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CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

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186 Chapter 7: VLAN Trunking Protocol (VTP)


The questions and scenarios in this book are more difficult than what you should experience on the actual exam. The questions do not attempt to cover more breadth or depth than the exam; however, they are designed to make sure that you know the answers. Rather than allowing you to derive the answers from clues hidden inside the questions themselves, the questions challenge your understanding and recall of the subject. Hopefully, these questions will help limit the number of exam questions on which you narrow your choices to two options and then guess.

The answers to these questions can be found in Appendix A.

1.True or false: You can use VTP domains to separate broadcast domains.

2.What VTP modes can a Catalyst switch be configured for? Can VLANs be created in each of the modes?

3.How many VTP management domains can a Catalyst switch participate in? How many VTP servers can a management domain have?

4.What conditions must exist for two Catalyst switches to be in the same VTP management domain?

5.On a VTP server switch, identify what you can do to reset the VTP configuration revision number to 0.

6.How can you clear the configuration revision number on a VTP client?

7.Complete this command to make all VLANs other than 30 and 100 eligible for pruning on the trunk interface:

switchport trunk pruning vlan

8.Which VLAN numbers are never eligible for VTP pruning? Why?

9.What does the acronym VTP stand for?

10.What VTP domain name is defined on a new switch with no configuration?

11.In a network of switches, VTP domain Engineering has been configured with VLANs 1, 10 through 30, and 100. The VTP configuration revision number is currently at 23. Suppose a new switch is connected to the network, and it has the following configuration: VTP domain Engineering, VTP server mode, only VLANs 1 and 2 are defined, and the configuration revision number is 30.

12.What happens when the switch is connected to the network?

Q&A 187

13.A VTP client switch has VLANs 1, 2, 3, 10, and 30 configured as part of a VTP domain; however, the switch has users connected only to access switch ports defined on VLANs 3 and 30. If VTP pruning is enabled and all VLANs are eligible, which VLANs will be pruned on the upstream switch?

14.The VTP domain Area3 consists of one server and several clients. The server’s VTP configuration revision number is at 11. A new switch is added to the network. It has VTP domain name Area5 and a configuration revision number of 10. What happens when the new switch is added to the network? What happens when the VTP domain name is changed to Area3 on the new switch?

15.What command shows information about the VTP configuration on a Catalyst 3550?

This chapter covers the following topics that you need to master for the CCNP BCMSN exam:

Switch Port Aggregation with Ether- Channel—This section discusses the concept of aggregating, or “bundling,” physical ports into a single logical link. Methods for load-balancing traffic across the physical links are also covered.

EtherChannel Negotiation Protocols

This section discusses two protocols that dynamically negotiate and control EtherChannels: Port Aggregation Protocol (PAgP), a Cisco proprietary protocol, and Link Aggregation Control Protocol (LACP), a standards-based protocol.

EtherChannel Configuration—This section discusses the Catalyst switch commands needed to configure EtherChannel.

Troubleshooting an EtherChannel—This section gives a brief summary of things to consider and commands to use when an aggregated link is not operating properly.

C H A P T E R 8

Aggregating Switch Links

In previous chapters, you learned about campus network design and connecting and organizing switches into blocks and common workgroups. Using these principles, end users can be given effective access to resources both on and off the campus network. However, today’s missioncritical applications and services demand networks that provide high availability and reliability.

This chapter presents technologies that you can use in a campus network to provide higher bandwidth and reliability between switches.

“Do I Know This Already?” Quiz

The purpose of the “Do I Know This Already?” quiz is to help you decide if you need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 13-question quiz, derived from the major sections in the “Foundation Topics” portion of the chapter, helps you determine how to spend your limited study time.

Table 8-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Table 8-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

Foundation Topics Section

Questions Covered in This Section



Switch Port Aggregation with






EtherChannel Negotiation




EtherChannel Configuration




Troubleshooting an EtherChannel




190 Chapter 8: Aggregating Switch Links

CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong. Giving yourself credit for an answer you correctly guess skews your selfassessment results and might give you a false sense of security.

1.If Fast Ethernet ports are bundled into an EtherChannel, what is the maximum throughput supported on a Catalyst switch?

a.100 Mbps

b.200 Mbps

c.400 Mbps

d.800 Mbps

e.1600 Mbps

2.Which of these distributes traffic over an EtherChannel?

a.Round robin

b.Least-used link

c.A function of address

d.A function of packet size

3.What type of interface represents an EtherChannel as a whole?





