Tài liệu Ethernet 1 LAN Switching Part I Lab Scenarios docx

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Date of Issue: 02-01-2001
LAN Switching Part I Lab Scenarios
by Dan Farkas
Lab Exercise 1
Goal
Configure deterministic root placement and port blocking.
Equipment
Three Catalyst(r) 4xxx, 5xxx, or 6xxx switches, two PCs with NICs, three crossover category 5 cables, two straight-
through category 5 cables, and a stopwatch or a watch with a second hand.
Initial Configuration
Task 1
l Determine which switch is the root and label all switch ports as either Root Port (RP), Designated Port (DP), or
Non-Designated Port (NDP).
l Label the Root Switch A.
l Label the switch with two DPs B.
l Label the remaining switch C.
l Connect PC 1 to switch B.
l Connect PC 2 to switch C.
Task 2
Lab Exercise 1
Goal
Equipment
Initial Configuration
Task 1
Task 2
Solution
Task 1
Task 2
Lab Exercise 2

Goal
Initial Configuration
Task 1
Task 2
Solution
Task 1
Task 2
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l Configure C to be the root switch. On the link between A and B, which switch has the DP?
l Configure the switch with the NDP to transition its NDP to the forwarding state. Verify it!
Solution
Task 1
Use the show spantree command for VLAN 1 on each switch. You will see:
Switch(enable) show spantree 1
Vlan 1
Spanning tree enabled
Spanning tree type ieee
Designated Root 00-50-bd-18-a8-00
Designated Root Priority 8192
Designated Root Cost 0
Designated Root Port 1/0
Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Bridge ID MAC ADDR 00-50-bd-18-a8-00
Bridge ID Priority 8192
Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Port VLAN Port-State Cost Priority Fast-Start

2/1 1 forwarding 19 32 disabled
2/2 1 forwarding 19 32 disabled

To determine the root switch, look for a designated root cost of zero. Also look for the switch with the Designated Root
field equal to the Bridge ID.
To determine which ports are in a blocking state and which are in a forwarding state look at the "Port-State" field at
the bottom of the output.
Task 2
On Switch C, use the set spantree root command to set it as the root switch.
Switch(enable) set spantree root 1 dia 2
VLAN 1 bridge priority set to 8192
VLAN 1 bridge max aging time set to 10 seconds.
VLAN 1 bridge hello time set to 2 seconds.
VLAN 1 bridge forward delay set to 7 seconds.
Switch is now the root switch for active VLAN 1.
The set spantree root command sets C's priority to 8192. Being the switch with the lowest priority makes it the root
of the spanning tree. The diameter of 2 is used for optimizing the spanning tree timers.
Using the show spantree command, we see that A is the DP for the link between A and B.
To make B the DP we could use the set spantree priority or set spantree portcost commands. Lowering B's priority
to 30,000 will make its BID lower. BID is used as the deciding factor if path costs are equal. The portcost command
could be used to lower the cost of B's interface. In a 10-megabit network, set the cost to something less than 100. In a
100-megabit network, lower the cost to something less than 19.
Switch(enable) set spantree portcost mod-num/port-num cost
Switch (enable) set spantree priority 30000 1
The show spantree command will verify your configuration. Look to see that B is forwarding and A is blocking on
their connected ports.
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Lab Exercise 2
Goal
Optimize Spanning Tree convergence by using PortFast and UplinkFast.
Initial Configuration
From Lab 1, C is the root of the spanning tree, and A is the leaf. PC 1 is connected to B; PC 2 to C. (Note: the term

"leaf" refers to the switch that is furthest away from the root; it has blocked ports.)
Task 1
l On PC 1, send a stream of pings to PC 2 (from a DOS prompt, type ping -t ip_address_pc2).
l Disconnect the cable between PC 1 and B. The pings will start to fail on PC 1.
l Reconnect the cable between PC 1 and B.
Why do the pings still fail after the network is reconnected?
l Time how long it takes for spanning tree to transition PC 1's port to forwarding and for the pings to succeed
again.
l Configure PortFast on B for PC 1's port.
l On PC 1, send a stream of pings to PC 2 (from a DOS prompt, type ping -t ip_address_pc2).
l Disconnect the cable between PC 1 and B. The pings will start to fail on PC 1.
l Reconnect the cable between PC 1 and B.
l Time how long it takes for spanning tree to transition PC 1's port to forwarding and for the pings to succeed
again.
How long did the port transition take? Was it faster? Why?
Task 2
l On PC 1, send a stream of pings to PC 2 (from a DOS prompt, type ping -t ip_address_pc2).
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l Disconnect the cable between C and B. The pings will start to fail on PC 1.
Why do the pings still fail even though there is an alternate path through A?
l Time how long it takes for spanning tree convergence and for the pings to succeed again.
l Reconnect the cable between C and B.
l Configure UplinkFast on B.
Which switch is the root of the spanning tree? Which switch is the leaf? Why?
l On PC 1, send a stream of pings to PC 2 (from a DOS prompt, type ping -t ip_address_pc2).
l Disconnect the cable between C and B. The pings will start to fail on PC 1.
l Time how long it takes for spanning tree convergence and for the pings to succeed again.
How long did spanning tree convergence take? Was it faster? Why?
Solution

Task 1
The pings still fail because spanning tree requires the port to go through the listening and learning states before it can
go to the forwarding state. It should have taken about 30 seconds for the pings to succeed again.
The command to configure PortFast is:
Switch (enable) set spantree portfast mod_num/port_num enable
The pings should now have started to succeed again after a few seconds. This is because PortFast allows a port
connected to a workstation or server to transition directly to a forwarding state without having to go through the
listening and learning states.
Task 2
The pings will fail because spanning tree convergence requires that all ports transition through the listening and
learning states before they can go to the forwarding state. This is done to ensure a loop-free topology. It should have
taken roughly 30 seconds for reconvergence.
The command to configure UplinkFast is:
Switch (enable) set spantree uplinkfast enable
[rate station-update-rate][all-protocols off | on]
VLANs 1-1005 bridge priority set to 49152.
The port cost and portvlancost of all ports set to above 3000.
Station update rate set to 15 packets/100ms.
UplinkFast all-protocols field set to off.
UplinkFast enabled for bridge.
C is still the root of the spanning tree, but now B is the leaf. UplinkFast causes the priority of B to be set to 49,152 and
its port costs to be set above 3000. As a result, when spanning tree converges, on the link between A and B, A will be
the DP and B will block. Being a leaf switch is a requirement for UplinkFast.
The pings should now succeed again after only a few seconds. UplinkFast lets its NDP ports transition almost
immediately to the forwarding state. B also begins sending out dummy update multicast frames at a rate of 15/100ms.
This allows A and C to quickly learn the new path to PC 1's MAC address.
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[IE-LANS1-LS1-F04]
[2001-01-26-01]

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