Tables Used in Switching

Tables Used in Switching

Catalyst switches maintain several types of tables to be used in the switching process. The tables are tailored for Layer 2 switching or MLS and are kept in very fast memory so that many fields within a frame or packet can be compared in parallel.

Content-Addressable Memory

All Catalyst switch models use a CAM table for Layer 2 switching. As frames arrive on switch ports, the source MAC addresses are learned and recorded in the CAM table. The port of arrival and the VLAN both are recorded in the table, along with a time stamp. If a MAC address learned on one switch port has moved to a different port, the MAC address and time stamp are recorded for the most recent arrival port. Then, the previous entry is deleted. If a MAC address is found already present in the table for the correct arrival port, only its time stamp is updated.

Switches generally have large CAM tables so that many addresses can be looked up for frame forwarding. However, there is not enough table space to hold every possible address on large networks. To manage the CAM table space, stale entries (addresses that have not been heard from for a period of time) are aged out. By default, idle CAM table entries are kept for 300 seconds before they are deleted. You can change the default setting using the following configuration command:

Switch(config)# mac address-table aging-time seconds

By default, MAC addresses are learned dynamically from incoming frames. You also can configure static CAM table entries that contain MAC addresses that might not be learned otherwise. To do this, use the following configuration command:

Switch(config)# mac address-table static mac-address vlan vlan-id interface type mod/num

Exactly what happens when a host’s MAC address is learned on one switch port, and then the host moves so that it appears on a different switch port? Ordinarily, the host’s original CAM table entry would have to age out after 300 seconds, while its address was learned on the new port. To avoid having duplicate CAM table entries during that time, a switch purges any existing entries for a MAC address that has just been learned on a different switch port. This is a safe assumption because MAC addresses are unique, and a single host should never be seen on more than one switch port unless problems exist in the network. If a switch notices that a MAC address is being learned on alternating switch ports, it generates an error message that flags the MAC address as “flapping” between interfaces.

Ternary Content-Addressable Memory

In traditional routing, ACLs can match, filter, or control specific traffic. Access lists are made up of one or more access control entities (ACEs) or matching statements that are evaluated in sequential order. Evaluating an access list can take up additional time, adding to the latency of forwarding packets.

In multilayer switches, however, all the matching process that ACLs provide is implemented in hardware called a TCAM. With a TCAM, a packet can be evaluated against an entire access list within a single table lookup. Most switches have multiple TCAMs so that both inbound and outbound security and QoS ACLs can be evaluated simultaneously, or entirely in parallel with a Layer 2 or Layer 3 forwarding decision.

The Catalyst IOS Software has two components that are part of the TCAM operation:

■ Feature Manager (FM): After an access list has been created or configured, the Feature Manager software compiles, or merges, the ACEs into entries in the TCAM table. The TCAM then can be consulted at full frame-forwarding speed.

■ Switching Database Manager (SDM): On some Catalyst switch models, the TCAM is partitioned into several areas that support different functions. The SDM software configures or tunes the TCAM partitions, if needed, to provide ample space for specific switching functions. (The TCAM is fixed on Catalyst 4500 and 6500 platforms and cannot be repartitioned.)

TCAM Structure

The TCAM is an extension of the CAM table concept. Recall that a CAM table takes in an index or key value (usually a MAC address) and looks up the resulting value (usually a switch port or VLAN ID). Table lookup is fast and always based on an exact key match consisting of binary numbers made up of two possible values: 0 and 1 bits.

TCAM also uses a table-lookup operation but is greatly enhanced to allow a more abstract operation. For example, binary values (0s and 1s) make up a key into the table, but a mask value also is used to decide which bits of the key are actually relevant. This effectively makes a key consisting of three input values: 0, 1, and X (do not care) bit values—a threefold or ternary combination.

