1 Page 1 of 18 Networking Fundamentals for Dante Overview Audinate s Dante is an audio solution that runs on top of standard Ethernet networks. With Audio- Technica s entrance into Dante-powered products, it is important to understand the fundamentals behind basic computer networking. This document focuses on basic networking knowledge that will help with designing, deploying, and supporting a network for Dante equipment. Contents Background Theory... 3 OSI Model... 3 Layer 1: Physical... 3 Layer 2: Data Link... 3 Layer 3: Network... 3 Layer 4: Transport... 4 Layers 5, 6, and 7: Session, Presentation, and Application... 4 Encapsulation... 4 Addressing... 5 MAC Addresses... 5 IP Addressing... 5 Components and Technologies... 8 Network Devices... 8 Hubs... 8 Switches... 9 Routers Network Interface Cards Cabling and Ethernet Network Speeds Power over Ethernet DHCP VLANs Quality of Service Basic Network Configuration Configuring Network Equipment... 13
2 Page 2 of 18 Configuring Hosts Audinate Dante Dante Performance and Reliability Quality of Service Considerations Congestion and Bottlenecks Security Troubleshooting Layer Layer Layer Layer Layers 5, 6, and
3 Page 3 of 18 Background Theory A computer network is a group of computers and devices that can communicate and share information with each other. The way these devices communicate is governed by a variety of protocols. Protocols in computer networking serve the same purpose as protocols in the physical world: they are agreed sets of procedures that all devices understand in order to accomplish a task. Typically, computers use multiple protocols at any one time to complete a single task. This is because protocols are designed to be modular in nature. These protocols are all organized in layers. The model that best illustrates these layers is the OSI model. OSI Model The Open Systems Interconnection (OSI) model is a model developed by the International Organization for Standardization (ISO) that splits up the functions of network communication into seven layers. The layers are numbered from bottom to top. As data from an application moves from the top of the model down, the data is encapsulated by the layer below it with information that will get it to its destination application. When the encapsulated data arrives at the destination, it moves up the OSI model where each layer is then decapsulated until the original data reaches the destination application. Layer 1: Physical The physical layer determines what kind of physical medium communication occurs over. It is responsible for how electrical impulses are transmitted over a copper cable or how light is modulated over a fiber-optic cable. In the context of computer networking, this layer is concerned with the transmission and reception of 1s and 0s and nothing else. Layer 2: Data Link The data link layer determines how two or more devices talk to each other on a network. In most cases, the Ethernet protocol is used here. Every Ethernet device has its own unique address that does not change, called a MAC address. It usually looks something like 01:23:45:67:89:ab. Devices use MAC addresses to communicate with each other on a network 1. The data link layer also provides redundancy to transmission problems on the physical layer. For example, if two devices transmit at the same time, it will end in a collision and no data will be received. The data link layer provides a way to detect this and retry the transmission. Layer 3: Network The network layer is where routing occurs. In layer 2, communication can only occur among devices on the same network segment. When a destination exists on another network, the communication must be directed by a router to the other network 2. A router joins two or more networks. 1 A layer 2 address is similar in function to a person s name. Anyone nearby can communicate with them using that name. However, if they are not nearby then something like a mailing address or a phone number must be used to reach them. 2 A layer 3 address is analogous to a mailing address or a phone number. The post office or the telephone company acts as the router and connects the parties using their assigned addresses or numbers.
4 Page 4 of 18 The address in layer 3 is what routers use to get communication to its destination. The layer 3 protocol of the Internet and the vast majority of computer networks is the Internet Protocol (IP). Each device is given an IP address. Unlike the MAC address, this address can change, though it usually stays the same once assigned. This address usually looks something like Layer 4: Transport The transport layer is where end-to-end communication is established. In the IP suite, there are two primary transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). The main difference between these two protocols is that TCP creates connections and always verifies that traffic arrives at its destination. UDP has no concept of connections and delivers data on a besteffort basis. Both TCP and UDP use the concept of ports to sort traffic going to higher level services. For example, a server may listen on port 21 for file transfers and on port 80 for a website. When another computer wants to transfer files, it will connect on port 21. When it wants the website, it will connect on port 80. The connecting computer will also specify a return port. The server will send its traffic to that port when responding. Layers 5, 6, and 7: Session, Presentation, and Application Layers 5 7 are layers that include the protocols that applications themselves use to communicate. In the case of Dante, these layers might contain the digital audio or timing information. A network engineer, however, is not normally concerned with these layers as they don t affect routing. Encapsulation Data from the sending application flows down from the application layer all the way to the physical layer where it is transmitted out on the network. The final recipient of the data receives it on the physical layer and sends it up to the application layer where the receiving application resides. This process is illustrated in Figure 1.
