Transcript CCNA Guide to Cisco Networking Fundamentals Chapter 13 Advanced Switching Concepts

CCNA Guide to Cisco
Networking Fundamentals
Fourth Edition
Chapter 13
Advanced Switching Concepts
Objectives
• Explain how the Spanning Tree Protocol works and
describe its benefits
• Describe the benefits of virtual LANs
• Configure a VLAN
• Understand the Purpose of the VLAN trunking
protocol (VTP)
• Configure VTP
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Spanning Tree Protocol
• Physical path loops
– A physical connection created when network devices
are connected to one another by two or more physical
media links
– Help improve a network’s fault tolerance
• Drawback
– Can result in endless packet looping
• Spanning Tree Protocol (SP)
– A layer 2 link management protocol designed to
prevent looping on bridges and switches
– The specification for STP is IEEE 802.1d
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Spanning Tree Protocol (continued)
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Spanning Tree Protocol (continued)
• STP uses the Spanning Tree Algorithm (STA)
– To interrupt the logical loop created by a physical loop
in a bridged/switched environment
– STP does this by ensuring that certain ports on some
of the bridges and switches do not forward frames
• Building a logical path
– Switches and bridges on a network use an election
process to configure a single logical path
– First, a root bridge (root device) is selected
– Then, the other switches and bridges configure their
ports, using the root bridge as a point of reference
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Spanning Tree Protocol (continued)
• Bridges use STP to transfer the information about
each bridge’s MAC address and priority number
• Bridge protocol data units (BPDU) or
configuration bridge protocol data units (CBPDU)
– The messages the devices send to one another
• Each bridge or switch determines which of its own
ports offers the best path to the root bridge
• Root ports
– The BPDU messages are sent between the root
bridge and the best ports on the other devices
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Spanning Tree Protocol (continued)
• If BPDUs are not received for a certain period of time
– The non-root-bridge devices will assume that the root
bridge has failed, and a new root bridge will be elected
• Once the root bridge is determined and the switches
and bridges have calculated their paths to the root
bridge
– The logical loop is removed by one of the switches or
bridges
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Spanning Tree Protocol (continued)
• Port states
– STP will cause the ports on a switch or bridge to
settle into a stable state
• Stable states
– The normal operating states of ports when the root
bridge is available and all paths are functioning as
expected
• Transitory states
– Prevent logical loops during a period of transition from
one root bridge to another
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Spanning Tree Protocol (continued)
• The stable states are as follows:
– Blocking
– Forwarding
– Disabled
• The transitory states are as follows:
– Listening
– Learning
• STP devices use the transitory states on ports while
a new root bridge is being elected
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Spanning Tree Protocol (continued)
• Ports on STP-enabled devices move through the
different states as indicated in the following list:
–
–
–
–
–
From bridge/switch bootup to blocking
From blocking to listening (or to disabled)
From listening to learning (or to disabled)
From learning to forwarding (or to disabled)
From forwarding to disabled
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Spanning Tree Protocol (continued)
• Topology changes
– When the topology is changed, STP-enabled devices
react automatically
– If a device in an STP-enabled network stops receiving
CBPDUs, then that device will claim to be the root
bridge
• Will begin sending CBPDUs describing itself as such
• Per-VLAN STP (PVSTP)
– Operates on VLANs and treats all VLANs connected
as separate physical networks
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Spanning Tree Protocol (continued)
• Spanning Tree PortFast
– Allows you to configure a switch to bypass some of
the latency (delay)
• Associated with the switch ports transitioning through
all of the STP transitory states before they reach the
forwarding state
• Configuring STP
– See Table 13-1
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Spanning Tree Protocol (continued)
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Spanning Tree Protocol (continued)
• Rapid STP (RSTP) 802.1w
– Takes the basis of 802.1d (STP) and incorporates
some additional features (such as portfast) that
overcome some of the flaws of STP
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Virtual LANs
• Virtual LAN (VLAN)
– A grouping of network devices that is not restricted to
a physical segment or switch
– Can be configured on most switches to restructure
broadcast domains
• Broadcast domain
– Group of network devices that will receive LAN
broadcast traffic from each other
• Management VLAN (also known as the default
VLAN)
– By default, every port on a switch is in VLAN 1
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Virtual LANs (continued)
• You can create multiple VLANs on a single switch
– Or even create one VLAN across multiple switches
• A VLAN is a layer 2 implementation, and does not
affect layer 3 logical addressing
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Virtual LANs (continued)
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Virtual LANs (continued)
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Benefits of VLANs
• Benefits:
–
–
–
–
Ease of adding and moving stations on the LAN
Ease of reconfiguring the LAN
Better traffic control
Increased security
• VLANs help to reduce the cost of moving employees
from one location to another
– Many changes can be made at the switch
– Physical moves do not necessitate the changing of IP
addresses and subnets
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Benefits of VLANs (continued)
• Because the administrator can set the size of the
broadcast domain
– The VLAN gives the administrator added control over
network traffic
• Dividing the broadcast domains into logical groups
increases security
– Requires a hacker to perform the difficult feat of
tapping a network port and then figuring out the
configuration of the LAN
• VLANs can be configured by network administrators
to allow membership only for certain devices
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Dynamic vs. Static VLANs
• Static VLANs
– Configured port-by-port, with each port being
associated with a particular VLAN
– The network administrator manually types in the
mapping for each port and VLAN
• Dynamic VLAN
– Ports can automatically determine their VLAN
configuration
– Uses a software database of MAC address-to-VLAN
mappings that is created manually
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Dynamic vs. Static VLANs (continued)
• Dynamic VLAN could prove to be more timeconsuming than the static VLAN
• Dynamic VLAN allows the network administration
team to keep the entire administrative database in
