HPE6-A47 Exam Dumps : Designing Aruba Solutions

 A collision domain is a logical area of the network where devices share the same wireless medium and may interfere with each other’s transmissions. In a WLAN, a collision domain is defined by the channel and the coverage area of an AP or a client. Devices that operate on the same channel and are within range of each other belong to the same collision domain and must contend for access to the medium using the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. Devices that operate on different channels or are out of range of each other belong to different collision domains and do not interfere with each other.

In this scenario, the architect plans 12 APs for an auditorium that is 325 square meters (3,498 square feet). Each AP has one 2.4 GHz radio and one 5 GHz radio. Both types of radios use 20 MHz channels. Assuming that DFS channels can be used in this design, the question is how many 5 GHz collision domains does this design provide.

To answer this question, we need to consider the following factors:

• The number of available 5 GHz channels in the regulatory domain of the auditorium. Different countries have different regulations on the use of the 5 GHz band, which affects the number and availability of channels for WLANs. For example, in the United States, there are 25 non-overlapping 20 MHz channels in the 5 GHz band, but some of them are subject to Dynamic Frequency Selection (DFS) requirements, which means that they must avoid interfering with radar systems by detecting and switching to another channel if radar signals are present. In other countries, such as Japan, there are 19 non-overlapping 20 MHz channels in the 5 GHz band, but some of them are restricted to indoor use only. Therefore, the number of available 5 GHz channels for the design depends on the regulatory domain of the auditorium.
• The channel assignment and distribution of the 12 APs in the auditorium. The channel assignment and distribution of the APs affect the number and size of the collision domains in the WLAN. Ideally, the APs should be assigned to non-overlapping channels and distributed evenly across the auditorium to provide optimal coverage and capacity, while minimizing co-channelinterference and adjacent channel interference. Co-channel interference occurs when devices on the same channel interfere with each other, while adjacent channel interference occurs when devices on nearby channels interfere with each other. Both types of interference degrade the performance and quality of the WLAN. Therefore, the channel assignment and distribution of the APs should follow the best practices of RF design, such as using a channel reuse plan, avoiding channel overlap, and adjusting the transmit power and antenna gain of the APs to match the coverage and capacity requirements of the auditorium.
• The coverage and capacity requirements of the auditorium. The coverage and capacity requirements of the auditorium affect the number and placement of the APs and the transmit power and antenna gain settings of the radios. The coverage requirement refers to the minimum signal strength and signal-to-noise ratio (SNR) that the APs and clients need to maintain a reliable wireless connection. The capacity requirement refers to the maximum number of clients and the minimum throughput and quality of service (QoS) that the APs and clients need to support the applications and services of the auditorium. The coverage and capacity requirements of the auditorium depend on the size and shape of the space, the density and distribution of the clients, the types and characteristics of the applications and services, and the performance and capabilities of the devices. Therefore, the coverage and capacity requirements of the auditorium should be determined by conducting a site survey and a network analysis, and then used to guide the design and configuration of the APs and radios.

Based on these factors, we can estimate the number of 5 GHz collision domains in the design as follows:

• Assuming that the auditorium is located in the United States and that DFS channels can be used in the design, there are 25 non-overlapping 20 MHz channels in the 5 GHz band. However, not all of them may be available or suitable for the design, depending on the presence and activity of radar systems in the area, the compatibility and support of the APs and clients, and the interference and noise level of the environment. Therefore, we will use a conservative estimate of 15 available 20 MHz channels in the 5 GHz band for the design.
• Assuming that the 12 APs are assigned to non-overlapping channels and distributed evenly across the auditorium, each AP will have its own 5 GHz collision domain, as there will be no co-channel interference or adjacent channel interference among the APs. Therefore, there will be 12 5 GHz collision domains in the design, corresponding to the 12 APs.
• Assuming that the coverage and capacity requirements of the auditorium are moderate and that the transmit power and antenna gain settings of the radios are adjusted accordingly, each AP will cover an area of about 27 square meters (290 square feet), which is roughly the size of a small classroom. This means that the APs will provide adequate signal strength and SNR for the clients, while avoiding excessive overlap and interference among the APs. Therefore, there will be no need to further divide or merge the 5 GHz collision domains in the design, as they will match the coverage and capacity requirements of the auditorium.

