Switching Types

Multilayer switching is a function performed by a multilayer switch. A
multilayer switch brings the functions of a switch and a router together in one
device. Traditionally, a switch forwards traffic based on Layer 2 information and a
router forwards traffic based on Layer 3 information. With a multilayer switch, the
device can forward traffic that is in the same VLAN at Layer 2 and that same
device has the ability to do inter-VLAN routing and forward traffic between
VLANs at Layer 3.
Multilayer switches can forward traffic at line speed. (Line speed is a nice
way of saying that it forwards the traffic with very little delay.) The forwarding
lookups are done in hardware rather than in software. Software lookups require
that the forwarding decisions be made by the central processor, a process that
takes considerably longer. The hardware that is used to accomplish this is called
an application-specific integrated circuit (ASIC).
The routing and required forwarding information are held in the hardware.
The information is built into tables that are stored in content-addressable memory
(CAM) and ternary content-addressable memory (TCAM). Having this
information readily available in hardware makes the forwarding process much
more efficient, hence the reason for multilayer switches having wide spread use. In
devices today, Cisco Express Forwarding (CEF) is the mechanism that performs
this forwarding and uses these tables. We are going to look at CEF in a little more
detail in the next section.
In order for a multilayer switch to have the ability to route, it needs a route
processor. However, remember that on routers each interface is a separate subnet;
this is a fundamental part of its functionality. A router routes traffic from one
network or subnet to another. On a switch, when we think about the physical ports,
they are Layer 2 ports and cannot have an IP address configured on them. So
where do we put Layer 3 information on a switch?
If you remember from chapters 10 and 11, we always assign the
management IP address for a switch to the VLAN 1 interface by default. This
interface exists by default on all switches. The interface itself is called a switched
virtual interface (SVI). The SVI provides the Layer 3 path and default gateway for
hosts in the VLAN. These SVI are the Layer 3 interfaces that the route processor
uses to route between subnets. On a Layer 2 switch such as 2950 or 2960, there is
only one SVI that can be used at a time. On a multilayer switch, however, you can
create an SVI for each of the VLANs that you wish to route to.
The maximum number of SVIs that you can have on a switch depends on a
few things:
 Switch platform
 Number of supported VLANs
 Processing overhead

TIP: Processing overhead isn’t so much a hard limit but something you will have
to watch. If your switch is acting slow or has a constantly maxed out processor,
then you may have too many interfaces configured or too much data being
routed across the device.
Multilayer switches can also be configured with true routed ports, which
means you can configure the port to act just as a port on a real router would. You
can place the IP address and router interface commands directly on the interface.
The port when configured this way is no longer part of any VLAN; it represents a
subnet just as a router interface would.
Cisco Express Forwarding (CEF)
CEF is the newest Cisco proprietary switching or forward type. CEF makes
the forwarding process in multilayer switches and routers much faster and more
efficient. The history of switching includes a couple of different forwarding types.
Routing
The lineage of switching really started in routers. When you hear switching, you
typically think Layer 2 switches; however, switching is really moving a piece of
data from one interface to another. So, when a router moved a packet from one
interface to another internally, it is really switching the traffic. Routing was the
process the router used to determine which interface had to be the outgoing
interface to move the data toward its destination.
Process Switching
The first switching was just process switching; all of the data packets had to be
processed through the central processor. Process switching was very time
consuming and delayed the packets.
Cache-Based Switching
Cache-based switching has had a few different names—Netflow switching, “route
one switch many”—but they refer to the same thing. When the first packet enters
the device, it is process switched so that all the appropriate information can be
determined. Once the device has the forwarding information, it is cached in a fast
lookup table. When the rest of the packets in the flow enter the device, the
forwarding lookup can be accomplished in RAM by looking at the information in
the cache table.
Cisco Express Forwarding
The current forwarding type is CEF. CEF is also called topology-based switching.
CEF allows for fast forwarding of data at Layers 2 and 3 by building new tables to
hold the information in hardware.

Building the Forwarding Information Base (FIB) Table
CEF builds a new routing table called the forwarding information base
(FIB). The FIB table holds all of the routes the same way the routing table does;
the FIB is constructed from the routing table. CEF is not a routing protocol itself.
It relies on a routing protocol to build and update the routing table from which the
FIB is built. When CEF is enabled, the FIB table has the same authority as the
routing table. If a destination network is not in the FIB when a packet comes into
the device, then the packet is dropped just as it would be with the standard routing
table.
Building the Adjacency Table
CEF also builds an adjacency table that houses the Layer 2 information for
forwarding decisions. The adjacency table is stored in CAM and lookup time is
improved through the use of search algorithms. A key is created to compare the
frame to the table content. Let me give you an example. The destination MAC
address and VLAN ID (VID) of a frame constitute the key for Layer 2 table
lookup. This key is fed into a hashing algorithm, which produces a pointer into the
table. The system uses the pointer to access a smaller specific area of the table
without requiring a search of the entire table.
Preparing to Transmit
Once the forwarding decision has been made and the packet has been
switched across the routing device, the frame must be rewritten to be transmitted
on the new network. IP unicast packets are rewritten on the output interface like
this. First, the device must change the source MAC address from the sender’s to its
own. Second, the device must change the destination MAC address from its
address to the next hop’s address. Third, the time to live (TTL) must be
decremented by one, which means that the IP header checksum must be
recalculated. Last, the frame checksum must be recalculated.

Switching Types

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Switching Types