TCAM entries are composed of Value, Mask, and Result (VMR) combinations. Fields from frame or packet headers are fed into the TCAM, where they are matched against the value and mask pairs to yield a result. As a quick reference, these can be described as follows:

■ Values are always 134-bit quantities, consisting of source and destination addresses and other relevant protocol information—all patterns to be matched. The information concatenated to form the value depends on the type of access list, as shown in Below Table. Values in the TCAM come directly from any address, port, or other protocol information given in an ACE, up to a maximum of 134 bits.

TCAM Value Pattern Components

Access List Type	Value and Mask Components (Number of Bits)
Ethernet	Source MAC (48), destination MAC (48), EtherType (16)
ICMP	Source IP (32), destination IP (32), protocol (16), ICMP code (8), ICMP type (4), IP type of service (ToS) (8)
Extended IP using TCP/UDP	Source IP (32), destination IP (32), protocol (16), IP ToS (8), source port (16), source operator (4), destination port (16), destination operator (4)
Other IP	Source IP (32), destination IP (32), protocol (16), IP ToS (8)
IGMP	Source IP (32), destination IP (32), protocol (16), IP ToS (8), IGMP message type (8)

■ Masks are also 134-bit quantities, in exactly the same format, or bit order, as the values. Masks select only the value bits of interest; a mask bit is set to mark a value bit to be exactly matched or is not set to mark a value bit that does not matter. The masks used in the TCAM stem from address or bit masks in ACEs.

■ Results are numeric values that represent what action to take after the TCAM lookup occurs. Whereas traditional access lists offer only a permit or deny result, TCAM lookups offer a number of possible results or actions. For example, the result can be a permit or deny decision, an index value to a QoS policer, a pointer to a next-hop routing table, and so on.

The TCAM is always organized by masks, where each unique mask has eight value patterns associated with it. For example, the Catalyst 6500 TCAM (one for security ACLs and one for QoS ACLs) holds up to 4096 masks and 32,768 value patterns. The trick is that each of the mask-value pairs is evaluated simultaneously, or in parallel, revealing the best or longest match in a single table lookup.

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Multilayer Switch Operation

Multilayer Switch Operation

Many Cisco Catalyst switches can also forward frames based on Layers 3 and 4 information contained in packets. This is known as multilayer switching (MLS). Naturally, Layer 2 switching is performed at the same time because even the higher-layer encapsulations still are contained in Ethernet frames.

Types of Multilayer Switching

Catalyst switches have supported two basic generations or types of MLS: route caching (first-generation MLS) and topology based (second-generation MLS). This section presents an overview of both, although only the second generation is supported in the Cisco IOS Software-based switch families, such as the Catalyst 2960, 3750, 4500, and 6500. You should understand the two types and the differences between them:

■ Route caching: The first generation of MLS, requiring a route processor (RP) and a switch engine (SE). The RP must process a traffic flow’s first packet to determine the destination. The SE listens to the first packet and to the resulting destination, and then sets up a “shortcut” entry in its MLS cache. The SE forwards subsequent packets belonging to the same traffic flow based on shortcut entries in its cache.

This type of MLS also is known by the names NetFlow LAN switching , flow-based or demand-based switching , and route once, switch many . The RP must examine each new traffic flow and set up shortcut entries for the SE. Even if this method isn’t used to forward packets in Cisco IOS–based Catalyst switches, the technique can still be used to generate traffic flow information and statistics.

■ Topology based: The second generation of MLS, utilizing specialized hardware, is also organized with distinct RP and SE functions. The RP uses Layer 3 routing information to build and prepopulate a single database of the entire known network topology. This database becomes an efficient table lookup in hardware, and is consulted so that packets can be forwarded at high rates by the SE. The longest match found in the database is used as the correct Layer 3 destination. As the routing topology changes over time, the database contained in the hardware can be updated dynamically with no performance penalty.

This type of MLS is known as Cisco Express Forwarding (CEF). A routing process running on the switch downloads the current routing table database into the Forwarding Information Base (FIB) area of hardware.

