CCNAv30 – Semester 1 – Module 8 - Ethernet Switching

1. CCNAv30 – Semester 1 – Module 8 - Ethernet Switching Reiner Nitsch  r.nitsch@fbi.h-da.de

2. Layer 2 switching • A switch is simply a bridge with many ports. • Each portcreates 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 Network Layer

3. 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 transmissionafter 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. Network Layer

4. 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 initialization disabled listening forwarding learning Spanning-Tree Protocol Network Layer

5. 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 address0xFFFFFFFFFFFF. • 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. Network Layer

6. 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. Conclusion: It's necessary to have devices on a network that control the extent of broadcast domains Network Layer

7. 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? Network Layer

8. So, das war´s erst mal! Network Layer