1. Saeed Darvish Pazoki – MCSE, CCNA Abstracted From: Cisco Press – ICND 1 – Chapter 7 Ethernet LAN Switching Concepts 1

2. LAN Switching Concepts Historical Progression: Hubs, Bridges, and Switches Ethernet started out with standards that used a physical electrical bus created with coaxial cabling. 10BASE-T Ethernet came next. It offered improved LAN availability, because a problem on a single cable did not affect the rest of the LAN—a common problem with 10BASE2 and 10BASE5 networks. 10BASE-T allowed the use of unshielded twisted-pair (UTP) cabling, which is much cheaper than coaxial cable. Also, many buildings already had UTP cabling installed for phone service, so 10BASE-T quickly became a popular alternative to 10BASE2 and 10BASE5 Ethernet networks. 2

3. LAN Switching Concepts Historical Progression: Hubs, Bridges, and Switches Although using 10BASE-T with a hub improved Ethernet as compared to the older standards, several drawbacks continued to exist, even with 10BASE-T using hubs: Any device sending a frame could have the frame collide with a frame sent by any other device attached to that LAN segment. Only one device could send a frame at a time, so the devices shared the (10-Mbps) bandwidth (Half-Duplex). Broadcasts sent by one device were heard by, and processed by, all other devices on the LAN. 3

4. LAN Switching Concepts Historical Progression: Hubs, Bridges, and Switches Over time, the performance of many Ethernet networks started to degrade. People developed applications to take advantage of the LAN bandwidth. More devices were added to each Ethernet. Eventually, an entire network became congested. Ethernet bridges were created to solve some of the performance issues. Bridges solved the growing Ethernet congestion problem in two ways: They reduced the number of collisions that occurred in the network. They added bandwidth to the network. If one LAN segment is busy, and the bridge needs to forward a frame onto the busy segment, the bridge simply buffers the frame (holds the frame in memory) until the segment is no longer busy. Adding a bridge between two hubs really creates two separate 10BASE-T networks. 4

5. LAN Switching Concepts Historical Progression: Hubs, Bridges, and Switches The following figure shows the basic premise behind an Ethernet transparent bridge. The top part of the figure shows a 10BASE-T network before adding a bridge, and the lower part shows the network after it has been segmented using a bridge. The bridge creates two separate collision domains. 5

6. LAN Switching Concepts Historical Progression: Hubs, Bridges, and Switches LAN switches perform the same basic core functions as bridges, but with many enhanced features. Like bridges, switches segment a LAN into separate parts, each part being a separate collision domain. Switches have potentially large numbers of interfaces, with highly optimized hardware, allowing even small Enterprise switches to forward millions of Ethernet frames per second. 6

7. LAN Switching Concepts Switching Logic The role of a LAN switch is to forward Ethernet frames. To achieve that goal, switches use logic—logic based on the source and destination MAC address in each frame’s Ethernet header. The IEEE defines three general categories of Ethernet MAC addresses: Unicast addresses: MAC addresses that identify a single LAN interface card. Broadcast addresses: A frame sent with a destination address of the broadcast address (FFFF.FFFF.FFFF) implies that all devices on the LAN should receive and process the frame. Multicast addresses: Multicast MAC addresses are used to allow a dynamic subset of devices on a LAN to communicate. 7

8. LAN Switching Concepts Switching Logic The primary job of a LAN switch is to receive Ethernet frames and then make a decision: either forward the frame out some other port(s), or ignore the frame. To accomplish this primary mission, transparent bridges perform three actions: Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address Learning MAC addresses by examining the source MAC address of each frame received by the bridge Creating a (Layer 2) loop-free environment with other bridges by using Spanning Tree Protocol (STP) 8

9. LAN Switching Concepts Switching Logic The Forward Versus Filter Decision To decide whether to forward a frame, a switch uses a dynamically built table that lists MAC addresses and outgoing interfaces. Switches compare the frame’s destination MAC address to this table to decide whether the switch should forward a frame or simply ignore it. A switch’s MAC address table is also called the switching table, or bridging table, or even the Content Addressable Memory (CAM), in reference to the type of physical memory used to store the table. 9