Tip Although the RP and SE functions within a multilayer switch do interact, they can operate independently, as if they are on different “planes.” The control plane of a switch includes the RP and any process that runs to control or manage the switch, whereas the data plane exists in the SE, where data is forwarded.

Follow That Packet!

The path that a Layer 3 packet follows through a multilayer switch is similar to that of a Layer 2 switch. Obviously, some means of making a Layer 3 forwarding decision must be added. Beyond that, several, sometimes unexpected, things can happen to packets as they are forwarded. Below figure shows a typical multilayer switch and the decision processes that must occur. Packets arriving on a switch port are placed in the appropriate ingress queue, just as in a Layer 2 switch.

Operations Within a Multilayer Catalyst Switch

Each packet is pulled off an ingress queue and inspected for both Layer 2 and Layer 3 destination addresses. Now, the decision of where to forward the packet is based on two address tables, whereas the decision of how to forward the packet still is based on access list results.

All the multilayer switching decisions are performed simultaneously in hardware, using the following functions:

■ L2 forwarding table: The destination MAC address is used as an index into the CAM table. If the frame contains a Layer 3 packet that needs to be forwarded from one subnet to another, the destination MAC address will contain the address of a Layer 3 port on the switch itself. In this case, the CAM table results are used only to decide that the frame should be processed at Layer 3.

■ L3 forwarding table: The FIB table is consulted, using the destination IP address as an index. The longest match in the table is found (both address and mask), and the resulting next-hop Layer 3 address is obtained. The FIB also contains each next-hop router’s Layer 2 MAC address and the egress switch port (and VLAN ID) so that further table lookups are not necessary.

■ Security ACLs: Inbound and outbound access lists are compiled into TCAM entries so that decisions of whether to forward a packet can be determined as a single table lookup.

■ QoS ACLs: Packet classification, policing, and marking all can be performed as single table lookups in the QoS TCAM.

As with Layer 2 switching, the packet finally must be placed in the appropriate egress queue on the appropriate egress switch port.

During the multilayer switching process, some portions of the frame must be modified or rewritten, just as any router would do. For example, the destination MAC address in the inbound frame contains the address of the next-hop destination, which is the ingress Layer 3 interface on the multilayer switch. Once the FIB table is consulted, the next-hop router IP and MAC addresses are found.

The next-hop Layer 2 address must be put into the frame in place of the original destination address (the multilayer switch). The frame’s Layer 2 source address also must become that of the multilayer switch’s egress interface before the frame is sent on to the next hop. As any good router must do, the time-to-live (TTL) value in the Layer 3 packet must be decremented by one.

Because the contents of the Layer 3 packet (the TTL value) have changed, the Layer 3 header checksum must be recalculated. And because both Layers 2 and 3 contents have changed, the Layer 2 checksum must be recalculated. In other words, the entire Ethernet frame must be rewritten before it goes into the egress queue. This also is accomplished efficiently in hardware.

Multilayer Switching Exceptions

To forward packets using the simultaneous decision processes, the packet must be “MLS ready” and must require no additional decisions. For example, CEF can directly forward most IP and IPv6 packets between hosts. This occurs when the source and destination addresses (both MAC and IP) are already known and no other IP parameters must be manipulated.

Other packets cannot be directly forwarded by CEF and must be handled in more detail. This is done by a quick inspection during the forwarding decisions. If a packet meets criteria such as the following, it is flagged for further processing and sent or “punted” to the switch CPU for process switching :

■ ARP requests and replies

■ IP packets requiring a response from a router (TTL has expired, maximum transmission unit [MTU] is    exceeded, fragmentation is needed, and so on)

■ IP broadcasts that will be relayed as unicast (Dynamic Host Configuration Protocol [DHCP] requests, IP helper-address functions)

■ Routing protocol updates

■ Cisco Discovery Protocol (CDP) packets

■ Packets needing encryption

■ Packets triggering Network Address Translation (NAT)

■ Legacy multiprotocol packets (IPX, AppleTalk, and so on)

Packets that are punted to the CPU cannot be forwarded as efficiently as ones that can be forwarded in hardware directly. The additional processing takes additional time and consumes CPU resources. Ideally, all packets should be forwarded in hardware, but that is not always possible.

