Transparent Inter-Connection of Lots of Links (TRILL) is a technology that addresses the same requirements as the Cisco FabricPath and has almost the same benefits as FabricPath. Unlike FabricPath TRILL, is an IETF standard that was proposed to address the various STP challenges including inefficient utilizations of links, long convergence times, and MAC address table scaling in the Datacenter networks.
Like FabricPath, TRILL uses IS-IS as the control protocol with the idea of taking the advantages of a Layer 3 routing protocol and at the same time maintain the simplicity of a Layer 2 network.
To forward frames, TRILL uses a MAC-in-MAC encapsulation format. The ingress RBridge encapsulates the original Layer 2 frame with a new source and destination MAC (the MAC addresses of the source RBridge, and the next-hop RBridge, respectively), a TRILL Header, (which has the Ingress and Egress nicknames that identify the source and destination RBridge, respectively), and the original Layer 2 frame with a new CRC. The incoming 802.1q or q-in-q tag needs to be preserved in the inner header. Egress RBridge removes the headers added by the ingress RBridge and will forward them based on the inner frame.
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