Hence, the number of 5 GHz collision domains in the design is 12, which is the answer option C. References:

• RF Design | Validated Solution Guide - Aruba
• Configuring 2.4 Ghz and 5 Ghz Radios - Aruba
• Chapter T-5: Understanding RF Collision Domains - Aruba
• [Aruba Certified Design Professional Official Certification Study Guide (HPE6-A47)]

This answer is correct because it matches the scenario for which the Aruba backplane stacking technology is designed. Aruba backplane stacking technology allows up to 10 switches to be interconnected using high-speed backplane cables, creating a single logical device with a single IP address and configuration. This simplifies the management and operation of the network, as well as the redundancy design, as the stack can operate in ring or chain topology and provide fast failover in case of link or switch failure. This technology is suitable for customers who need to deploy a pair of aggregation switches in the same closet, as it reduces the number of devices and links to manage, and provides high availability and scalability for the network.

The other options are incorrect because:

• B. The customer needs to simplify the deployment of core switches which are located in different closets for redundancy purposes. This scenario is not suitable for Aruba backplane stacking technology, as it requires the switches to be physically close to each other and connected with backplane cables. A better option for this scenario would be to use Aruba Virtual Switching Framework (VSF) technology, which allows up to twoswitches to be interconnected using standard Ethernet links, creating a virtual switch that can span across different closets or racks.
• C. The customer needs to expand the network and make it possible for the current core switches to support more MAC forwarding and ARP entries. This scenario is not suitable for Aruba backplane stacking technology, as it does not increase the MAC or ARP table size of the switches. The MAC and ARP table size is determined by the hardware specifications of each switch model, and stacking them together does not change that. A better option for this scenario would be to upgrade the core switches to a higher-end model that supports more MAC and ARP entries, or to segment the network into smaller subnets to reduce the MAC and ARP traffic.
• D. The customer needs to deploy a switch at a branch office and have the switch automatically discover and join the network stack at the main office. This scenario is not suitable for Aruba backplane stacking technology, as it requires the switches to be on the same LAN and connected with backplane cables. A better option for this scenario would be to use Aruba Central, which is a cloud-based management platform that allows remote provisioning and monitoring of Aruba switches and other devices. Aruba Central can automatically discover and join the switch to the network, and apply the configuration and policies from the main office.

References:

• Aruba 2930M Switch Series - Data sheet)
• ArubaOS-Switch Virtual Switching Framework - Configuration guide)
• Aruba Central - Data sheet)

The customer needs a solution that can terminate VPN tunnels for Aruba RAPs, which are remote access points that connect to the corporate network over the Internet. The customer has only one public IP address, which means that they need to use NAT to forward the IPsec traffic to the device that will terminate the VPN tunnel. The customer also wants N+1 redundancy, which means that they need at least two devices that can act as backup for each other in case of failure.

The solution that meets the customer requirements is to use two Aruba MCs on the same subnet that use VRRP without clustering. VRRP stands for Virtual Router Redundancy Protocol, and it is a protocol that allows multiple devices to share a virtual IP address and act as a single gateway for the network. This way, the customer can use the virtual IP address as the public IP address for the NAT, and the IPsec traffic will be forwarded to the device that is the VRRP master. If the master device fails, the backup device will take over the role of the master and continue to terminate the VPN tunnels. This provides N+1 redundancy for the customer.

The other options are not suitable for the customer requirements:

• Option B is not suitable because a Layer 3 cluster requires two public IP addresses, one for each MC. The customer only has one public IP address, so they cannot use this option.
• Option C is not suitable because VRRP does not work across different subnets. The customer needs to have the MCs on the same subnet to use VRRP.
• Option D is not suitable because a Layer 2 cluster does not support NAT. The customer needs to use NAT to forward the IPsec traffic to the MCs.

References:

• Aruba Remote Access Point (RAP) Networks Validated Reference Design)
• Aruba Mobility Controller Virtual IP Configuration Guide)
• ArubaOS 8.5 User Guide)