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VLAN Trunking Protocol (VTP)

VLAN Trunking Protocol (VTP)

VLAN configuration and trunking on a switch or a small group of switches is fairly intuitive. Campus network environments, however, usually consist of many interconnected switches. Configuring and managing a large number of switches, VLANs, and VLAN trunks quickly can get out of control.

Cisco has developed a method to manage VLANs across the campus network. The VLAN Trunking Protocol (VTP) uses Layer 2 trunk frames to communicate VLAN information among a group of switches. VTP manages the addition, deletion, and renaming of VLANs across the network from a central point of control. Any switch participating in a VTP exchange is aware of and can use any VLAN that VTP manages.

VTP Domains

VTP is organized into management domains , or areas with common VLAN requirements. A switch can belong to only one VTP domain, sharing VLAN information with other switches in the domain. Switches in different VTP domains, however, do not share VTP information.

Switches in a VTP domain advertise several attributes to their domain neighbors. Each advertisement contains information about the VTP management domain, VTP revision number, known VLANs, and specific VLAN parameters. When a VLAN is added to a switch in a management domain, other switches are notified of the new VLAN through VTP advertisements . In this way, all switches in a domain can prepare to receive traffic on their trunk ports using the new VLAN.

VTP Modes

To participate in a VTP management domain, each switch must be configured to operate in one of several modes. The VTP mode determines how the switch processes and advertises VTP information. You can use the following modes:

Server mode : VTP servers have full control over VLAN creation and modification for their domains. All VTP information is advertised to other switches in the domain, while all received VTP information is synchronized with the other switches. By default, a switch is in VTP server mode. Note that each VTP domain must have at least one server so that VLANs can be created, modified, or deleted, and VLAN information can be propagated.

Client mode : VTP clients do not allow the administrator to create, change, or delete any VLANs. Instead, they listen to VTP advertisements from other switches and modify their VLAN configurations accordingly. In effect, this is a passive listening mode. Received VTP information is forwarded out trunk links to neighboring switches in the domain, so the switch also acts as a VTP relay.

Transparent mode : VTP transparent switches do not participate in VTP. While in transparent mode, a switch does not advertise its own VLAN configuration, and it does not synchronize its VLAN database with received advertisements. In VTP version 1, a transparent mode switch does not even relay VTP information it receives to other switches unless its VTP domain names and VTP version numbers match those of the other switches. In VTP version 2, transparent switches do forward received VTP advertisements out of their trunk ports, acting as VTP relays. This occurs regardless of the VTP domain name setting.

Off mode : Like transparent mode, switches in VTP off mode do not participate in VTP; however, VTP advertisements are not relayed at all. You can use VTP off mode to disable all VTP activity on or through a switch.

Tip: While a switch is in VTP transparent mode, it can create and delete VLANs that are local only  to itself. These VLAN changes, however, are not propagated to any other switch.

VTP Advertisements

VTP has evolved over time to include three different versions. Cisco switches can support all three versions, but the versions are not fully backward compatible with each other. If a network contains switches that are running different VTP versions, you should consider how the switches will interact with their VTP information. By default, Cisco switches use VTP Version 1.

Each Cisco switch participating in VTP advertises VLANs, revision numbers, and VLAN parameters on its trunk ports to notify other switches in the management domain. VTP Versions 1 and 2 support VLAN numbers 1 to 1005, whereas only VTP Version 3 supports the full extended VLAN range 1 to 4094.

VTP advertisements are sent as multicast frames. A switch intercepts frames sent to the VTP multicast address and processes them locally. The advertisements can also be relayed or forwarded out trunk links toward neighboring switches in all VTP modes except off mode. Because all switches in a management domain learn of new VLAN configuration changes, a VLAN must be created and configured on only one VTP server switch in the domain.

