Sybex CCNA 640-802
Chapter 8: Layer-2 Switching
Instructor & Todd Lammle
Chapter 8 Objectives
The CCNA Topics Covered in this chapter include:
• What is layer-2 switching
• Switching services
• Bridges vs. LAN switching
• Three switch functions
• MAC table
• Switching loops Spanning-Tree Protocol (STP)
2
Layer 2 Switching
• Purposes for using switching
1. Breaks up collision domains
2. Cost-effective, resilient internetwork
• Purpose for Spanning-Tree Protocol (STP)
– Stops loops in layer 2 switched networks
Before Layer 2 Switching
Switched LANs
Typical Switched Designs
One link to the server!
Switching Services
Layer 2 switching provides:
–
–
–
–
Hardware-based bridging (ASIC)
Wire speed
Low latency
Low cost
Limitations of Layer 2 Switching
• Must break up the collision domains correctly.
• Rule of Thumb: Make sure that users spend
80 percent of their time on the local segment.
• Switches do not break up broadcast domains
by default.
– Or at all , unless they are layer 3 switches.
Bridging vs. LAN switching
• Bridges are software based, switches are hardware based
using ASIC chips to filter decisions.
• A switch can be viewed as a multiport bridge.
• Bridges can only have one spanning-tree instance per
bridge, and switches can have many.
• Switches have a higher number of ports than most bridges.
• Both of them forward layer 2 broadcasts.
• Both of them learn MAC addresses by examining the source
address of each frame received.
• Both of them make forwarding decisions based on layer 2
addresses.
Three Switch Functions at Layer-2
• Address learning
– Layer 2 switches and bridges remember the source hardware address
of each frame received on an interface, and they enter this information
into a MAC database called a forward/filter table.
• Forward/filter decisions
– When a frame is received on an interface, the switch looks at the
destination hardware address and finds the exit interface in the MAC
database. The frame is only forwarded out the specified destination
port.
• Loop avoidance
– If multiple connections between switches are created for redundancy
purposes, network loops can occur. Spanning Tree Protocol (STP) is
used to stop network loops while still permitting redundancy.
Empty MAC table
Nothing here at this point
How Switches Learn Hosts’ Locations (p 501)
Switching Loops
Switching Loop Problems
The MAC address filter table could be totally confused
about the device’s location because the switch can receive
the frame from more than one link.
Spanning-Tree Protocol (STP)
Solves Switching loops at layer 2
•
•
•
•
•
STP
Root Bridge
BPDU
Bridge ID
Nonroot Bridge
•Root port
•Designated port
•Port cost
•Nondesignated
port
•Forwarding port
•Block port
Spanning-Tree Terms
• Root bridge: The root bridge is the bridge with the best
bridge ID.
– With STP, the key is for all the switches in the network to elect a root
bridge that becomes the focal point in the network.
– All other decisions in the network—such as which port is to be blocked
and which port is to be put in forwarding mode—are made from the
perspective of this root bridge.
• BPDU: All the switches exchange information to use in the
selection of the root switch as well as in subsequent
configuration of the network.
– Each switch compares the parameters in the Bridge Protocol Data Unit
(BPDU) that it sends to one neighbor with the one that it receives from
another neighbor.
• Bridge ID: The bridge ID is how STP keeps track of all the switches in
the network. It is determined by a combination of the bridge priority
(32,768 by default on all Cisco switches) and the base MAC address.
• The bridge with the lowest bridge ID becomes the root bridge.
Spanning-Tree Terms
• Nonroot bridges: All bridges that are not the root bridge.
• Port cost: Port cost determines the best path when multiple
links are used between two switches and none of the links
is a root port.
– The cost of a link is determined by the bandwidth of a link.
• Root port: The root port is always the link directly connected
to the root bridge, or the shortest path to the root bridge.
– If more than one link connects to the root bridge, then a
port cost is determined by checking the bandwidth of each
link.
– The lowest-cost port becomes the root port.
– If multiple links have the same cost, the bridge with the
lower advertising bridge ID is used.
– Since multiple links can be from the same device, the
lowest port number will be used.
Spanning-Tree Terms
• Designated port: A designated port is one that has been
determined as having the best (lowest) cost.
– A designated port will be marked as a forwarding port.
• Nondesignated port: A nondesignated port is one with a
higher cost than the designated port.
– Nondesignated ports are put in blocking mode—they are
not forwarding ports.
• Forwarding port: A forwarding port forwards frames.
• Blocked port: A blocked port is the port that, in order to
prevent loops, will not forward frames.
– However, a blocked port will always listen to frames.