one location
• On a dynamic VLAN, moving a cable from one
switch port to another is not a problem
– Because the VLAN will automatically reconfigure its
ports on the basis of the attached workstation’s MAC
address
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VLAN Standardization
• Before VLAN was an IEEE standard
– Early implementations depended on the switch vendor
and on a method known as frame filtering
• Frame filtering
– Complex process that involved one table for each
VLAN
– Had a master table that was shared by all VLANs
• The IEEE 802.1q specification that defines VLANs
recommends frame tagging
– Also known as frame identification
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VLAN Standardization (continued)
• Frame tagging
– Involves adding a four-byte field to the actual Ethernet
frame to identify the VLAN and other pertinent
information
– Makes it easier and more efficient to ship VLAN
frames across network backbones
• Switches on the other side of the backbone can simply
read the frame instead of being required to refer back to
a frame-filtering table
• The two most common types of frame tagging
(encapsulation) are 802.1q and Inter-Switch Link
(ISL) protocol
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Creating VLANs
• You can create VLANs by entering the (configvlan)# mode and using the VLAN command
– Or you can enter the VLAN database and use the
VLAN configuration mode
• To use the config-vlan mode, you type the following:
– Rm410HL(config)#VLAN 2
– Rm410HL(config-vlan)name production
• To use the VLAN configuration mode, you start by
entering the VLAN database
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Creating VLANs (continued)
• The next step is to assign switch ports to the new
VLANs
– Ports can be assigned as static or dynamic
• To remove a VLAN, use the no parameter:
– Rm410HL(config)#no vlan 2
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Link Types and Configuration
• Two types of links are on Cisco switches: trunk links
and access links
• Trunk links
– Switch-to- switch or switch-to-router links that can
carry traffic from multiple VLANs
• Access links
– Links to non-VLAN-aware devices such as hubs and
individual workstations
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Link Types and Configuration
(continued)
• You choose from five different states for a trunk link:
–
–
–
–
–
Auto
Desirable
Nonegotiate
Off
On
• To configure a trunk link on a Catalyst 2950, you
must be in the appropriate interface configuration
mode
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Link Types and Configuration
(continued)
• Switch interface descriptions
– You can configure a name for each port on a switch
– This is useful when you begin to define roles for a
switch port on a more global basis
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VLAN Trunking Protocol
• VLAN trunking protocol (VTP)
– Created by Cisco to manage all of the configured
VLANs that traverse trunks between switches
– A layer 2 messaging protocol that manages all the
changes to the VLANs across networks
• VTP domains
– VTP devices are organized into domains
– Each switch can only be in one VTP domain at a time
• All devices that need to share information must be in the
same VTP domain
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VLAN Trunking Protocol (continued)
• VTP device modes
– Server
• Device can add, rename, and delete VLANs and
propagate those changes to the rest of the VTP devices
– Client
• Device is not allowed to make changes to the VLAN
structure, but it can receive, interpret, and propagate
changes made by a server
– Transparent
• A device is not participating in VTP communications,
other than to forward that information through its
configured trunk links
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VLAN Trunking Protocol (continued)
• VTP pruning option
– Reduces the number of VTP updates that traverse a
link
– Off by default on all switches
• If you turn VTP pruning on
– VTP message broadcasts are only sent through trunk
links that must have the information
• VLAN 1 is not eligible to be pruned because it is an
administrative (and default) VLAN
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Nonswitching Hubs and VLANs
• Important considerations:
– If you insert a hub into a port on the switch and then
connect several devices to the hub, all the systems
attached to that hub will be in the same VLAN
– If you must move a single workstation that is attached
to a hub with several workstations, you will have to
physically attach the device to another hub or switch
port to change its VLAN assignment
– The more hosts that are attached to individual switch
ports, the greater the microsegmentation and flexibility
the VLAN can offer
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Routers and VLANs
• Routers can be used with VLANs to increase
security
– Must be used to manage traffic between different
VLANs
• Routers can implement access lists
– Which increase inter-VLAN security
• A router allows restrictions to be placed on station
addresses, application types, and protocol types
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Routers and VLANs (continued)
• Router can either be an onboard Route Switch
Module (RSM) or an external router
• The router will accept the frame tagged by the
sending VLAN and determine the best path to the
destination address
– The router will then switch the packet to the
appropriate interface and forward it to the destination
address
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Routers and VLANs (continued)
• Router-on-a-stick
– If a single link is used to connect an external router
with the switch containing multiple VLANs
• Trunking is required for inter-VLAN routing
• Trunking is the process of using either ISL or 802.1q
to allow multiple VLAN traffic on the same link
– For instance, an ISL trunk link would encapsulate
each packet with the associated VLAN information
and allow the router to route the packet accordingly
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Summary
• The Spanning Tree Protocol (STP) allows
administrators to create physical loops between
bridges and switches
– Without creating logical loops that would pose a
problem for packet delivery
• The Rapid Spanning Tree Protocol (RSTP) has
enhanced STP to reduce the latency associated
with convergence
• Implementing VLANs via switches provides another
way to increase the performance, flexibility, and
security of a network
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Summary (continued)
• VLANs are separate broadcast domains that are
not limited by physical configurations
• Performance benefits associated with VLANs are
derived from limiting the amount of broadcast traffic
that would naturally pass through a switch without
filtration
• Because traffic on a VLAN broadcast can be
limited to a specific group of computers, security is
also enhanced by making it more difficult for
eavesdropping systems to learn the configuration
of a network
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Summary (continued)
• VLAN information is communicated to switches
using the VLAN trunking protocol (VTP)
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