By default, management domains are set to use nonsecure advertisements without a password. You can add a password to set the domain to secure mode. The same password must be configured on every switch in the domain so that all switches exchanging VTP           information use identical encryption methods.

VTP switches use an index called the VTP configuration revision number to keep track of the most recent information. Every switch in a VTP domain stores the configuration revision number that it last heard from a VTP advertisement. The VTP advertisement process always starts with configuration revision number 0.

When subsequent changes are made on a VTP server, the revision number is incremented before the advertisements are sent. When listening switches (configured as members of the same VTP domain as the advertising switch) receive an advertisement with a greater revision number than is stored locally, they assume that the advertisement contains new and updated information. The advertisement is stored and overwrites any previously stored VLAN information.

VTP advertisements usually originate from server mode switches as VLAN configuration changes occur and are announced. Advertisements can also originate as requests from client mode switches that want to learn about the VTP database as they boot.

VTP advertisements can occur in three forms:

Summary advertisements : VTP domain servers send summary advertisements every 300 seconds and every time a VLAN database change occurs. The summary advertisement lists information about the management domain, including VTP version, domain name, configuration revision number, time stamp, MD5 encryption hash code, and the number of subset advertisements to follow. For VLAN configuration

changes, summary advertisements are followed by one or more subset advertisements with more specific VLAN configuration data.

Subset advertisements : VTP domain servers send subset advertisements after a VLAN configuration change occurs. These advertisements list the specific changes that have been performed, such as creating or deleting a VLAN, suspending or activating a VLAN, changing the name of a VLAN, and changing a VLAN’s maximum transmission unit (MTU). Subset advertisements can list the following VLAN parameters: status of the VLAN, VLAN type (such as Ethernet or Token Ring), MTU, length of the VLAN name, VLAN number, security association identifier (SAID) value, and VLAN name. VLANs are listed individually in sequential subset advertisements.

Advertisement requests from clients: A VTP client can request any VLAN information it lacks. For example, a client switch might be reset and have its VLAN database cleared, and its VTP domain membership might be changed, or it might hear a VTP summary advertisement with a higher revision number than it currently has. After a client advertisement request, the VTP domain servers respond with summary and subset advertisements to bring it up to date.

Catalyst switches in server mode store VTP information separately from the switch configuration in NVRAM. VLAN and VTP data are saved in the vlan.dat file on the switch’s flash memory file system. All VTP information, including the VTP configuration revision number, is retained even when the switch power is off. In this manner, a switch can recover the last known VLAN configuration from its VTP database after it reboots.

VTP Synchronization

Whenever a switch receives a VTP advertisement with a configuration revision number that is greater than the value stored locally, it considers the advertisement to contain newer information. The switch will overwrite its own VLAN data with the newer version—even if the newer version contains irrelevant information. Because of this, it is very important to always force any newly added network switches to have revision number 0 before being attached to the network. Otherwise, a switch might have stored a revision

number that is greater than the value currently in use in the domain.

The VTP revision number is stored in NVRAM and is not altered by a power cycle of the switch; therefore, the revision number can be initialized to 0 only by using one of the following methods:

■ Change the switch’s VTP mode to transparent and then change the mode back to server.

■ Change the switch’s VTP domain to a bogus name (a nonexistent VTP domain), and then change the VTP domain back to the original name.

If the VTP revision number is not reset to 0, the switch might enter the network as a VTP server and have a preexisting revision number (from a previous life) that is higher than in previous legitimate advertisements. The new switch’s VTP information would be seen as more recent, so all other switches in the VTP domain would gladly accept its database of VLANs and overwrite their good VLAN database entries with null or deleted VLAN status information.

In other words, a new server switch might inadvertently cause every other working switch to flush all records of every VLAN in production. The VLANs would be deleted from the VTP database and from the switches, causing any switch port assigned to them to be returned to the default VLAN 1. This is referred to as a VTP synchronization problem. For critical portions of your network, you should consider using VTP transparent or off mode to prevent the synchronization problem from ever becoming an issue.