4.Which of the following is not a valid method for EtherChannel load balancing?

a.Source MAC address

b.Source and destination MAC addresses

c.Source IP address

d.IP precedence

e.UDP/TCP port

5. The EtherChannel load-balancing method can be set



a.Per switch port

b.Per EtherChannel

“Do I Know This Already?” Quiz 191

c.Globally per switch

d.Can’t be configured

6.What logical operation is performed to calculate EtherChannel load balancing as a function of two addresses?





7.Which one of the following is a valid combination of ports for an EtherChannel?

a.Two access links (one VLAN 5, one VLAN 5)

b.Two access links (one VLAN 1, one VLAN 10)

c.Two trunk links (one VLANs 1-10, one VLANs 1, 11-20)

d.Two Fast Ethernet links (both full-duplex, one 10 Mbps)

8.Which of these is a method for negotiating an EtherChannel?





9.Which of the following is a valid EtherChannel negotiation mode combination between two switches?

a.PAgP auto, PAgP auto

b.PAgP auto, PAgP desirable

c.on, PAgP auto

d.LACP passive, LACP passive

10.When would PagP’s “desirable silent” mode be useful?

a.When the switch should not send PAgP frames

b.When the switch should not form an EtherChannel

c.When the switch should not expect to receive PAgP frames

d.When the switch is using LACP mode

192Chapter 8: Aggregating Switch Links

11.Which of the following EtherChannel modes does not send or receive any negotiation frames?

a.channel-group 1 mode passive

b.channel-group 1 mode active

c.channel-group 1 mode on

d.channel-group 1 mode desirable

e.channel-group 1 mode auto

12.Two computers are the only hosts sending IP data across an EtherChannel between two switches. Several different applications are being used between them. Which of these loadbalancing methods would be more likely to use the most links in the EtherChannel?

a.Source and destination MAC addresses

b.Source and destination IP addresses

c.Source and destination TCP/UDP ports

d.None of the above

13.Which command can be used to see the status of an EtherChannel’s links?

a.show channel link

b.show etherchannel status

c.show etherchannel summary

d.show ether channel status

The answers to the quiz are found in Appendix A, “Answers to Chapter ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:

8 or less overall score—Read the entire chapter. This includes the “Foundation Topics,” “Foundation Summary,” and “Q&A” sections.

9–11 overall score—Begin with the “Foundation Summary” section and then follow up with the “Q&A” section at the end of the chapter.

12 or more overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the “Q&A” section at the end of the chapter. Otherwise, move to Chapter 9, “Traditional Spanning Tree Protocol.”

Switch Port Aggregation with EtherChannel 193

Foundation Topics

Switch Port Aggregation with EtherChannel

As discussed in Chapter 5, “Switch Port Configuration,” switches can use Ethernet, Fast Ethernet, or Gigabit Ethernet ports to scale link speeds by a factor of ten. Cisco offers another method of scaling link bandwidth by aggregating, or bundling, parallel links, termed the EtherChannel technology. Two to eight links of either Fast Ethernet (FE) or Gigabit Ethernet (GE) are bundled as one logical link of Fast EtherChannel (FEC) or Gigabit EtherChannel (GEC), respectively. This bundle provides a full-duplex bandwidth of up to 1600 Mbps (8 links of Fast Ethernet) or 16 Gbps (8 links of Gigabit Ethernet).

This also provides an easy means to “grow,” or expand, a link’s capacity between two switches, without having to continually purchase hardware for the next magnitude of throughput. For example, a single FastEthernet link (200-Mbps throughput) can be incrementally expanded up to eight FastEthernet links (1600 Mbps) as a single Fast EtherChannel. If the traffic load grows beyond that, the growth process can begin again with a single Gigabit Ethernet link (2-Gbps throughput). Up to seven additional Gigabit Ethernet links can be added to that Gigabit EtherChannel (16 Gbps). The process repeats again by moving to a single 10Gigabit Ethernet link, and so on.

Ordinarily, having multiple or parallel links between switches creates the possibility of bridging loops—an undesirable condition. EtherChannel avoids this situation by bundling parallel links into a single, logical link, which can act as either an access or a trunk link. Switches or devices on each end of the EtherChannel link must understand and use the EtherChannel technology for proper operation.

Although an EtherChannel link is seen as a single logical link, the link does not have an inherent total bandwidth equal to the sum of its component physical links. For example, suppose an FEC link is made up of four full-duplex, 100-Mbps Fast Ethernet links. Although it is possible for the FEC link to carry a throughput of 800 Mbps, the single resulting FEC link does not operate at this speed. Instead, traffic is balanced across the individual links within the EtherChannel. Each of these links operates at its inherent speed (200 Mbps full-duplex for FE) but carries only the frames placed on it by the EtherChannel hardware. The load-balancing process is explained further in the next section.