5 Page 5 of 18 Application Presentation Application Data Sesssion Transport TCP Header Application Data Network IP Header TCP Header Application Data Data Link Ethernet Header IP Header TCP Header Application Data FCS Physical Figure 1: Data as it travels down and is encapsulated by the various layers of the OSI model As the data moves down, it is encapsulated with a header from each layer. The data is split into segments at layer 4 and is given a TCP header. At layer 3, these segments are encapsulated with an IP header. At this point the segments are referred to as packets. The packets are then encapsulated with a MAC header and a frame check sequence (FCS) at layer 2. At this point the packets are referred to as frames. Finally, the raw data is transmitted out onto the physical medium at layer 1. Addressing Addressing of devices takes place at layer 2 and layer 3. At layer 2 a device has a MAC address, and at layer 3 it has an IP address. MAC Addresses MAC addresses are 48-bit in size. 48-bit means that when written in binary (base 2), there are 48 places. The bits are then split up in six 8-bit sections (octets). Each section is represented in hexadecimal (base 16) and ends up looking like 00:13:00:E1:11:11 3. IP Addressing IP addresses are more relevant when it comes to planning a network. An IP address is a 32-bit number that is split up in 4 octets represented in decimal (base 10). Unlike MAC addresses, IP addresses are hierarchical. In other words, any given address belongs to a network called a subnet. The subnet mask determines how big a network is. This makes the most sense when looked at in binary as shown in Figure 2. 3 While it is important to understand what MAC addresses are, the math behind them is not particularly important in simple network planning.
6 Page 6 of 18 Address: (Binary) Mask: AND Network: Figure 2: An IP address ( ) and subnet mask ( ) being combined to deduce the network's address There is a dividing line in the subnet mask where every bit on the left is a 1 and every bit on the right is a 0. This dividing line is applied to the IP address and splits it into two parts, the network portion (on the left) and the host portion (on the right). The network portion of a device s IP address determines what network it is in and the host portion uniquely identifies it from other devices on that network. Substituting the host portion of the address with all 0s leaves the network number 4. The network number represents the entire network. The network number is the first address in the network and cannot be assigned to a host. So in the case of Figure 2, a device configured with an address of and mask of is a member of the /24 network. The /24 represents the subnet mask and is just a count of the number of 1s in the mask. Address Classes In IP, there are five different classes of networks. Originally, these classes determined how large a network would be. This was later changed to allow subnet masks of variable lengths. The five classes are Class A Class E. Class A, B, and C are used to address any single host on the Internet. Class A networks always begin with a 0 as the leftmost binary digit and have a subnet mask of /8. Any address between and is in Class A. Class B networks always begin with 10 as the two leftmost binary digits and have a subnet mask of /16. Any address between and is in Class B. Class C networks always begin with 110 as the three leftmost binary digits and have a subnet mask of /16. Any address between and is in class C. Class D is used for multicast addresses. A multicast address represents multiple devices with one address. Class D addresses always begin with 1110 as the four leftmost digits and have no subnet mask. Any address from to is in Class D. Class E is marked as experimental and its addresses are usually considered invalid. The four leftmost bits of a Class E address are always 1111 and there is no subnet mask. Any address from to is in Class E. 4 This substitution is the same as performing a binary AND operation between the address and the mask, which is what is shown in the figure. A binary AND simply compares each place and returns a 1 if both numbers are a 1 and a 0 otherwise.