10. LAN Switching Concepts Switching Logic The Forward Versus Filter Decision 10

11. LAN Switching Concepts Switching Logic The Forward Versus Filter Decision 11

12. LAN Switching Concepts Switching Logic How Switches Learn MAC Addresses Switches build the address table by listening to incoming frames and examining the source MAC address in the frame. If a frame enters the switch and the source MAC address is not in the MAC address table, the switch creates an entry in the table. The MAC address is placed in the table, along with the interface from which the frame arrived. 12

13. LAN Switching Concepts Switching Logic Flooding Frames when there is no matching entry in the table, switches forward the frame out all interfaces (except the incoming interface). Switches forward these unknown unicast frames (frames whose destination MAC addresses are not yet in the bridging table) out all other interfaces, with the hope that the unknown device will be on some other Ethernet segment and will reply, allowing the switch to build a correct entry in the address table. The process of sending frames out all other interfaces, except the interface on which the frame arrived, is called flooding. Switches keep a timer for each entry in the MAC address table, called an inactivity timer. The switch sets the timer to 0 for new entries. Each time the switch receives another frame with that same source MAC address, the timer is reset to 0. The timer counts upward, so the switch can tell which entries have gone the longest time since receiving a frame from that device. If the switch ever runs out of space for entries in the MAC address table, the switch can then remove table entries with the oldest (largest) inactivity timers. 13

14. LAN Switching Concepts Switching Logic Avoiding Loops Using Spanning Tree Protocol The third primary feature of LAN switches is loop prevention, as implemented by Spanning Tree Protocol (STP). Without STP, frames would loop for an indefinite period of time in Ethernet networks with physically redundant links. To prevent looping frames, STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments (collision domains). STP causes each interface on a switch to settle into either a blocking state or a forwarding state. Blocking means that the interface cannot forward or receive data frames. Forwarding means that the interface can send and receive data frames. 14

15. LAN Switching Concepts Switching Logic Avoiding Loops Using Spanning Tree Protocol 15

16. LAN Switching Concepts Switching Logic Internal Processing on Cisco Switches As soon as a Cisco switch decides to forward a frame, the switch can use a couple of different types of internal processing variations. Almost all of the more recently released switches use store-and-forward processing, but all three types of these internal processing methods are supported in at least one type of currently available Cisco switch. Some switches, and transparent bridges in general, use store-and-forward processing. With store-and-forward, the switch must receive the entire frame before forwarding the first bit of the frame. Cisco also offers two other internal processing methods for switches: cut-through and fragment-free. 16

17. LAN Switching Concepts Switching Logic Internal Processing on Cisco Switches With cut-through processing, the switch starts sending the frame out the output port as soon as possible. Although this might reduce latency, it also propagates errors. Because the frame check sequence (FCS) is in the Ethernet trailer, the switch cannot determine if the frame had any errors before starting to forward the frame. So, the switch reduces the frame’s latency, but with the price of having forwarded some frames that contain errors. Fragment-free processing works similarly to cut-through, but it tries to reduce the number of errored frames that it forwards. One interesting fact about Ethernet carrier sense multiple access with collision detection (CSMA/CD) logic is that collisions should be detected within the first 64 bytes of a frame. Fragment-free processing works like cut-through logic, but it waits to receive the first 64 bytes before forwarding a frame. The frames experience less latency than with store-and-forward logic and slightly more latency than with cut-through, but frames that have errors as a result of collisions are not forwarded. 17

18. LAN Design Considerations Collision Domains and Broadcast Domains Collision Domains A collision domain is the set of LAN interfaces whose frames could collide with each other, but not with frames sent by any other devices in the network. 18

19. LAN Design Considerations Collision Domains and Broadcast Domains Broadcast Domains The term broadcast domain relates to where broadcasts can be forwarded. A broadcast domain encompasses a set of devices for which, when one of the devices sends a broadcast, all the other devices receive a copy of the broadcast 19

