Is It Routing or Is It Layer 3 Switching?
YES!
Learn It and Use It
Cisco Networking Academy, US/Canada
Pete Anderson
Steve Stiles
Assistant Professor
Davenport University
Instructor
James A Rhodes State College
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1
• Introductions – 5 minutes
• What is Layer 3 Switching?– 5
minutes
• How does it relate to Network
Design? – 5 minutes
• Comparison Routers/L3 Switches
– 5 minutes
• Technologies – 35 minutes
• Hands-On Labs - 35 minutes
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2
Introductions
What is Layer 3
Switching?
• Layer 3 switching speeds approximate
that of Layer 2 switches
• Hardware-based routing using
Application-Specific Integrated Circuits
(ASICs)
• Make use of TCAM (Ternary Content
Addressable Memory) for routing, ACL
lookups, policy etc.
• Make use of CEF (Cisco Express
Forwarding)
• RIP, OSPF, and EIGRP are supported
• Future: Pure Layer 3 environment
leveraging inexpensive L3 access layer
switches
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• Layer 4 switching enables
load balancing based on
Layer 4 port number
• Layer 7 switching uses
Network-Based Application
Recognition (NBAR) to
permit or deny traffic based
on data passed by an
application
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 CEF uses special strategies to switch data packets to their destinations
expediently. It caches the information generated by the Layer 3 routing engine
even before the switch encounters any data flows.
 CEF caches routing information in one table (FIB) and caches Layer 2 next-hop
addresses and frame header rewrite information for all FIB entries in another
table, called the adjacency table (AT).
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How does it relate to Network
Design?
• Layer 3 switches usually have packet-switching throughputs in the
millions of packets per second (pps)
• Traditional general-purpose routers provide packet switching in the
range of 100,000 pps to over 1 million pps
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• Aggregates distribution layer switches.
• Implements scalable protocols and technologies and load balancing.
• High-speed layer 3 switching using 10-Gigabit Ethernet.
• Uses redundant L3 links.
For small networks, a
core layer is not needed.
In this case, the
distribution layer switches
need to be fully meshed.
Recommended practice
is to deploy a dedicated
core layer to connect 3 or
more physical segments
or 4 or more pairs of
building distribution
switches.
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• High availability, fast path recovery, load balancing, QoS, and security
• Route summarization and packet manipulation
• Redistribution point between routing domains
• Packet filtering and policy routing to implement policy-based connectivity
• Terminate VLANs
• First Hop Redundancy Protocol
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• Convergence – provides inline Power over Ethernet (PoE) to support
IP telephony and wireless access points.
• Security – includes port security, DHCP snooping, Dynamic ARP
inspection, IP source guard.
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Comparison Routers/L3
Switches
Similarities
Differences
Both routers and multilayer
switches use routing
protocols or static routes to
maintain information about
reachability and direction to
network destinations
(prefixes) and record this
information in a routing table.
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Routers connect
heterogeneous networks and
support a wide variety of
media and interfaces.
Multilayer switches typically
connect homogenous
networks. Nowadays LAN
switches are mostly Ethernet
only.
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Similarities
Differences
Perform the same functional
packet switching actions:
Multilayer switches use
specialized hardware to achieve
wire-speed Ethernet-to-Ethernet
packet switching.
1. Receive a frame and strip
off the Layer 2 header.
2. Perform a Layer 3 lookup
to determine the outbound
interface and next hop.
3. Encapsulate the packet in
a new Layer 2 frame and
transmit the frame.
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Low- to mid-range routers use
multipurpose hardware to
perform the packet-switching
process.
On average, the packetswitching throughput of routers is
lower than the packet-switching
throughput of multilayer
switches.
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Similarities
Differences
Routers usually support a
wider range of features,
mainly because switches
need specialized hardware to
be able to support certain
data plane features or
protocols. On routers, you
can often add features
through a software update.
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Technologies
A Logical Demonstration of a Multilayer Switch:
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SVIs
Switched Virtual
Interfaces
• Configured on multilayer switches, one per VLAN.
• The management interface on an L2 switch is an SVI, but an
L2 switch is limited to one active SVI.
• An SVI associates with an L2 VLAN – a switch must have
an active L2 instance of a VLAN in order for an (L3) SVI to
function.
