different L2-ISIS Ethertype and different multicast destination address to exchange control plane information between RBridges as compared to L3 ISIS. TRILL ISIS control protocol packets do not have a TRILL header.

All RBridges must participate in the TRILL protocol using a single Level 1 ISIS area using the fixed area address 0.0.0.0. TRILL ISIS packets are never forwarded by an RBridge. All RBridges should be configured to use the same VLAN ID, called the Designated TRILL VLAN. All TRILL ISIS packets are sent over the Designated TRIILL VLAN except for some TRILL Hello packets.

Each RBridge is identified by its System ID, which defaults to its local MAC address. The System ID can be configured to any 6-octet value. A zero octet is appended to the end of the System ID to form the TRILL ISIS-ID. If the concatenated octet is non-zero, the TRILL ISIS-ID represents a TRILL pseudonode. Pseudonodes are used by TRILL ISIS to identify separate TRILL links over a shared Ethernet segment.

RBridges announce themselves to other RBridges by sending Hellos. There are two types of RBridge Hellos: one for P2P Ethernet links (P2P Hellos) and the other for shared Ethernet or bridged segments (TRILL Hellos). All RBridges must support TRILL Hellos unless specifically configured to use P2P Hellos on a per port basis. P2P links represent directly connected RBridges over a single Ethernet segment and have no directly connected Ethernet end stations. Any received non-TRILL Ethernet data traffic on a P2P link must never be encapsulated and forwarded over the TRILL network nor should native Ethernet packets encapsulated in a TRILL header be decapsulated and transmitted on the Designated VLAN.

The TRILL Hello protocol is used to determine the RBridge that is the Designated RBridge (DRB) on each link based on configured priority and RBridge System ID. If there are multiple RBridges that share the highest priority, the RBridge with the highest TRILL-ID becomes the DRB. If there are multiple links, the DRB will be represented as an RBridge pseudonode. A DRB is selected for both TRILL and native Ethernet links.

The Designated DRB has the following responsibilities:

Determine VLAN ID to use for inter-RBridge communication.
Appoint an RBridge as the appointed forwarder for each VLAN.

RBridges that are appointed forwarders for an Access VLAN are responsible for providing connectivity for all connected devices on the VLAN:

Implementing loop avoidance.
Learning MAC addresses for local connected devices (tuple of port, VLAN, MAC Address).
Learning MAC addresses for remote connected devices (tuple of egress RBridge, VLAN MAC Address).
Listening to STP BPDUs and reporting list of root bridges in its LSP.
Sending TRILL Hello packets on designated VLANs.
Sending Hellos on VLANS for which they are the appointed forwarder.

There are optional responsibilities that may also be performed by the RBridge appointed forwarder. These optional operations include:

Learning local MAC port bindings based on any registration or authentication protocol such as 802.1X.
Observing native IGMP, MLD, and or MRD packets to learn the presence of multicast routers and receivers.
Listening for ESADI messages for learning TRILL RBridge MAC bindings.
Advertising local RBridge MAC bindings in ESADI messages.

Published March 2020prev | next

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