1. Prof. Dr. R.Nitsch, FH Darmstadt CCNAv30 – Semester 1 – Module 8 - Ethernet Switching Reiner Nitsch  r.nitsch@fbi.h-da.der.nitsch@fbi.h-da.de

2. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer2 Layer 2 switching A switch is simply a bridge with many ports. Each port creates its own collision domain. When only one node is connected to a switch port, the collision domain on the shared media contains only two nodes. These small physical segments are called microsegments. When only two nodes are connected to a microsegment and communication is full duplex, a collision domain no longer exists. Theoretically, the bandwidth is doubled when using full duplex. A switch dynamically builds and maintains a Content-Addressable Memory (CAM) table, holding all of the necessary MAC information for each port. Content-addressable memory (CAM) is memory that essentially works backwards compared to conventional memory. Entering data into the memory will return the associated address. Using CAM allows a switch to directly find the port that is associated with a MAC address without using search algorithms. Segment 3

3. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer3 LAN Switch modes Asymmetric switching provides switched connections between ports of unlike bandwidths, such as a combination of 100 Mbps and 1000 Mbps. Symmetric switching provides switched connections between ports of equal bandwidths. How a frame is switched to the destination port is a trade off between latency and reliability. Store and Forward: –The switch receives the entire frame before sending it out the destination port. –The frame is discarded by the switch rather than at the ultimate destination if it contains a CRC error or if it is a runt (less than 64 bytes, including the CRC) or a giant (more than 1518 bytes, including the CRC). Cut-through switching –The switch starts to transfer the frame as soon as the destination MAC address is received. The MAC address determines the output port! –Results in the lowest latency through the switch. –No error checking is available. Invalid frames are forwarded and waste bandwidth. Fragment-free Mode: –compromise between the cut-through and store-and-forward modes –The switch starts frame transmission after it reads the first 64 bytes, which includes the frame header, and switching begins before the entire data field and checksum are read. –Runts were detected and discarded. A late collision is when a collision happens after the first 64 bytes of the frame are transmitted.

4. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer4 Spanning-Tree Protocol Switched networks are often designed with redundant paths to provide for reliability and fault tolerance. Switching loops can occur by design or by accident, and they can lead to broadcast storms that will rapidly overwhelm a network. How to avoid switching loops? –Allways arrange multiple switches in a simple hierarchical tree (difficult to administer) –Use switches with the standards-based protocol Spanning-Tree Protocol (STP) activated. LAN-Switches using STP send special messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know of its existence and to elect a root bridge for the network. The switches then use the Spanning-Tree Algorithm (STA) to resolve and shut down the redundant paths until they are needed. Each port on a switch using Spanning-Tree Protocol exists in one of the following five states: Blocking, Listening, Learning, Forwarding, Disabled A port moves through these five states as shown: blocking initializationlistening learningforwarding disabled

5. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer5 Layer 2 broadcasts Protocols use broadcast and multicast frames at Layer 2 of the OSI model. When a node needs to communicate with all hosts on the network, it sends a broadcast frame with a destination MAC address 0xFFFFFFFFFFFF. This is an address to which each network interface card (NIC) must respond. Layer-2 devices must flood all broadcast and multicast traffic which is referred to as broadcast radiation. The circulation of broadcast radiation can saturate the network so that there is no bandwidth left for other application data. The probability this event, which is also called a broadcast storm, increases as the switched network grows. Broadcast radiation affects the performance of hosts in the network, because the NIC must interrupt the CPU to process each broadcast or multicast group it belongs to. The figure shows the effect of broadcast radiation on the CPU performance of a Sun SPARCstation 2 with a standard built-in Ethernet card. Most often, the host does not benefit from processing the broadcast, as it is not the destination being sought.

6. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer6 Sources of Broadcasts The three sources of broadcasts and multicasts in IP networks are workstations, routers, and multicast applications. Workstations broadcast an Address Resolution Protocol (ARP) request every time they need to locate a MAC address that is not in the ARP table. The ARP rate for a typical workstation might be about 50 addresses every two hours or 0.007 ARPs per second. Thus, 2000 IP end stations produce about 14 ARPs per second. Routing protocols running on routers produce broadcast traffic. The Routing Information Protocol (RIP) broadcasts every 30 seconds the entire RIP routing table to other RIP routers. For a routing table that has a size of 50 packets, 10 RIP routers would generate about 16 broadcasts per second. It's necessary to have devices on a network that control the extent of broadcast domains Conclusion:

7. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer7 Broadcast Domains A broadcast domain is a grouping of collision domains that are connected by Layer 2 devices. Broadcasts have to be controlled at Layer 3, as layer-2 and layer-1 devices have no way of controlling them. It is layer 3 that allows layer-3 devices to limit broadcast domains: Layer-2 information is stripped off before the frame payload is handed over to the layer-3 forwarding processes. Their forwarding decision is based on layer-3 network addresses and not on MAC addresses. Routers actually work at Layers 1, 2, and 3. Therefore: Broadcast domains are controlled (or contained) at Layer 3 because routers do not forward layer-2 broadcasts. What are the collision domains? What are the broadcast domains?

8. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer8 So, das war´s erst mal!

9. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer9 Layer 2 bridging By increasing the number of nodes on a single segment, the probability of collisions increases, resulting in more retransmissions. A solution to the problem is to break the large segment into parts and separate it into isolated collision domains. To accomplish this a bridge keeps a table of MAC addresses and the associated ports. The bridge then forwards or discards frames based on the table entries. Generally, a bridge has only two ports and divides a collision domain into two parts. All decisions made by a bridge are based on MAC or Layer-2 addressing and do not affect the logical or Layer-3 addressing. Thus, a bridge will divide a collision domain but has no effect on a logical or broadcast domain. Port1 Bridge Port 2 00000CAAAAAA00000CCCCCCC 00000CBBBBBB00000CDDDDDD MAC Address Port 00000CBBBBBB2 No matter how many bridges are in a network, unless there is a device such as a router that works on Layer 3 addressing, the entire network will share the same logical broadcast address space. A bridge will create more collision domains but will not add broadcast domains. 00000CAAAAAA 100000CCCCCCC100000CDDDDDD2

10. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer10 Latency In a network, latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another. Sometimes latency is measured by sending a packet that is returned to the sender and the round-trip time is considered the latency. Media delays (Ausbreitungsdauer) caused by the finite speed that signals can travel through the physical media. Transmission time (Übertragungsdauer) which is the time it takes until a complete frame is received Circuit delays (Wartezeiten) caused by the electronics that process the signal along the path (e.g. waiting delays in buffers). Software delays (Wartezeiten) caused by the decisions that software must make to implement switching and protocols. Latency = Media Delays + Transmission Time + Circuit Delays + Software Delays Latency = Media Delays + Transmission Time + Circuit Delays + Software Delays

11. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer11 Collision Domains and Broadcast Domains The types of devices that interconnect the media segments influence collision domains. –Layer 1 devices (repeaters and hubs ) extend collision domains –Layer-2 (Bridge and switch) and Layer-3 (Router) devices do break up collision domains. –Breaking up, or increasing the number of collision domains with Layer 2 and 3 devices is also known as segmentation. A late collision is when a collision happens after the first 64 bytes of the frame are transmitted. These late collision frames add so called consumption delay. What are the collision domains?

12. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer12 What is meaning of segment in the data communication context? As with many terms and acronyms, segment has multiple meanings. The meaning depends on the context of the sentence. In the context of data communication, the following definitions are used: –Term used in the TCP specification to describe a single transport layer PDU. The terms datagram, frame, message, and packet are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles. –Section of a network that is bounded by bridges, routers, or switches. –In a LAN using a bus topology, a segment is a continuous electrical circuit that is often connected to other such segments with repeaters.

13. Prof. Dr. R.Nitsch, FH Darmstadt 4.7.2002Network Layer13 Q—A multilayer switch mimics the actions of a router when an initial frame passes through a router. What things does the multilayer switch do to the Layer 2 and Layer 3 headers to thoroughly imitate the router? A—The switch must modify the source and destination MAC addresses in the Layer 2 header so that the frame appears to come from/to the router/workstation. Furthermore, the switch must change things in the Layer 3 header such as the IP time-to-live value. Q—A LAN switch most closely resembles what type of internetworking device? A—A LAN switch behaves like a multiport bridge. Q—Which switching method protects network segment bandwidth from errored frames? Store-and-forward transmits frames only if the frame's integrity is assured. If the switch receives an errored frame, then the switch discards it. Q—How does a store-and-forward switch know if a frame is errored? A—The switch uses the CRC to determine whether any changes occurred to the frame since the source generated it. The switch calculates CRC for the received frame and compares it with the CRC transmitted with the frame. If they differ, the frame changed during transit and will be discarded in a store-and-forward switch.