LAB. Spanning Tree Protocol

Transcription

1 LAB Spanning Tree Protocol

2 Scenario 1 Attention to the switch model / / /24

3 Scenario 1 Configuration 1. Define the scenario according to the figure. Be careful when selecting switches 0 and 1. Those switches need to have Gigabit-Ethernet ports (1000Mbits/s). 2. Don t use the automatic connection (ray symbol) for the switches 0 and 1. Instead, use the dashed connection (------) and select the Gigabit port in both switches (end of the port list) 3. Assign IP addresses to the computers, assuming all of them in the same subnet.

4 Spanning Tree Protocol Once the switches are connected, you can observe one of the switches ports will turn to yellow. This port is blocked by software. This happens because switches cannot form a closed path (LOOP). The spanning tree protocol (STP) is a standard protocol present in most switches, that is able to detect and eliminate loops in networks formed by cascaded switches. Loops are removing by deactivating one or more ports of the switches. The goal of this lab is to understand the operation of the STP.

5 STP Operation Root Bridge ID = XXX 01 Bridge ID = XXX 02 Bridge ID = XXX 03 Once the network is created, an election will occur and one of the switches will be chosen as root. All switches transmit its Bridge ID (based on a MAC address pre-recorded by the vendor) to each other. The winner of the election is the one with the lowest Bridge ID

6 Report Part 1 Determine who won the election to Root on your network. The result will not be, compulsorily, the same for all students, because the MAC of switches can be chosen differently by the Simulator. Open the CLI tab of each Switch and enter the following command: show spanning-tree active OBS. The CLI prompt must be Switch# Copy the contents obtained from the three switches in your report. The content of each switch will be similar to that below. Root ID: BD.A9EC Bridge ID: 0007.EC78.94A0 Interface Role Sts Cost Prio.Nbr Type Fa0/2 Root FWD P2p Fa0/1 Desg FWD P2p Gi1/1 Desg FWD P2p Indicate in the report the number of the switch that became root in your scenario. The root switch is the one whose ID is equal to Root Bridge ID.

7 Understanding the Result Desg Root Bridge ID = XXX 01 Desg Root X Root Desg Root Bridge ID = XXX 02 Bridge ID = XXX 03 The root switch sends permanently STP messages to other switches. Messages are sent from one switch to another so that the port from where the switch sends the message is called ROOT and the port that receives the message is called DESIGNATED. Switches can only have one ROOT port. If they have two or more, they exceeding ports are blocked by software.

8 Understanding the Result 1G Custo 4 Root Bridge ID = XXX M Custo M Bridge ID = XXX 02 Custo 19 Bridge ID = XXX 03 Each of the switch ports receives a cost that is defined with respect to its transmission rate. The cost is determined by the administrator. The default cost of Cisco switches is: 100 Mbit/s = 19 and 1 Gbit/s = 4. When a switch has more than one ROOT port, only the port of the lowest cost is permitted to operate. All other ROOT ports are blocked.

9 Scenario 2 To configure the scenario 2 simply enter two more switches as shown in the figure. Don't forget to remove the old connection between switches 1 and 2. Note that the connection between switches 1 and 3 must also be Gigabit.

10 Scenario 2 Configuration In this scenario, we will choose the root switch. This can be done by increasing the priority of the switch we want to win the election. Select a switch different switch that won the last election (preferably, the switch 0, or the next lowest number switch). enter in the CLI tab the following commands: enable config terminal spanning-tree vlan 1 priority 4096 end Note that prompt must be in the Switch> level before typing the commands above.

11 Report Part 2 Identify the resulting network topology. To do this, use the numbers of switches, and indicate that there is a connection between them only if both ports of the link is green. For example, suppose your result was as follows: The result in this case would be: Note that there is no connection between 3 and 4 because the port of switch 3 is blocked. In a network with STP there will never be a switch with two different paths to the Root.