7 Page 7 of 18 Currently, the address space for standard hosts on the Internet is between and The space between to is used for multicast and the remaining space is unused. Private Addresses There are three ranges of IP addresses in the usable address space of IP that are considered private. These addresses do not exist on the Internet and are reserved for use inside of a private network. Most home networks and corporate networks use these private addresses as it reduces the need for a large number of public IP addresses. The three ranges of private addresses are /8, /12, and /16. Additionally, the /8 network is reserved for loopback addresses, addresses that are local within a single host. Subnetting The subnet mask determines the size of the network. The number of addresses that can fit in a network are determined by the host portion of the mask. For example, with a subnet mask of /24 ( ), the host portion is eight bits long. Eight bits is enough space for 256 unique numbers (0 256 in decimal or in binary). That means that in any /24 network, there are 256 unique addresses. There can only be at most 254 hosts, however, as the first address is the network number and the last address is reserved as a broadcast address to address all hosts in the network. Subnetting is the splitting up of a network into smaller networks. For example, a business may have one large network assigned to it. But since it is not practical to have every device on the same network, the business would most likely subnet the address into smaller networks for different floors of the building or for different device types like servers, workstations, and phones. An example of subnetting a /16 network is shown in Figure 3. Address: /16 Mask (/16): Binary: Need at least 230 hosts per subnet (8 bits) - borrow 8 network bits First subnet: /24 Increment first bit in new network portion to calculate each subnet Second subnet: /24... Last subnet: /24 Figure 3: Subnetting the /16 network for at least 230 hosts per subnet In this example, the /16 needs to be subnetted to have at least 230 hosts per subnet. Adding two for the network and broadcast addresses results in 232 total addresses required per subnet. This is in binary. Since 232 is eight bits long in binary, each subnet needs to have at least eight bits in the host portion of the address. In the /16 network there are 16 total host bits available. Each subnet only needs eight bits, which leaves eight bits that will be borrowed from the network portion to form the new subnets. Adding
8 Page 8 of 18 the eight borrowed bits to the original 16 network bits leaves /24 as the new subnet mask of each subnet. To calculate each possible subnet, the borrowed bits are incremented from 0 until the maximum is reached. In this case, the maximum is eight bits long at in binary or 256 in decimal. These subnets can then be further subnetted into smaller sections if needed. Unicast, Multicast, and Broadcast Unicast represents one-to-one communication. Multicast represents one-to-many communication. Broadcast represents one-to-all communication. Any traffic between two hosts is unicast. Broadcast and multicast differ in that broadcast is sent to everyone and multicast is only sent to those registered under that multicast address 5. In any network, the last available address is the broadcast address. For example, in the /24 network the broadcast address is Multicast addresses exist in the space of Multiple devices can register for an address in this range and will then receive any traffic destined to that address. Components and Technologies Network Devices Network devices are special devices in a network because they forward traffic for others. They do not normally send or receive their own traffic except for management. Because they only need to direct the traffic, these devices do not examine all seven layers. Each type of device operates in a specific layer and only processes data up to that layer. Hubs Hubs are the simplest of any network device. They operate only at the physical layer, layer 1. When they receive data on a port they do nothing more than duplicate that data out every other port as seen in Figure 4. 5 In the case of Dante, broadcasting and multicasting is preferable any time a device is sending audio to multiple outputs. Instead of sending multiple copies, only one copy is sent that is received by multiple devices. This reduces the overall bandwidth requirement.
