nx.os.and.cisco.nexus.switching.2nd.edition.1587143046

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Ethernet1/1 TBM12224047 1 Ethernet1/2 b
Ethernet1/3 TBM12224047 1 Ethernet1/4 b
Cisco FabricPath
Historically, Layer 2 networks did not require high levels of scalability because network
administrators designed networks based on the limitations of spanning tree, hardware MAC
address tables, and L2 flooding characteristics. Doing so limited Layer 2 to small pockets
and separated them from one another via L3 routing devices. Leveraging L3 boundaries
around these small pockets of L2 infrastructure allowed network administrators to rely on
more scalable resilient routing protocols to scale their networks. Trends today in the areas
of high-performance computing (HPC), virtualization, and a demand for a more flexible
infrastructure requires many organizations to revisit Layer 2 technologies. New technologies
such as virtual Port Channel (vPC) are emerging to help customers build larger L2 domains;
however, these technologies still fundamentally rely on spanning tree. Spanning tree creates
topologies where local problems such as a link flap can have catastrophic impacts to the
entire infrastructure, whereas more intelligent Layer 3 routing protocols such as OSPF and
EIGRP assume these types of failures and have sophisticated algorithms to handle such
events. These challenges have created the demand for a more flexible protocol, which takes
the best characteristics of Layer 2 and Layer 3 and combines them to meet these demands.
Cisco FabricPath introduces a new way to build and operate Layer 2 networks. FabricPath
enables network operators to build large-scale L2 networks and eliminate challenges
associated with various spanning tree implementations. FabricPath takes well-understood
technologies that have been leveraged at Layer 3 for routing IP packets and uses them to
carry Layer 2 reachability information. In this manner, network administrators gain the
flexibility of large L2 networks and still have the resiliency of large L3 networks.
Cisco FabricPath can be broken down into two distinct areas of operations: control plane
and data plane. The control plane is responsible for building and disseminating information
about the reachability of devices within the network, whereas the data plane is responsible
for the encapsulation, de-encapsulation, and forwarding of data packets across a FabricPath
network.
The FabricPath control plane uses Intermediate System-to-Intermediate System (IS-IS) to
replace spanning tree. IS-IS is a multi-area (level) routing protocol that has no IP
dependencies and is easily extensible using custom TLVs. These characteristics make IS-IS
an attractive way to exchange routing information at Layer 2. The FabricPath control-plane
functions can be broken down into two main areas. First, the FabricPath domain is
composed of a number of switches, which must be uniquely identified. The FabricPath
domain may also consist of a number of disparate logical topologies for isolation of various
types of traffic. Dynamic Resource Allocation Protocol (DRAP) provides the mechanism
for switches to be auto-assigned a unique 12-bit switch ID (SID). This protocol is built on
IS-IS adjacencies, which are formed when an interface first connects to another FabricPath
switch. In addition, the network administrator may choose to statically configure the SID.