Answers to Chapter 6 Hands-On Lab

Answers to Chapter 6 Hands-On Lab

Build a network that resembles Figure 6-27.

Using only VLAN 1, complete the following steps:

1:

Start a continuous ping (tip: under Microsoft Windows, use the ping -t ip_address command) between PC-1 and PC-3. Break the link connecting PC-3 to Cat-2. After reconnecting the link, how long does it take for the pings to resume?

A:

It takes 30-35 seconds. Some people might observe a period of approximately 50 seconds. This is most often due to a feature called PAgP that runs on Etherchannel-capable ports by default. Disable PAgP with the set port channel 2/1-4 off command and repeat the measurements. Note that some hardware requires that you specify a different range of ports than 2/1-4.

2:

Start a continuous ping between PC-1 and PC-2. As in Step 1, break the link between PC-3 and Cat-2. Does this affect the traffic between PC-1 and PC-2?

A:

No. Because the traffic was not using this link, no disruption to traffic between PC-1 and PC-2 occurs (in other words, don't fall trap to the notion that every change in the Spanning Tree disrupts the entire network).

3:

Use the show spantree command on Cat-1 and Cat-2. What bridge is acting as the Root Bridge? Make a note of the state of all ports.

A:

The Root Bridge is the Catalyst where the Designated Root field matches the Bridge ID MAC ADDR field. Notice that the Designated Root Cost equals zero and the Designated Root Port equals 1/0.

All ports should be in the Forwarding state except the 1/2 port on the non-Root Bridge.

4:

Why is the 1/2 port on the non-Root Bridge Blocking? How did the Catalyst know to block this port?

A:

Recall that STP always refers back to the same four-step decision sequence. Assume Cat-1 is the Root Bridge (if you have Cat-2 as the Root Bridge, just reverse the Cat-1 and Cat-2 names in the following discussion). In this case, both bridges are in agreement that Cat-1 is the Root Bridge, causing Root Path Cost to be considered next. However, because both 1/1 and 1/2 have a Root Path Cost of 19, Cat-2 must consider the Sender BID field next. Because Cat-1's BID is listed in BPDUs received on both 1/1 and 1/2, there is once again a tie. This causes Port ID to be evaluated next. Because 1/1 has a lower Port ID (0x8001) than port 1/2 (0x8002), port 1/1 is preferred. This places 1/1 in the Forwarding state and 1/2 in the Blocking state.

5:

Start a continuous ping between PC-1 and PC-3. Break the 1/1 link connecting Cat-1 and Cat-2. How long before the traffic starts using the 1/2 link?

A:

It takes about 30-35 seconds. If you observe a period of approximately 50 seconds, disable PAgP with the set port channel 1/1-2 off command and repeat the measurements. Note that some hardware requires that you specify a different range of ports than 1/1-2.

6:

Reconnect the 1/1 link from Step 2. What happens? Why?

A:

The traffic stops once again for 30-35 seconds! This surprises many people you just fixed the network but STP still blocked traffic for approximately 30 seconds. As soon as the 1/1 link is reconnected, BPDUs start flowing from the Root Bridge to the non-Root Bridge. As soon as the non-Root Bridge sees a lower BPDU arrive on port 1/1, it realizes that port 1/2 is no longer a valid Root Port. Therefore, port 1/2 is immediately put into the Blocking state, but port 1/1 must still spend 15 seconds in the Listening state and 15 second in the Learning state (assuming default timer values). After 30 seconds, port 1/1 takes over as the Root Port.

7:

With the continuous ping from Step 3 still running, break the 1/2 link connecting Cat-1 and Cat-2. What effect does this have?

A:

No effect. As with Question 2, there is no disruption because the traffic is not using this link.