Networking Multiple Choice Questions and Answers (MCQ)

Networking Multiple Choice Questions and Answers (MCQ)

Question 1. Where would a router boot from if a configuration register was set to 0x0101?

A.Flash

B.ROM

C.Boot ROM

D.NVRAM

Answer 1.

B. The configuration register is used to tell the router how to load the IOS and configuration. The value 0x0101 tells the router to boot from ROM. 

Question 2.What is the correct command to set the router identification to the name Atlanta, which an administrator would see when connecting with Telnet or through the console?

A.Description Atlanta Router

B.Banner Motd $

C.Hostname Atlanta

D.Host name Atlanta

E.Set prompt Atlanta

Answer 2.

C. The command hostname (one word) is used to set the name of the router.

Question 3. Which of the following is the valid host range for the IP address 192.168.168.188 255.255.255.192?
A. 192.168.168.129–190
B. 192.168.168.129–191
C. 192.168.168.128–190
D. 192.168.168.128–192

Answer 3.
A. Start by using 256, the subnet mask, which is 256–192=64. The first subnet is 64. The next subnet would be 128. This host is in the128 subnet, the broadcast address is 191, and the valid host range is 129 through 190.

Question 4.What are the access list numbers used for IP standard access lists?
A.1–10
B.1–99
C.100–199
D.1000–1999

Answer 4.
B. Standard IP access lists use the numbers 1–99. 

Question 5.If you type show interface serial 0 and receive the following response, what could the problem be?
RouterA#sh int s0
Serial0 is up, line protocol is down

A. The keepalives could be set wrong between the point-to-point links.
B.  No cable is attached to the interface.
C. The administrator needs to issue a no shutdown request to the interface.
D. The interface is defective.

Answer 5.
A. If you see the line is up, but the protocol is down, you are having a clocking (keepalive) or framing issue. Check the keepalives on both ends to make sure they match; the clock rate is set, if needed; and the encapsulation type is the same on both ends.

Question 6.How do you create a default route?
A.By using all 1s in place of the network and mask
B.By defining a static route and using all 0s in place of the network and mask
C.By using 255 in place of the network and mask
D.Login <name, password>

Answer 6.
B. Default routes are created by using all 0s in place of the network and mask IDs.

Question 7.What is a trunked link?
A.A link that is only part of one VLAN and is referred to as the native VLAN of the port
B.A link that can carry multiple VLANs
C.A switch port connected to the Internet
D.Data and voice capability on the same interface

Answer 7.
B. Trunked links carry multiple VLANs across the same link.

Question 8.Which protocol is used to look up an IP address from a known Ethernet address?
A.IP
B.ARP
C.RARP
D.TCP

Answer 8.
C. The protocol at the Network layer that finds an IP address from a known Ethernet address is Reverse ARP (RARP).

Question 9.What is the broadcast address of the subnet address 192.168.99.20 255.255.255.252?
A.192.168.99.127
B.192.168.99.63
C.192.168.99.23
D.192.168.99.31

Answer 9.
C. Start by using 256, the subnet mask, which is 256–25=4. The first subnet is 4. The next subnet would be 8, then 12, 16, 20, and 24. The broadcast address is 23, and the valid host range is 21 and 22.

Question 10.What is the valid host range that the host ID 192.168.10.33 255.255.255.224 is a part of?
A.192.168.10.32–63
B.192.168.10.33–63
C.192.168.10.33–62
D.192.168.10.33–61

Answer 10.
C. Start by using 256, the subnet mask, which is 256–224=32. The first subnet is 10.32. The next subnet would be 10.64. This host is in
the 10.32 subnet, the broadcast address is 10.63, and the valid host range is 10.33 through 10.62.

Question 11.What does a switch do with a multicast frame received on an interface?
A.Forwards the switch to the first available link
B.Drops the frame
C.Floods the network with the frame looking for the device
D.Sends back a message to the originating station asking for a name resolution

Answer 11.
C. The switch will flood the network with the frame looking for the device.

