CN2668
Routers and Switches (V2)
Kemtis Kunanuraksapong
MSIS with Distinction
MCTS, MCDST, MCP, A+
Agenda
• Chapter 13: Advanced Switching Concepts
• Exercise
• Quiz
Spanning Tree Protocol
• Physical path loops
▫ A physical connection created when connected with
two or more physical media links
▫ Help improve a network’s fault tolerance
▫ Drawback
 Can result in endless packet looping
▫ See Figure 13-1 on Page 366
• Spanning Tree Protocol (STP)
▫ A layer 2 link management protocol designed to
prevent looping on bridges and switches
▫ The specification for STP is IEEE 802.1d
Spanning Tree Protocol (continued)
• Spanning Tree Algorithm (STA)
▫ To interrupt the logical loop created by a physical loop
in a bridged/switched environment
 Some or certain ports on are configured to discard the
frames
• Building a logical path
▫ Switches and bridges on a network use an election
process to configure a single logical path
 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
 The decision based on the lowest number of priority or
lowest MAC address if both have same priority number
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
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
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
• Transitory states
▫ Prevent logical loops during a period of transition
from one root bridge to another while a new root
bridge is being elected
Spanning Tree Protocol (continued)
• The stable states are as follows:
▫ Blocking
 Send and receive STP messages, not the data frames
▫ Forwarding
 Receive and send all data frames, STP messages, and
learn new MAC address
▫ Disabled
• The transitory states are as follows:
▫ Listening
▫ Learning
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
Spanning Tree Protocol (continued)
• Topology changes
▫ When the topology is changed, STP-enabled
devices react automatically
 If CBPDUs are not received, Device will claim to be
the root bridge and continue the path build process
• Per-VLAN STP (PVSTP)
▫ Operates on VLANs and treats all VLANs
connected as separate physical networks
Spanning Tree Protocol (continued)
• Spanning Tree PortFast
▫ Allows you to configure a switch to bypass some of
the latency (delay)
 Only if that device will not create the loop
• Configuring STP
▫ See Table 13-1 on Page 369
• Rapid STP (RSTP)
▫ Use IEEE 802.1w
Virtual LANs
• 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
Virtual LANs (continued)
• Management VLAN (also known as the default
VLAN)
▫ By default, every port on a switch is in VLAN 1
▫ You can create multiple VLANs on a single switch
▫ Or create one VLAN across multiple switches
• A VLAN is a layer 2 implementation, and does
not affect layer 3 logical addressing
• See Figure 13-2 and 13-3 on Page 371
Benefits of VLANs
• Benefits:
▫ Ease of adding and moving stations on the LAN
▫ Ease of reconfiguring the LAN
▫ Better traffic control
 Because the administrator can set the size of the
broadcast domain
▫ Increased security
 VLANs can be configured by network administrators
to allow membership only for certain devices
 See Figure 13-4 on Page 373
Dynamic vs. Static VLANs
• Static VLANs
▫ Configured port-by-port
▫ The network administrator manually types in the
mapping for each port and VLAN
• Dynamic VLAN
▫ Uses a software database of MAC address-toVLAN mappings that is created manually
 Ports can automatically determine their VLAN
configuration
VLAN Standardization
• Frame filtering
▫ One table for each VLAN
▫ Master table that was shared by all VLANs
• Frame tagging
▫ Also known as frame identification
▫ IEEE 802.1q specification
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
• Inter-Switch Link (ISL) protocol
▫ Cisco-based that supports only
Creating VLANs
• 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
▫ Rm410HL #VLAN database
▫ Rm410HL(vlan)#vlan 2 name production
Creating VLANs (continued)
• To assign switch ports to the new VLANs
▫ Ports can be assigned as static or dynamic
▫ Rm410HL(config)#int f0/5
▫ Rm410HL(config-if)#switchport access
vlan 2
• To remove a VLAN, use the no parameter:
▫ Rm410HL(config)#no vlan 2
Link Types and Configuration
• 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
Link Types and Configuration
(continued)
• 5 different states for a trunk link:
▫ Auto – if connected device is set to on or
desireable
▫ Desirable – if connected device is set to on or
desireable, or auto
▫ Nonegotiate – will not negotiate that status with
any other device
▫ Off – is not a trunk interface and will try to disable
▫ On – is not a trunk interface and will try to disable
Link Types and Configuration
(continued)
• To configure a trunk link on a Catalyst 2950
▫ Rm401HL# Conf t
▫ Rm401HL(config)# int fastethernet 0/24
▫ Rm401HL(config-if)# switchport mode trunk
• You can prevent ports from using VLAN
▫ Rm401HL# Conf t
▫ Rm401HL(config)# int fastethernet 0/24
▫ Rm401HL(config-if)# switchport trunk allowed
vlan remove 4
Link Types and Configuration
(continued)
• Switch interface descriptions
▫ You can configure a name for each port on a
switch
▫ Rm401HL# Conf t
▫ Rm401HL(config)# int f0/1
▫ Rm401HL(config-if)# description productionVlan
▫ Rm401HL(config-if)# int f0/24
▫ Rm401HL(config-if)# description trunkbldg777
VLAN Trunking Protocol
• To manage all of the configured VLANs that
traverse trunks between switches
• A layer 2 messaging protocol
• 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
 Rm410HL#vlan database
 Rm410HL(vlan)#vtp domain domainname
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
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
Nonswitching Hubs and VLANs
• Important considerations:
▫ All the systems attached to the hub will be in the
same VLAN
▫ You will have to physically attach the device to
another hub or switch port to change its VLAN
assignment, if you move the device
Routers and VLANs
• To increase security
▫ Must be used to manage traffic between different
VLANs
▫ Implement access lists
 Increase inter-VLAN security
• A router allows restrictions to be placed on
station addresses, application types, and
protocol types
Routers and VLANs (continued)
• 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
Routers and VLANs (continued)
30
CCNA Guide to
Cisco
Networking
Fundamentals,
Fourth Edition
• 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
Assignment
• Review Questions
▫ 1 – 25
• Exercise
Exercise
• Place 2 switches and 2 workstation, do not
connect the cables
• On first switch rename it to server (for easier
understanding)
• Create VLAN as shown on Page 374-375
• Set the VTP domain
▫ Server#vlan database
▫ Server (vlan)#VTP domain RM403
• Create the trunk link on Server. See page 376.
Exercise
• On second switch rename it to client (for easier
understanding)
• Connect UTP cable from WS1 to port 5
• Connect cross-over cable to port 24 on Server
and Client
• Type the following on Client switches
▫ Enable
▫ Show vlan