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Switch(config)# ip routing
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
Switch(config)# interface vlan 10
Switch(config-if)# ip address 10.10.1.1 255.0.0.0
Switch(config-if)# no shutdown
Switch(config-if)# interface vlan 20
Switch(config-if)# ip address 10.20.1.1 255.255.255.0
Switch(config-if)# no shutdown
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Layer 2 Etherchannels
• Up to 8 physical links can be
bundled into a single logical
EtherChannel link.
• Usually EtherChannel is used for
trunk links.
• Configuration applied to port
channel interface affects all
physical interfaces assigned to
the port channel.
• Load balancing takes place
between the physical links in an
EtherChannel.
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• Port Aggregation Protocol (PAgP) is a Cisco-proprietary
protocol that aids in the automatic creation of Fast
EtherChannel links.
• Link Aggregation Control Protocol (LACP) is part of an
IEEE specification (802.3ad) that also enables several
physical ports to be bundled together to form an
EtherChannel.
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Mode
Purpose
Auto
Places an interface in a passive negotiating state in which the interface responds to the
PAgP packets that it receives but does not initiate PAgP negotiation (default).
Desirable
Places an interface in an active negotiating state in which the interface initiates negotiations
with other interfaces by sending PAgP packets. Interfaces configured in the “on” mode do
not exchange PAgP packets.
On
Forces the interface to channel without PAgP.
Non-silent
If a switch is connected to a partner that is PAgP-capable, configure the switch interface for
non-silent operation. The non-silent keyword is always used with the auto or desirable mode.
If you do not specify non-silent with the auto or desirable mode, silent is assumed. The silent
setting is for connections to file servers or packet analyzers; this setting enables PAgP to
operate, to attach the interface to a channel group, and to use the interface for transmission.
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Mode
Purpose
Passive
Places a port in a passive negotiating state. In this state, the port responds
to the LACP packets that it receives but does not initiate LACP packet
negotiation (default).
Active
Places a port in an active negotiating state. In this state, the port initiates
negotiations with other ports by sending LACP packets.
On
Forces the interface to the channel without PAgP or LACP.
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Switch(config)# interface fastethernet 0/23
Switch(config-if)# channel-group 2 mode active
Switch(config)# interface fastethernet 0/24
Switch(config-if)# channel-group 2 mode active
Switch(config)# interface port-channel 2
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk native VLAN 99
Switch(config-if)# switchport trunk allowed VLAN 2,3,99
Remote Switch configuration
RSwitch(config)# interface fastethernet 0/23
RSwitch(config-if)# channel-group 5 mode on
RSwitch(config)# interface fastethernet 0/24
RSwitch(config-if)# channel-group 5 mode on
RSwitch(config)# interface port-channel 5
RSwitch(config-if)# switchport mode trunk
RSwitch(config-if)# switchport trunk native VLAN 99
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• When several port-channel interfaces are configured on the same device, the
show etherchannel summary command is useful for displaying one-line
information per port-channel.
• Layer 2 EtherChannels are all in use (SU next to the port-channel number).
Switch# show etherchannel summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use f - failed to allocate aggregator
M - not in use, minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
Number of channel-groups in use: 2
Number of aggregators: 2
Group
Port-channel Protocol
Ports
------+-------------+-----------+-------------------------------------------2
Po2(SU)
LACP g0/49(P) g0/50(P) g0/51(P) g0/52(P)
7
Po7(SU)
LACP g0/47(P) g0/48(P)
9
Po9(SU)
PAgP g0/8(P) g0/9(P)
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Routed Ports
• Use the no switchport
command to configure a
physical switch port as a
routed port. (3560)
• Routed ports are used in
conjunction with SVI’s.
• Routed ports connect point-
to-point (L3) links between
distribution layer and core
layer switches.
• A 48-port L3 switch can be
configured as a 48-port router.
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Switch(config)# interface GigabitEthernet 1/1
Switch(config-if)# no switchport
Switch(config-if)# ip address 10.10.1.1 255.255.255.252
Switch(config-if)# exit
** Switch will let us know when we get forgetful **
Switch(config)# interface GigabitEthernet 1/2
Switch(config-if)# ip address 10.20.1.254 255.255.255.252
% IP addresses may not be configured on L2 links.
Switch(config-if)# no switchport
Switch(config-if)# ip address 10.20.1.254 255.255.255.252
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Layer 3 Etherchannels
• Just as with physical
interfaces on multilayer
switches, bundles of
interfaces (port channels)
can be configured as routed
ports.
• Port channels configured as
routed ports are called L3
EtherChannels.
• L2 EtherChannels are
normally used only when
connecting from an access
layer switch.
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