Saturday, September 15, 2012

Data Center Switching: Cisco Fabric Path

Fabric Path is an innovative designed to bring the stability and performance of routing to Layer 2 and overcome the existing spanning tree challenges in the data center space. Each device in the topology is identified by a switch-id and all Layer 2 forwarding tables are built based on reachability to each switch-id (as opposed to reachability to a MAC address). The switch-id is dynamically assigned via the Dynamic Resource Allocation Protocol (DRAP),However we can manually define the switch IDs also. Fabric Path uses Intermediate-System-to-Intermediate-System (IS-IS) routing protocol which is independent of layer 3 and adds the capability of layer 3 routing in layer 2 domain. The Fabric Path is plug and play and you need to enable “Switchport mode fabricpath” command which automatically starts the protocol. To run this feature one must not have the information of IS-IS prior to this. This is one of the utmost advantage I can see.

There are two types of functions defined in Fabric Path
1. Spine Devices:- These are devices which interconnecting devices by using fabric path and forwarding is done on the basis of switch IDs.
2. Leaf Devices:- Edge devices are part of fabric path as well as of customer facing devices too. In fabric path technology the ports are known to classical Ethernet where we use the dot1q encapsulation. There after mac-in-mac is used to forward the frames on the basis of destination switch IDs.

Below are the terms used in Fabric Path:-
Fabric Path domain – It is layer 2 domain which runs the fabric path and all the traffic which is entering into this domain is fabric path encapsulated.
Fabric Path core port – It is the interface connected to the Fabric Path domain. Normally this interface carries all the vlans which are required to pass thru fabric path.
Fabric Path edge port – It is the classical Ethernet interface where normal Ethernet hits and thereafter is encapsulated into the fabric path.
Spine – All the interfaces which participates in spine must use the fabric path.
Leaf – It is having mix of interfaces like fabric path as well as classical Ethernet.

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