Introduction to Fabric Path

What is Fabric Path ?

        Cisco Fabric Path is an innovation in Cisco NX-OS software that brings the stability and performance of Layer 3 routing to Layer 2 switched networks to build a highly resilient and scalable Layer 2 fabric. Cisco FabricPath is a foundation for building massively scalable and flexible data centers.. Thus it is a layer 2 routing protocol also sometimes called MAC in MAC Routing.

This is an alternative to running STP. Thus if there is a layer 2 failure due to link cut or layer 2 crash, the entire spanning tree does not need to re-converge which is a feature of STP thus increasing the availabilty of the system. In addition to that, there are no redundant paths as in STP which gives higher bandwidth between the switches by implementing equal cost paths among the links.

How does it work ?

        Cisco Fabric  Path introduces entirely new Layer 2 data plane by encapsulating the frames entering the fabric with a header that consists of routable source and destination addresses. These addresses are called Switch IDs. These addresses are the address of switch in which the frame was received and the address of the destination switch to which the frame is heading. From there the frame is routed until it reaches the remote switch where it is decapsulated and delivered in its original Ethernet format. The figure below shows this simple process.

Fabric Path Terminologies:

Classical Ethernet (CE): It is the regular ethernet with regular flooding and regular STP etc.

The classical ethernet domain has been marked with green border in the figure above.

Leaf Switches : These switches connect CE(Classical Ethernet) domain to FP(Fabric Path) domain.

The switches in the middle of the picture are the leaf switches which are implemented using nexus 5k devices, more precisely nexus 5500 and 7000 F1 and F2 line card modules as nexus 5000 does not support fabric path because fabric path is implemented in hardware.

Spine Swtiches: These switches FP backbone switch with all ports in the FP domain only. The switches on the top of the picture are the spine switches which are implemented using nexus 7k devices.

FP Core Ports: Links the leaf up to spine or between spine switches. The ports in nexus switches are by default in classical ethernet mode ie access mode. Ports within the red border in above picture.To configure it for fabric path the following command should be used.

N7K-1#conf t

N7K-1(config)#int ethernet 1/1

N7K-1(config-if)#switchport mode fabricpath

CE Edge Ports: These ports link the leaf switches connecting to regular classical ethernet domain.

Ports within the green border in above picture.

Fabric Path Switch ID:

           This is analogous to OSPF router id or IS-IS NET id. This identifies the node in IS-IS SPT(Shortest path tree).  This id is automatically generated by default. It can also be assigned manually using the following command in global config mode.

N7K-1#conf t

N7K-1(config)#fabricpath switch-id

Fabric Path Control Plane:

FabricPath IS-IS replaces STP as the control-plane protocol within the FabricPath domain. In other words, FabricPath IS-IS determines the forwarding topology STP(Shortest path tree) instead of STP(Spanning Tree Protocol).    

IS-IS is an industry standard link-state routing protocol. The FabricPath IS-IS implementation is implemented as a single-level IS-IS domain and extended through the definition of FabricPath-specific Type-Length-Value (TLV) fields.

Several characteristics of IS-IS make it ideally suited for use as a Layer 2 forwarding protocol:

●   Has no IP dependency - IS-IS does not require IP reachability in order to form adjacency between devices. While the majority of modern networks do provide IP connectivity for network infrastructure, using IS-IS ensures that no strict requirement for in-band IP connectivity among switches exists.It does not use IP to carry routing information messages. IS-IS is neutral regarding the type of network addresses for which it can route.

●   Easily extensible - Using custom TLVs, IS-IS devices can exchange information about virtually anything. In the case of Layer 3 IS-IS, routers exchange IP prefix reachability. In the case of FabricPath, switches exchange Switch ID reachability.

●   Provides SPF routing - SPF routing protocols have been proven to be scalable, flexible, and fast to converge. In addition, IS-IS supports equal-cost multipath (ECMP) forwarding, allowing data-plane packets to follow any available parallel path rather than restricting forwarding to a single path.