Question 12.What is an access link?
A. A link that is only part of one VLAN and is referred to as the native VLAN of the port
B. A link that can carry multiple VLANs
C. A switch port connected to the Internet
D. Data and voice capability on the same interface

Answer 12.
A. Access links connect hosts to a switch and are part of only one VLAN.

Question 13.What command can you use to back up the Cisco router configuration to a TFTP host?
A. copy run tftp
B. copy flash tftp
C. copy nvram startup
D. copy tftp flash

Answer 13.
A. The command copy running-config tftp (copy run tftp for short) will copy the router’s configuration to a TFTP host for backup purposes.

OSI Layer Questions and Answers

OSI Layer Questions and Answers

Question 1. Which layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery?

Answer 1. The Application layer is responsible for finding the network resources broadcast from a server and adding flow control and error control (if the application developer chooses).

Question 2. Which layer is responsible for converting data packets from the Data Link layer into electrical signals?

Answer 2. The Physical layer takes frames from the Data Link layer and encodes the 1s and 0s into a digital signal for transmission on the network medium.

Question 3. At which layer is routing implemented, enabling connections and path selection between two end systems?

Answer 3. The Network layer provides routing through an internetwork and logical addressing.

Question 4. Which layer defines how data is formatted, presented, encoded, and converted for use on the network?

Answer 4. The Presentation layer makes sure that data is in a readable format for the Application layer.

Question 5. Which layer is responsible for creating, managing, and terminating sessions between applications?

Answer 5. The Session layer sets up, maintains, and terminates sessions between applications.

Question 6. Which layer ensures the trustworthy transmission of data across a physical link and is primarily concerned with physical addressing, line discipline, network topology, error notification, ordered delivery of frames, and flow control?

Answer 6. PDUs at the Data Link layer are called frames and provide physical addressing, plus other options to place packets on the network medium.

Question 7. Which layer is used for reliable communication between end nodes over the network and provides mechanisms for establishing, maintaining, and terminating virtual circuits; transport-fault detection and recovery; and controlling the flow of information?

Answer 7. The Transport layer uses virtual circuits to create a reliable connection between two hosts.

Question 8. Which layer provides logical addressing that routers will use for path determination?

Answer 8. The Network layer provides logical addressing, typically IP addressing and routing.

Question 9. Which layer specifies voltage, wire speed, and pinout cables and moves bits between devices?

Answer 9. The Physical layer is responsible for the electrical and mechanical connections between devices.

Question 10. Which layer combines bits into bytes and bytes into frames, uses MAC addressing, and provides error detection?

Answer 10. The Data Link layer is responsible for the framing of data packets.

Question 11. Which layer is responsible for keeping the data from different applications separate on the network?

Answer 11. The Session layer creates sessions between different hosts’ applications.

Question 12. Which layer is represented by frames?

Answer 12. The Data Link layer frames packets received from the Network layer.

Question 13. Which layer is represented by segments?

Answer 13. The Transport layer segments user data.

Question 14. Which layer is represented by packets?

Answer 14. The Network layer creates packets out of segments handed down from the Transport layer.

Question 15. Which layer is represented by bits?

Answer 15. The Physical layer is responsible for transporting 1s and 0s (bits) in a digital signal.

Question 16. Put the following in order of encapsulation:

Packets

Frames

Bits

Segments

Answer 16. Segments, packets, frames, bits

Question 17. Which layer segments and reassembles data into a data stream?

Answer 17. Transport

Question 18. Which layer provides the physical transmission of the data and handles error notification, network topology, and flow control?

Answer 18. Data Link

Question 19. Which layer manages device addressing, tracks the location of devices on the network, and determines the best way to move data?

Answer 19. Network

Question 20. What is the bit length and expression form of a MAC address?

Answer 20. 48 bits (6 bytes) expressed as a hexadecimal number