9 Page 9 of 18 L3 L2 Data Source IP: Destination IP: Source MAC: AA:AA:AA:AA:AA:AA Destination MAC: CC:CC:CC:CC:CC:CC PC2 BB:BB:BB:BB:BB:BB Frame is ignored: Doesn t match MAC address Receives Data Matches IP address Matches MAC address PC1 AA:AA:AA:AA:AA:AA Figure 4: Behavior of a hub in a network Hub Hub replicates incoming signal to all ports PC3 CC:CC:CC:CC:CC:CC In this example, PC1 addresses a packet and frame for PC3 and sends it out on the wire. When the hub receives the signal, it replicates the signal out to all other ports. The hub does not examine the incoming data in any way. It simply reflects the bits it receives to its other ports. Both PC2 and PC3 receive the signal coming from PC1. When PC2 examines the frame, it sees that the frame is not addressed to its MAC address and discards it. When PC3 examines the frame, it sees that the frame is destined for its MAC address, and so it moves up to layer 3 to examine the destination IP address. When it sees that the packet is addressed to its IP address, it then moves the data up to the higher layers where it is received by the application. In effect, the hub creates one big link that all the connected devices share. Everyone hears everyone else s traffic. These are very rarely used anymore because of security and performance implications. Switches Switches have effectively taken the place of hubs in modern networks. From a high level, they appear to do the same thing as hubs, however, they have one key difference. Switches examine traffic up to layer 2 and make forwarding decisions based on MAC addresses as seen in Figure 5.
10 Page 10 of 18 PC2 BB:BB:BB:BB:BB:BB L3 L2 Data Source IP: Destination IP: Source MAC: AA:AA:AA:AA:AA:AA Destination MAC: CC:CC:CC:CC:CC:CC 1 Switch examines the frame and sends it only to the correct destination as noted in its MAC address table 2 3 Receives Data Matches IP address Matches MAC address PC1 AA:AA:AA:AA:AA:AA Figure 5: Behavior of a switch in a network MAC Address Port AAAA.AAAA.AAAA 1 BBBB.BBBB.BBBB 2 CCCC.CCCC.CCCC 3 PC3 CC:CC:CC:CC:CC:CC The switch knows what devices are attached to each port based on their MAC addresses and therefore doesn t have to duplicate the data on every port. In this example, when the switch receives data from PC1, it examines the frame and the destination MAC address. It then looks up the destination MAC address in a table stored on the switch to find the corresponding port. It then forwards the data only out of the correct port to PC3. PC2 never receives any data. This behavior does two important things. First, it increases security because only the intended recipients can see the data. But more importantly, it breaks up what is known as collision domains. A collision domain is effectively a collection of devices that share the same physical link. With a hub, every connected device shares the same physical link and collision domain. With a switch, each device has its own collision domain. Since there are only two devices on any given link, collisions are significantly less likely and in most cases impossible. This significantly improves performance and scalability of the network. Routers Routers move one step higher in the model, operating at layer 3. These devices use IP addresses to make forwarding decisions. Each port on a router is a member of a different network. When a router receives traffic from one network, it uses the destination IP address to determine which port to forward to as seen in Figure 6.
11 Page 11 of 18 Network Gateway / /24 Port /24 Port /24 Port 1 BB:BB:BB:BB:BB:BB R1 Port 2 CC:CC:CC:CC:CC:CC SRC: DST: SRC: AA:AA:AA:AA:AA:AA DST: BB:BB:BB:BB:BB:BB SRC: DST: SRC: CC:CC:CC:CC:CC:CC DST: DD:DD:DD:DD:DD:DD SRV1 DD:DD:DD:DD:DD:DD Gateway: PC1 AA:AA:AA:AA:AA:AA Gateway: Figure 6: Behavior of a router connecting two networks In this example, PC1 is sending traffic destined for SRV1. PC1 specifies its own IP address as the source address and SRV1 s as the destination address in the layer 3 packet. In the layer 2 frame, it works differently. Since SRV1 is on a different network, PC1 knows that it cannot communicate with it directly. Instead, it has to forward the data to its default gateway (in this case, R1). A default gateway is configured on a device to tell it where to send any data destined for an outside network. For layer 2, the device specifies its own MAC address as the source and R1 s address as the destination. When R1 receives the data, it first checks at layer 2 to see if it is addressed for itself. In this case, it is. Then it moves up to layer 3 and examines the destination IP address. It then consults its routing table to look up where the data should be forwarded. It finds an entry for the /24 network to which the destination IP address belongs. From this entry it knows that it must forward the data out onto port 2. It then re-encapsulates the layer 3 packet into a new layer 2 frame, this time specifying its own MAC address (of port 2) as the source address and SRV1 s address as the destination address. The data in layer 3 and above stay the same. Network Interface Cards For a device to be able to communicate on a network, it needs to have a network interface card (NIC) installed. In the case of an Ethernet network, this card would have an Ethernet jack on it. In the case of a wireless network, it would have an antenna of some type. Most modern computers include an Ethernet NIC built directly into the motherboard. However, additional cards can be added through expansion slots. It may be useful to have a second NIC for an audio network if it is not part of the normal building network.
