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Dissecting Cisco's FabricPath Ethernet technology

Cisco's data center network architecture has many vital features and moving parts

By , Network World
March 29, 2012 03:58 PM ET

Network World - Cisco's FabricPath data center Ethernet technology is designed to combine traditional, Spanning Tree-based Ethernet with a next-generation architecture that uses a link-state protocol to allow for multiple active paths. Deploying multiple active paths in a data center network is required to flatten the infrastructure to reduce latency and better support traffic flow between server racks.

The following attempts to boil down the key aspects of FabricPath on a Cisco Nexus 7000 core data center switch, as described in a Cisco whitepaper. Several features of FabricPath are vital to its operation, including:

• Replacing Spanning Tree with a link-state protocol;

• Interacting with Spanning Tree domains;

• Use of virtual PortChannel (vPC) to activate parallel paths and remove Spanning Tree blocks;

• Defining processes as virtual device contexts (VDC);

• Conversational MAC address learning and use of switch IDs (SID), and;

• VLAN designations

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Use of a link-state protocol

Using or replacing Spanning Tree with a link-state protocol is intended to overcome the limitations of Spanning Tree in data center and cloud environments - chiefly, the inability to use multiple active parallel paths in an Ethernet network. Spanning Tree allows only one path to be active between any two nodes and blocks the rest, which is unsuitable for low latency, Ethernet-based fabrics in data centers and cloud environments.

Virtually every vendor addressing the data center fabric switching market proposes augmenting or replacing Spanning Tree with a link-state protocol, and standards like TRILL and Shortest Path Bridging are being defined to do just that. Brocade uses TRILL in the data plane of its VCS architecture but the control plane is based on Fabric Shortest Path First, an ANSI standard used by all Fibre Channel SAN fabrics as the link-state routing protocol.

Juniper chose to implement a tagging mechanism in the Broadcom silicon in its QFabric switches rather than a link-state protocol. The company says this approach flattens the fabric, simplifies management, reduces latency and reduces cost.

Cisco pitches FabricPath as a "superset" of TRILL. FabricPath also uses a Shortest Path First (SPF) routing protocol to determine reachability and path selection in the FabricPath domain. Cisco says its SPF implementation is an industry standard IS-IS routing protocol with FabricPath-specific extensions, like exchanging switch ID (SID) reachability instead of IP prefixes. FabricPath also employs equal-cost multipath (ECMP) forwarding to make use of all available bandwidth.

Arista Networks also uses ECMP for Layer 3 fabrics, and Multichassis Link Aggregation for Layer 2 deployments.

Spanning Tree interaction

Even though it replaces Spanning Tree with a link-state protocol, FabricPath can also support and interact with Spanning Tree domains. FabricPath edge switches transmit and process Spanning Tree Bridge Protocol Data Units (BPDU) and participate in building the Spanning Tree forwarding topology in each connected Spanning Tree domain.

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