Spanning-Tree Operations
• Selecting the root bridge
• Selecting the designated port
• Blocking:
Spanning-Tree Port States
A blocked port won’t forward frames; it just
listens to BPDUs. The purpose of the blocking state is to
prevent the use of looped paths.
• Listening:
The port listens to BPDUs to make sure no loops
occur on the network before passing data frames. A port in
listening state prepares to forward data frames without
populating the MAC address table.
• Learning:
The switch port listens to BPDUs and learns all the
paths in the switched network. A port in learning state populates
the MAC address table but doesn’t forward data frames.
• Forwarding: The port sends and receives all data frames
on the bridged port. If the port is still a designated or root port at
the end of the learning state, it enters the forwarding state.
• Disabled: port in the disabled state (administratively) does not
participate in the frame forwarding or STP.
Spanning-Tree Example
Written Labs and Review Questions
– Open your books and go through all the written labs and the
review questions.
– Review the answers in class.
23
Post-Class Material: Collapsed Backbone, (pp 494ff)
The figure below shows the old model of a network, prior to switches. It was called a
collapsed backbone because all hosts would need to go to the corporate backbone to
reach any network services—both LAN and mainframe.
Root Bridge Elections:
(pp 508ff)
• Each switch will have a Bridge ID Priority value, more
commonly referred to as a BID.
• This BID is a combination of a default priority value and the
switch's MAC address, with the priority value listed first!
• For example, if a Cisco switch has the default priority value of
32,768 and a MAC address of 11-22-33-44-55-66, the BID
would be 32768:11-22-33-44-55-66. This is the KEY!
• Therefore, if the switch priority is left at the default, the MAC
address is the deciding factor.
• Switches are a lot like people - when they first arrive, they
announce that they are the center of the universe! Unlike some
people, the switches will soon get over it. BPDUs will be
exchanged until one switch is elected Root Bridge, and it's the
switch with the lowest BPDU that will end up being the Root
Bridge.
Root Bridge Elections
VLAN 10
VLAN 20
VLAN 30
BID Election 2
• If STP is left totally alone, a single switch is going to be the
root bridge for every single VLAN in your network!
• Worse, that single switch is going to be selected because it has
a lower MAC address than every other switch, not the criteria
you want to use to select a single root bridge.
• You will prefer to determine a particular switch to be the root
bridge for your VLANs, or you may want to spread the root
bridge workload around.
• [There are 2 ways to change the BID. The first is below:]
• You can do this with the cmd: spanning-tree vlan root
SW1(config)#spanning-tree vlan 1 ?
forward-time Set the forward delay for the spanning tree
hello-time
Set the hello interval for the spanning tree
max-age
Set the max age interval for the spanning tree
priority
Set the bridge priority for the spanning tree
root
Configure switch as root
• In this example, we've got two switches
– SW1 has been elected the root bridge for VLANs 10, 20, & 30. We'll use
the spanning-tree vlan root command on SW2 to make it the root bridge
for VLANs 20 and 30.
– SW2(config)#spanning-tree vlan 20 root primary
SW2(config)#spanning-tree vlan 30 root primary
• SW2#show spanning vlan 20
– VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 24596
Address 000f.90e2.1300
This bridge is the root
• SW2#show spanning vlan 30
– VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 24606
Address 000f.90e2.1300
This bridge is the root
– SW 2 is now the root bridge for both VLAN 20 and 30. Notice that the
priority value has changed from the default of 32768.
Second way to change the BID
• Change the priority of the switch that you
want to be the root bridge. For example:
– SW2(config)#spanning-tree vlan 3 priority 8192
• Remember, the priority can be anything from
0 to 65535.
– The priority must be in increments of 4096, starting
from 0
– Because the BID is read from left to right, the
priority is read first!
– So if the priority of one switch is lower than that of
another switch, the switch with the lower BID wins
and the MAC address is never considered!
Convergence:
Point 3, pg 511
• Switches are converged when they are all
in either a blocking or forwarding state.
• Also note: if a switch is blocking and then
must become the root bridge because of a
topology change, it must end up in the
forwarding state …
– But it must go through the listening and the
learning states on the way.
Redundant Link Convergence:
pg 512 ff
• The typical spanning-tree topology’s time to convergence from
blocking to forwarding on a switch port is 50 seconds. This
could create time-out problems on your servers or hosts—for
example, when you reboot them.
• To address this hitch, you can disable spanning tree on
individual ports using PortFast (or similar commands).
• Portfast – enables fast connectivity to be
established on access layer switch ports to
workstations
• UplinkFast --- enables fast uplink failover on an
access layer switch when dual uplinks are
connected to distribution layer
• BackboneFast – enables fast convergence in
network backbone (core) after STP change