12 Page 12 of 18 Cabling and Ethernet In a wired network, devices are connected together with cabling. In most networks, this cabling is copper, although network backbones are typically made up of fiber optics. A standard copper Ethernet cable is made up of eight individual strands of copper. These strands are twisted into pairs at various rates to reduce interference as much as possible. There are different grades of cable and the grade determines the speed of network traffic and the maximum length. Corporate copper networks consist mostly of Category 5 (Cat5), Category 5 enhanced (Cat5e), and Category 6 (Cat6) cable. Network Speeds Depending on the cabling and the individual devices, a network can support a variety of speeds. Generally, higher-numbered cable yields higher speeds and longer distances but is thicker or less flexible. For example, Cat5 cable can only support 100 megabits per second (Mb/s) over a maximum of 100 meters, while Cat5e can support 1 gigabit per second (Gb/s) over short distances. Cat6 can support full gigabit speeds over a full 100 meters and 10 Gb/s speeds over short distances. Power over Ethernet Normally, copper Ethernet is only used for transmitting data. However, some devices can also be powered over the cable, as defined in the IEEE 802.3af standard. Typically this power is enough to support small devices such as an IP phone or a wireless access point. In the case of Dante, this is sufficient power to power microphones and some small speakers. The advantage to this technology is that it uses only a single cable and equipment can be placed in areas where electrical outlets do not usually exist, such as ceilings. DHCP The Dynamic Host Configuration Protocol (DHCP) is an application that enables automatic assignment of IP addresses. Instead of manually assigning IP address to each device, a DHCP server can be put in place to automatically hand out IP addresses in a configured range. It can also specify options denoting the location of default routes, name resolution servers, and time servers. Most home and small office routers include a preconfigured DHCP server so that no configuration is necessary to get a working network and Internet connection. In a large network, a central DHCP server is likely to exist. When a DHCP server does not exist on a network and no IP address is manually configured, a device in some cases can automatically assign itself a link-local IP address in the range of /16. These addresses can only be used to address devices on the same local network. In the case of Dante devices, this may be suitable as long as audio does not need to be routed to another network. A self-contained network of audio devices would be a scenario in which this kind of addressing is sufficient. VLANs A Virtual LAN (VLAN) is a logical separation of a network into multiple, separate broadcast domains. Normally, a broadcast domain is only defined by a router. Under this model, each network exists on a separate switch attached to a port on a router. VLANs allow for these multiple networks to exist on a switch. Essentially, it breaks up a switch into multiple pieces.