While IS-IS forms the basis of FabricPath, enabling FabricPath in your network requires no specific knowledge of IS-IS - the configuration is plug-and-play. Much as a network operator simply “turns on” STP and interconnects switches, you can enable FabricPath on interfaces and begin forwarding through the FabricPath fabric with minimal configuration.    

IS-IS Commands

IS-IS status

„Show clns : Information about the CLNS network

„Show clns protocol : Lists the protocol-specific information

„Show clns is-neighbor : IS-IS neighbors

„Show clns neighbor : Both ES and IS information

„Show isis database : LSDB

„Show isis topology : ISIS least-cost paths to destinationIS-IS command

„

Router#conf t

Router(config)#router isis      : enable IS-IS

Router(config-router)# net [network entity title]

Router(config-router)#net  49.0401.2101.8302.8174.00

Router(config-router)# is-type  [level-1/level-1-2/level-2-only]

Router(config-router)#

exit

„

Router(config)#interface[interface type] [interface number]

Router(config-if)#ip router isis

Router(config-if)#isis circuit-type[level-1/level-1-2/level-2-only]

: need to repeat for all interfaces that need to run IS-IS

Router#show ip route isis     : check routing table

Spanning Tree Protocol: Port states and roles

Spanning-Tree Port States

1.            Disabled - The port in this state does not participate in the STP operation (it is shut down).

2.            Blocking (BPDU processing state)- The port does NOT forward any Ethernet frames, does NOT accept any Ethernet frames (discards arriving frames), does NOT learn any MAC addresses. However, the portDOES process BPDU frames received from a neighboring switch. If the port transitions to this state (blocking), it can stay blocked for 20 seconds by default (max_age).

3.            Listening (Loop free technology learning state)- The port in this state CAN send and receive the BPDU frames. However, the port in this state does NOT learn any MAC addresses, and does NOT forward or process incoming frames either. All Ethernet frames are being discarded. The computation of loop free topology takes place in this state. If the port transitions to this state (listening), it can stay in this state for 15 seconds by default (forward_delay).

4.            Learning (MAC addresses learning state)- The port in this state already knows its role (root port or designated port ) in the STP domain. However, the port will not forward any Ethernet frames yet. It will be learning MAC addresses from the frames arriving at the port in order to populate MAC address table. This helps avoid too much flooding when the port transition to the forwarding state. If the port transitions to this state (learning), it can stay in this state for 15 seconds by default (forward_delay).

5.            Forwarding (Frames forwarding state)- The port in this state will forward all Ethernet frames as per switch operation. Also, the port will process all incoming Ethernet frames and will actively learn MAC addresses from the arriving traffic.

Spanning-Tree Port Roles

Root Port (RP) - It is a port on a non-root switch, which is the shortest (the best) path towards the root bridge. Root bridge does NOT have any root ports. (no shortest path to itself ;-))

Designated Port (DP) - It is a port that is in the forwarding state. All ports of the root bridge are designated ports (they are never in a blocking state). BPDU frames our sent out this port.

Non-Designated Port (NDP) or Blocking Port - It is a port that is in a blocking state in the STP topology.


                                                             STP PortFast 

PortFast allows switch ports that connect a host device (such as a printer or 
a workstation), to bypass the usual progression of STP states. Theoretically, 
a port connecting to a host device can never create a switching loop. Thus, 
Port Fast allows the interface to move from a blocking state to a forwarding 
state immediately, eliminating the normal 30 second STP delay. 

To configure PortFast on an interface: 

Switch(config)# int fa0/10 
Switch(config-if)# spanning-tree portfast 

To enable PortFast globally on all interfaces: 

Switch(config)# spanning-tree portfast default 

PortFast should not be enabled on switch ports connecting to another 
hub/switch, as this may result in a loop. Note that PortFast does not disable 
STP on an interface - it merely speeds up the convergence. 

PortFast additionally reduces unnecessary BPDU traffic, as TCN BPDU’s 

will not be sent out for state changes on a PortFast-enabled interface.