20. LAN Design Considerations Collision Domains and Broadcast Domains General definitions for a collision domain and a broadcast domain are as follows: A collision domain is a set of network interface cards (NIC) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain. A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain. 20

21. LAN Design Considerations Collision Domains and Broadcast Domains The Impact of Collision and Broadcast Domains on LAN Design 21

22. LAN Design Considerations Virtual LANs (VLAN) A LAN consists of all devices in the same broadcast domain. With VLANs, a switch can put some interfaces into one broadcast domain and some into another based on some simple configuration. Essentially, the switch creates multiple broadcast domains by putting some interfaces into one VLAN and other interfaces into other VLANs. 22

23. LAN Design Considerations Virtual LANs (VLAN) 23

24. LAN Design Considerations Campus LAN Design Terminology The term campus LAN refers to the LAN created to support larger buildings, or multiple buildings in somewhat close proximity to one another. When planning and designing a campus LAN, the engineers must consider: The types of Ethernet available and the cabling lengths supported by each type. Choose the speeds required for each Ethernet segment Some thought needs to be given to the idea that some switches should be used to connect directly to end-user devices, whereas other switches might need to simply connect to a large number of these end-user switches. Most projects require that the engineer consider the type of equipment that is already installed and whether an increase in speed on some segments is worth the cost of buying new equipment. 24

25. LAN Design Considerations 25

26. LAN Design Considerations Campus LAN Design Terminology Cisco uses three terms to describe the role of each switch in a campus design: access, distribution, and core. The roles differ mainly in two main concepts: Whether the switch should connect to end-user devices Whether the switch should forward frames between other switches by connecting to multiple different switches Access switches connect directly to end users, providing access to the LAN. Access switches should not, at least by design, be expected to forward traffic between two other switches. 26

27. LAN Design Considerations Campus LAN Design Terminology In larger campus LANs, distribution switches provide a path through which the access switches can forward traffic to each other. By design, each of the access switches connects to at least one distribution switch. However, designs use at least two uplinks to two different distribution switches for redundancy. Core switches provide extremely high forwarding rates—these days into the hundreds of millions of frames per second. The reasons for core switches are generally the same as for distribution switches. 27

28. LAN Design Considerations Campus LAN Design Terminology The following list summarizes the terms that describe the roles of campus switches: Access: Provides a connection point (access) for end-user devices. Does not forward frames between two other access switches under normal circumstances. Distribution: Provides an aggregation point for access switches, forwarding frames between switches, but not connecting directly to end- user devices. Core: Aggregates distribution switches in very large campus LANs, providing very high forwarding rates. 28

29. LAN Design Considerations Ethernet LAN Media and Cable Lengths The three most common types of Ethernet today (10BASE-T, 100BASE-TX, and 1000BASE-T) have the same 100-meter cable restriction, but they use slightly different cables. Each Ethernet standard that uses UTP cabling lists a cabling quality category as the minimum category that the standard supports. For example, 10BASE-T allows for Category 3 (CAT3) cabling or better, whereas 100BASE-TX calls for higher-quality CAT5 cabling, and 1000BASE-TX requires even higher-quality CAT5e or CAT6 cabling. Several types of Ethernet define the use of fiber-optic cables. UTP cables include copper wires over which electrical currents can flow, whereas optical cables include ultra-thin strands of glass through which light can pass. 29

30. LAN Design Considerations Ethernet LAN Media and Cable Lengths Optical cables support a variety of much longer distances than the 100 meters supported by Ethernet on UTP cables. Optical cables experience much less interference from outside sources as compared to copper cables. Lasers allow for even longer cabling distances, up to 100 km today, at higher cost, whereas less-expensive LEDs may well support plenty of distance for campus LANs in most office parks. The type of optical cabling can also impact the maximum distances per cable. Of the two types, multimode fiber supports shorter distances, but it is generally cheaper cabling, and it works fine with less-expensive LEDs. The other optical cabling type, single-mode fiber, supports the longest distances but is more expensive. 30

31. LAN Design Considerations Ethernet LAN Media and Cable Lengths 31