13 Page 13 of 18 Typically, VLANs are only a feature of managed switches. A managed switch is configurable by a user to do more advanced things than simply switching frames. Each port on the switch can be assigned a VLAN. Any traffic on that port then belongs to that VLAN. Additionally, a port can also be configured as a trunk port that carries all VLANs. Trunk links are usually only used between switches. When the switch sends a frame out on a trunk port, it tags the frame with its VLAN ID so that the switch on the other end knows what VLAN it belongs to. In an enterprise network, VLANs are traditionally used to segregate traffic. For example, a separate VLAN could be created for IP phones, computers, servers, device management, etc. In the case of Dante, creating a VLAN for audio can help isolate the traffic for security or performance reasons. Quality of Service Quality of Service is a technology that allows the prioritization of some traffic over another. Due to the nature of IP-based networks, some packets may take longer to reach their destination than others. This is usually not an issue for network traffic such as web browsing or file transfers. However, it is a primary concern for any type of streaming audio or video applications as they are sensitive to latency and jitter. Latency is the time it takes a single packet to reach its destination after it is sent. Typically, this is expressed in milliseconds (ms). On a local network, latency will be close to or under 1 ms. Over the Internet, latency could jump to as high as 500 ms or even higher. In the case of live audio, minimal latency is critical. Jitter is the variance in latency for a group of packets to reach their destination. If one packet takes 1 ms to arrive and another takes 3 ms, the jitter in that case is 2 ms. High jitter can cause audio to break up or stutter as some packets don t arrive in time. Prioritizing voice and video traffic will ensure that it always is first in line to be switched and/or routed. This ensures minimal latency and jitter. To differentiate the priority needs of different types of traffic, devices on the network can mark a priority level on their traffic. Basic Network Configuration A basic Dante network does not require complex configurations to operate. If the only requirement of the network is to run Dante devices, such as in a test environment, setup can be as simple as the Dante devices themselves, a PoE switch, and a PC as a controller. In a larger deployment, where the existing network infrastructure will be used, configuration may be necessary in order to coexist nicely with other traffic. Configuring Network Equipment If all of the switches that will be used to connect Dante devices and any switches in between are all connected with layer 2 trunk links, the simplest way to separate the traffic would be to create a separate VLAN just for the audio devices. This keeps all of the devices on the same subnet and keeps non-audio traffic off the audio network. If all of the audio devices are not able to be connected to the same subnet, routers in the network will need to be configured to forward multicast traffic so that the devices can discover each other across network boundaries.
14 Page 14 of 18 Configuring Hosts Dante devices are configured from a Mac or PC. If the devices are to have dynamic IP addresses either from a DHCP server or with link-local addresses, the network adapter needs to be configured appropriately. In Windows, this can be configured from the adapter s Properties page as shown in Figure 7. Select Internet Protocol Version 4 (TCP/IPv4) from the list of items and click the Properties button below it. Figure 7: The Properties page of a network adapter in Windows 8.1 The Properties page for IPv4 will then be displayed where the IP settings can be changed as seen in Figure 8. By default, a network adapter is configured for dynamic assignment. In most cases, this will be appropriate for a Dante network, so make sure Obtain an IP address automatically is selected. If it is a requirement to use static IP addresses instead, specify the addresses manually by selecting Use the following IP address.
15 Page 15 of 18 Figure 8: IPv4 Properties page configured for dynamic address assignment Audinate Dante Audinate Dante is a technology built on top of standard Ethernet networks. As a result, the underlying network needs to be configured correctly for the Dante devices to perform at a high level. If the network is not functioning properly, the audio equipment will not function properly either. Dante Performance and Reliability The performance of Dante is heavily reliant on the network configured underneath it. Any networking equipment that adheres to typical standards will adequately support Dante. In most cases, Audinate recommends running on gigabit Ethernet. Most networking equipment will work well with Dante out of the box. However, certain configurations that may be present in an enterprise network could cause issues. In any case it is appropriate to communicate with the IT department when planning and configuring a Dante network. Quality of Service Considerations Most network equipment will honor the priority markings that a device puts in its traffic. However, in an enterprise setting, this is likely to be tightly controlled. For security reasons, only certain devices can be trusted to set their priorities. If no checks were in place, malicious devices or users could improperly flag their own traffic as important and potentially degrade the performance of the rest of the network.