EtherChannel also provides redundancy with several bundled physical links. If one of the links in the bundle fails, traffic sent through that link moves to an adjacent link. Failover occurs in less than a few milliseconds and is transparent to the end user. As more links fail, more traffic moves to further adjacent links. Likewise, as links are restored, the load redistributes among the active links.

194 Chapter 8: Aggregating Switch Links

Bundling Ports with EtherChannel

EtherChannel bundles can consist of up to eight physical ports of the same Ethernet media type and speed. Some configuration restrictions exist to ensure that only similarly configured links are bundled.

Generally, all bundled ports must first belong to the same VLAN. If used as a trunk, bundled ports must all be in trunking mode, have the same native VLAN, and pass the same set of VLANs. Each of the ports should also have the same speed and duplex settings before they are bundled. Bundled ports must also be configured with identical Spanning Tree settings.

Distributing Traffic in EtherChannel

Traffic in an EtherChannel is distributed across the individual bundled links in a deterministic fashion. However, the load is not necessarily balanced equally across all the links. Instead, frames are forwarded on a specific link as a result of a hashing algorithm. The algorithm can use source IP address, destination IP address, or a combination of source and destination IP addresses, source and destination MAC addresses, or TCP/UDP port numbers. The hash algorithm computes a binary pattern that selects a link number in the bundle for each frame.

If only one address or port number is hashed, a switch forwards each frame by using one or more low-order bits of the hash value as an index into the bundled links. If two addresses or port numbers are hashed, a switch performs an exclusive-OR (XOR) operation on one or more low-order bits of the addresses or TCP/UDP port numbers as an index into the bundled links.

For example, an EtherChannel consisting of two links bundled together requires a one-bit index. Either the lowest order address bit or the XOR of the last bit of the addresses in the frame is used. A four-link bundle uses a hash of the last two bits. Likewise, an eight-link bundle uses a hash of the last three bits. The hashing operation’s outcome selects the EtherChannel’s outbound link. Table 8-2 shows the results of an XOR on a two-link bundle, using the source and destination addresses.

The XOR operation is performed independently on each bit position in the address value. If the two address values have the same bit value, the XOR result is 0. If the two address bits differ, the XOR result is 1. In this way, frames can be statistically distributed among the links with the assumption that MAC or IP addresses are statistically distributed throughout the network. In a four-link EtherChannel, the XOR is performed on the lower two bits of the address values resulting in a 2-bit XOR value (each bit is computed separately) or a link number from 0 to 3.



Switch Port Aggregation with EtherChannel 195

Table 8-2 Frame Distribution on a Two-Link EtherChannel





Binary Addresses

Two-Link EtherChannel XOR and Link Number





Addr1: ... xxxxxxx0



Addr2: ... xxxxxxx0

... xxxxxxx0: Use link 0





Addr1: ... xxxxxxx0



Addr2: ... xxxxxxx1

... xxxxxxx1: Use link 1





Addr1: ... xxxxxxx1



Addr2: ... xxxxxxx0

... xxxxxxx1: Use link 1





Addr1: ... xxxxxxx1



Addr2: ... xxxxxxx1

... xxxxxxx0: Use link 0




As an example, consider a packet being sent from IP address to Because EtherChannels can be built from two to eight individual links, only the rightmost (least significant) three bits are needed as a link index. These bits are 001 (1) and 110 (6), respectively. For a two-link EtherChannel, a one-bit XOR is performed on the rightmost address bit: 1 XOR 0 = 1, causing Link 1 in the bundle to be used. A four-link EtherChannel produces a two-bit XOR: 01 XOR 10 = 11, causing Link 3 in the bundle to be used. Finally, an eight-link EtherChannel requires a three-bit XOR: 001 XOR 110 = 111, where Link 7 in the bundle is selected.

A conversation between two devices is always sent through the same EtherChannel link because the two endpoint addresses stay the same. However, when a device talks to several other devices, chances are that the destination addresses are equally distributed with 0s and 1s in the last bit (even and odd address values). This causes the frames to be distributed across the EtherChannel links. Note that a conversation between two end devices to create a load imbalance is possible using one of the links in a bundle because all traffic between a pair of stations will use the same link.

Configuring EtherChannel Load Balancing

The hashing operation can be performed on either MAC or IP addresses, and can be based solely on source or destination addresses, or both. Use the following command to configure frame distribution for all EtherChannel switch links:

Switch(config)# port-channel load-balance method

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