FabricPath provides a multipath Layer 2 domain that does not require STP
for a loop-free environment. Using the Intermediate System-to-Intermediate
System (IS-IS) protocol, the device provides multiple paths for Layer 2
Each FabricPath interface can learn multiple parallel paths to the other
nodes in the FabricPath network. Because you do not need to use STP, all the
paths are available for forwarding traffic. The device assigns the optimal path
The flow for known unicast packets is determined by the hierarchical
FabricPath outer destination address (ODA) and the outer source address (OSA)
value (see “Configuring FabricPath Switching,” for more information about
FabricPath hierarchical encapsulation). The system uses IS-IS Equal Cost
Multipathing (ECMP) to choose the forwarding path for these flows using
FabricPath Layer 2 IS-IS.
For multidestination traffic (unknown unicast, broadcast, and
multicast), the FabricPath system creates two paths or trees. The broadcast and
unknown unicast traffic flows through one of these trees. The system
distributes the multicast traffic between the two trees based on a hash. The
system load balances multicast traffic in the FabricPath network (see the
“Forwarding Trees for Broadcast, Unknown Unicast, and Multicast Packets”
section for more information).
FabricPath Layer 2 IS-IS defines the trees. The highest system ID is
chosen for the root and the tree flows from that. The second tree is the same
but with a different root priority. After the system chooses the root switch,
the tree is built with that as the root for the first tree. Then, the root
switch for the first tree elects the root of the second tree, again based on
the system ID, and the second tree flows from that root switch. All of this
information is advertised to the FabricPath network using Layer 2 IS-IS, so all
the devices in the network have the same information.
The system assigns the path at ingress and encodes that path in the FTag
portion of the FabricPath header. The system assigns one FTag per tree. Once
decided and tagged, the packet uses the same tree throughout the entire
FabricPath network. All the nodes in the FabricPath network forward traffic
based on this same information because all nodes have the same information
using Layer 2 IS-IS.
The FabricPath frame has a Reverse Path Forwarding (RPF) mechanism for
multidestination packets, which verifies that the packet is arriving on an
interface that leads to the source switch. RPF drops the packet if it is
received from an interface that is not part of the tree.
The FabricPath Layer 2 IS-IS protocol floods the link-state information
across the FabricPath network. Each device sends hello packets on each
FabricPath link and discovers its neighbors. When a neighbor is discovered, the
system creates an IS-IS adjacency. Each device also sends advertisements and
updates to the link-state database through all the existing adjacencies.