16 Page 16 of 18 As a result, in most corporate networks, a corresponding configuration needs to be made on network equipment to trust these priority markings. If these configurations are not made, the Dante devices could experience higher than normal latency or jitter, causing audio degradation or dropouts. Even if all the devices are on a separate VLAN, they still need to be given priority so that their traffic is switched ahead of less critical applications. The actual configuration steps are dependent on the vendor of the network equipment as well as the environment and should be performed by IT staff. Congestion and Bottlenecks Congestion on a network occurs when a link reaches 100% capacity. Once at capacity, any additional traffic will wait in a buffer until there is room for it on the link. If the buffer fills up, traffic will start to be dropped. In the case of Dante, this could cause dropouts if audio traffic is being dropped. The most common source of congestion is uplink ports between switches. For example, if 20 devices connected to a switch are each sending traffic at 100 Mbps and that traffic needs to be switched over a 1 gigabit uplink, congestion will occur because there is a total of 2 Gbps of traffic. Security Dante is easy to use by design. Any computer running the Dante controller is able to discover Dante devices on the network and reroute audio. While this makes the setup very simple, it presents a security concern as unauthorized users could potentially mess up the network or eavesdrop on communications. As a result, it is important to properly isolate Dante traffic from the rest of the network using VLANs, much like when optimizing for performance. Troubleshooting Generally, most problems with Dante are caused by a problematic network underneath. There are a variety of methods to troubleshoot a network, but most administrators take a structured approach based on the OSI model. When an issue occurs, they will step through each layer until they find the problem. Typically, they will start at the physical layer and work up or start at the application layer and work down. Sometimes it may also be appropriate to start in the middle and split the model, working up the layers on one half and down the layers on the other. Layer 1 Problems at the physical layer usually are a result of faulty cabling. For example, if the link lights on the back of a network card don t come on, the problem is physical and could either be a bad cable, a faulty or disconnected network card on one end, or a port disabled on a network device. To ensure that layer 1 is functioning as expected, physical connectivity must be ensured between every link between devices. Layer 2 At the data link layer problems usually result from the misconfiguration of network equipment. For example, if two devices are configured to belong to the same network, but their ports on the switch are configured to be in separate VLANs, communication won t be successful. Additionally, if the devices exist on different switches but are configured on the same VLAN, that VLAN must be allowed to traverse
17 Page 17 of 18 a VLAN trunk link between the switches. If a VLAN isn t able to be trunked as it should be, communication will fail 6. At layer 2, successful communication can be verified by ensuring communication is possible between devices on the same subnet. If the device is running a full operating system such as Microsoft Windows or is a network device that can be managed, the ping utility can be used to test connectivity. The ping utility utilizes the Internet Control Message Protocol (ICMP) echo command to send a request to a remote device that will respond with an echo reply 7. The basic syntax of the ping command is simple, as seen in Figure 9. In most devices, the command is followed only by the IP address of the remote host. C:\Users\Administrator>ping Pinging with 32 bytes of data: Reply from : bytes=32 time<1ms TTL=128 Reply from : bytes=32 time<1ms TTL=128 Reply from : bytes=32 time<1ms TTL=128 Reply from : bytes=32 time<1ms TTL=128 Ping statistics for : Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms Figure 9: Example of the syntax and successful output of the ping command when run from the Command Prompt on Windows Layer 3 At the network layer problems result from routing issues. If two devices are in separate subnets, a router must exist between the devices to route the traffic between networks. Routing issues can be caused by mistyped IP addresses or subnet masks as well as by missing routes in a device s routing table. A device only knows about the networks that are defined in its routing table. For an end device like a Dante microphone or a desktop computer there is a route for the local network and a default route if configured. A default route is simply a route for the /0 network that points at a router on the local network, known as the default gateway. This causes any traffic not destined for the local network to be forwarded to the default gateway. Routers are more complex in that they maintain a table of many more routes. If there are multiple routers in a network, they must all know about the others routes. Typically, if a device cannot reach a host in a remote network, the problem is with a router s routing table. Layer 4 At the transport layer communication problems are usually the fault of a firewall. This can be either the firewall that exists on a PC or an appliance that exists on the network between the devices. A firewall will block traffic on ports to mitigate possible attacks. If it blocks a port that is needed by an application, 6 A variety of other technologies can cause problems at layer 2, including Spanning Tree Protocol (STP) as well as link aggregation. These technologies are beyond the scope of this document. 7 In some corporate networks, ICMP may be filtered by the network or disabled on the remote device.
18 Page 18 of 18 that application may break. To troubleshoot communication at layer 4, make sure that any firewalls between two devices are configured to allow the necessary ports. Layers 5, 6, and 7 Problems above layer 4 are usually a result of the application itself. In rare instances, firewalls that do deep inspection into network traffic may be the culprit, but problems are normally a fault of the program.
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