| 1. | What are some benefits of an EtherChannel? |
| Answer: | Increased bandwidth Link redundancy |
| 2. | How many links can be aggregated into an EtherChannel? |
| Answer: | 2 to 8 |
| 3. | Traffic between two hosts will be distributed across all links in an EtherChannel. True or false? |
| Answer: | False. Packets are distributed onto the EtherChannel links based on the load-distribution method or algorithm that is configured. It is entirely possible that traffic passing between two hosts will end up on just one link within the bundle if a method involving source or destination addresses is used. |
| 4. | Which methods can you use to distribute traffic in an EtherChannel? |
| Answer: | MAC address IP address Layer 4 port |
| 5. | How does an EtherChannel distribute broadcasts and multicasts? |
| Answer: | Broadcasts and multicasts are distributed across the links within an EtherChannel, just like any other traffic. The broadcast or multicast addresses are used in the hash or load-balancing algorithm to determine the link index. |
| 6. | When load balancing, what hashing functions choose a link for a frame? |
| Answer: | If the hashing function is based on a single address (MAC, IP, or port), the low-order bits of that address are used as a link index. If two addresses or port numbers are used, the exclusive-OR (XOR) of those two values is used to derive the low-order bits that form a link index. |
| 7. | What protocols can negotiate an EtherChannel between two switches? |
| Answer: | PAgP and LACP |
| 8. | Suppose a switch at one end of an EtherChannel is configured to use source MAC addresses for load balancing. The switch on the other end is configured to use both source and destination IP addresses. What happens? |
| Answer: | The EtherChannel successfully will transport traffic between the two switches. However, the traffic load will not be distributed evenly or symmetrically across the links in the channel. |
| 9. | Two switches have a four-port EtherChannel between them. Both switches are load balancing using source and destination IP addresses. If a packet has the source address 192.168.15.10 and destination address 192.168.100.31, what is the EtherChannel link index? |
| Answer: | The link index is computed by an XOR of the source and destination IP addresses. Because this is a four-port link, only the 2 low-order bits are needed. 10 XOR 31 can be computed by using binary values: 00001010 XOR 00011111 = 00010101. The lowest 2 bits (01) give a link index of 1. |
| 10. | What does the acronym PAgP stand for? |
| Answer: | Port Aggregation Protocol |
| 11. | Two switches should be configured to negotiate an EtherChannel. If one switch is using PAgP auto mode, what should the other switch use? |
| Answer: | PAgP desirable mode |
| 12. | What is the LACP system priority value used for? |
| Answer: | The switch with the lowest system ID (system priority + MAC address) is allowed to make decisions regarding which ports actively will participate in an EtherChannel and which ones will be held in a standby state. |
| 13. | Complete the following command to put an interface into EtherChannel group 3 and to use PAgP to ask the far-end switch to participate in the EtherChannel. This switch port also should require PAgP packets back from the far-end switch. Switch(config-if)# channel-group __________
|
| Answer: | Switch(config-if)# channel-group 3 mode desirable non-silent
|
| 14. | What interface configuration command is needed to select LACP as the EtherChannel negotiation protocol? |
| Answer: | channel-protocol lacp
|
| 15. | What command could you use to see the status of every port in an EtherChannel? |
| Answer: | show etherchannel summary
|
| 16. | What command could you use to verify the hashing algorithm used for EtherChannel load balancing? |
| Answer: | show etherchannel load-balance
|
| 17. | Suppose that a switch is used in a small data center where one server offers an IP-based application to many clients throughout the campus. An EtherChannel connects the data-center switch to a Layer 3 core switch, which routes traffic to all clients. What EtherChannel load-balancing method might be most appropriate at the data-center switch? Source MAC address Source IP address Destination MAC address Destination IP address Source and destination MAC address Source and destination IP address
|
| Answer: | d. Remember that the EtherChannel will load-balance only outbound traffic, or that from the server toward the clients. Here are brief explanations of each of the choices, to help clarify the answer. a. Source MAC addressNot very useful. The source MAC address (the server) always is the same. One link in the channel always would be selected. b. Source IP addressNot very useful. Again, the source IP address (the server) is constant. c. Destination MAC addressNot very useful. Because a Layer 3 switch is positioned in the core layer, it always appears as the destination MAC address for all client destinations. d. Destination IP addressThis is the best choice because the destination IP addresses of the clients are diverse and are not modified along the path. e. Source and destination MAC addressNot very useful. Because the destination MAC address always is the Layer 3 switch, only one link will be used. f. Source and destination IP addressThis would make a good choice, although the combination of addresses doesn't add anything. The source IP address (the server) always is constant. Therefore, source XOR destination always yields something similar to the destination address itself. |
| 18. | Suppose that a mainframe is connected to a switch that has an EtherChannel uplink to a campus network. The EtherChannel has been configured with the port-channel load-balance src-dst-ip command. Most of the mainframe traffic is SNA (non-IP). What will happen to the SNA frames when they are switched? Would it be better to reconfigure the channel with port-channel load-balance src-dst-mac? |
| Answer: | The SNA frames are non-IP, so only MAC addresses are relevant. The switch has been configured to load-balance according to the XOR of the source and destination IP addresses. Obviously, the SNA frames will have neither of these values present. The switch will realize this and fall back to the "lower" method of src-dst-mac (XOR of the source and destination MAC addresses) for each SNA frame. No configuration changes are necessary for this to occur. There really isn't a good reason to reconfigure for src-dst-mac because the switch can forward the SNA frames already. However, if it were reconfigured, any IP packets would be encapsulated in Ethernet frames, providing MAC addresses within the frames. |
| 19. | What attributes of a set of switch ports must match to form an EtherChannel? |
| Answer: | Port speed Port duplex Trunking mode Trunking encapsulation Access or native VLAN |
| 20. | What happens if one port of an EtherChannel is unplugged or goes dead? What happens when that port is reconnected? |
| Answer: | Traffic on the disconnected port is moved to the next available link in the EtherChannel bundle. When the port is reconnected, traffic does not automatically move back to the bundle's original port. Instead, new traffic is learned and applied to the restored link. |
