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Transcript of 9781418837051pptch13-110810212650-phpapp02 (1).ppt
CCNA Guide to Cisco Networking Fundamentals
Chapter 13Advanced Switching Concepts
CCNA Guide to Cisco Networking Fundamentals 2
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
CCNA Guide to Cisco Networking Fundamentals 3
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
CCNA Guide to Cisco Networking Fundamentals 4
Spanning Tree Protocol (continued)
CCNA Guide to Cisco Networking Fundamentals 5
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
CCNA Guide to Cisco Networking Fundamentals 6
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
CCNA Guide to Cisco Networking Fundamentals 7
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
CCNA Guide to Cisco Networking Fundamentals 8
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
CCNA Guide to Cisco Networking Fundamentals 9
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
CCNA Guide to Cisco Networking Fundamentals 10
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
CCNA Guide to Cisco Networking Fundamentals 11
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
CCNA Guide to Cisco Networking Fundamentals 12
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
CCNA Guide to Cisco Networking Fundamentals 13
Spanning Tree Protocol (continued)
CCNA Guide to Cisco Networking Fundamentals 14
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
CCNA Guide to Cisco Networking Fundamentals 15
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
CCNA Guide to Cisco Networking Fundamentals 16
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
CCNA Guide to Cisco Networking Fundamentals 17
Virtual LANs (continued)
CCNA Guide to Cisco Networking Fundamentals 18
Virtual LANs (continued)
CCNA Guide to Cisco Networking Fundamentals 19
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
CCNA Guide to Cisco Networking Fundamentals 20
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
CCNA Guide to Cisco Networking Fundamentals 21
CCNA Guide to Cisco Networking Fundamentals 22
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
CCNA Guide to Cisco Networking Fundamentals 23
Dynamic vs. Static VLANs (continued)
• Dynamic VLAN could prove to be more time-consuming 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
CCNA Guide to Cisco Networking Fundamentals 24
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
CCNA Guide to Cisco Networking Fundamentals 25
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
CCNA Guide to Cisco Networking Fundamentals 26
Creating VLANs
• You can create VLANs by entering the (config-vlan)# 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
CCNA Guide to Cisco Networking Fundamentals 27
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
CCNA Guide to Cisco Networking Fundamentals 28
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
CCNA Guide to Cisco Networking Fundamentals 29
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
CCNA Guide to Cisco Networking Fundamentals 30
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
CCNA Guide to Cisco Networking Fundamentals 31
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
CCNA Guide to Cisco Networking Fundamentals 32
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
CCNA Guide to Cisco Networking Fundamentals 33
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
CCNA Guide to Cisco Networking Fundamentals 34
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
CCNA Guide to Cisco Networking Fundamentals 35
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
CCNA Guide to Cisco Networking Fundamentals 36
CCNA Guide to Cisco Networking Fundamentals 37
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
CCNA Guide to Cisco Networking Fundamentals 38
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
CCNA Guide to Cisco Networking Fundamentals 39
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
CCNA Guide to Cisco Networking Fundamentals 40
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
CCNA Guide to Cisco Networking Fundamentals 41
Summary (continued)
• VLAN information is communicated to switches using the VLAN trunking protocol (VTP)