Which of the following frame fields does Cisco FabricPath use to identify the unique FabricPath topology that a unicast frame is traversing?
Of the available choices, Cisco FabricPath uses the forwarding tag (FTAG) frame field to identify the unique FabricPath topology that a unicast frame is traversing. Each topology in FabricPath is assigned a unique tag. For multicast or broadcast traffic, the 10-bit FTAG field contains an ID for a forwarding tree that contains multiple destinations within the topology. The FTAG field is the second of three fields that reside in the FabricPath tag field. It is preceded by the 16-bit Ethertype field and succeeded by the Time to Live (TTL) field.
Cisco FabricPath frames are classic Ethernet frames that are encapsulated with a 16-byte FabricPath header. This header contains a 48-bit outer destination address (ODA), which is a Media Access Control (MAC) address, and a 48-bit outer source address (OSA). In addition, the field contains the 32-bit FabricPath tag. The classic Ethernet frame’s cyclic redundancy check (CRC) field is replaced by a new CRC field that is updated to reflect the additional header data in the frame.
Cisco FabricPath does not use the TTL frame field to identify the unique FabricPath topology that a unicast frame is traversing. The TTL field is the Cisco FabricPath frame field that FabricPath uses to mitigate temporary Open Systems Interconnection (OSI) networking model Layer 2 loops. The TTL field in a FabricPath frame operates similarly to the TTL field in IP networking in that the field is decremented by a value of 1 each time it traverses a new hop. If the TTL expires, the frame is discarded. The TTL field is a 6-bit field that resides at the end of the FabricPath tag field.
Cisco FabricPath does not use an Intermediate System-to-Intermediate System (IS-IS) frame field to identify the unique FabricPath topology that a unicast frame is traversing. In addition, FabricPath does not use a Spanning Tree Protocol (STP) frame field. However, the IS-IS routing protocol is used as a Layer 3 replacement for traditional STP in a Cisco FabricPath topology. In traditional networking, STP is used to prevent Layer 2 switching loops in a topology that contains redundant links. The use of the IS-IS routing protocol ensures that Cisco FabricPath operates as a multipath environment for Layer 2 packets. In other words, IS-IS ensures that Cisco FabricPath is capable of Layer 2 multipath forwarding.
Cisco FabricPath does not use the local ID (LID) frame field to identify the unique FabricPath topology that a unicast frame is traversing. Instead, the LID field stores a 16-bit value that identifies the edge port that a packet is either destined to or sent from. The LID field is the last field in the ODA and OSA fields of a Cisco FabricPath header. The edge port can be either a physical port or a logical port. In addition, the value in the LID field is locally significant to the switch that the frame is traversing.