Prof. Dr. R.Nitsch, FH Darmstadt CCNAv30 – Semester 1 – Module 8 - Ethernet Switching Reiner Nitsch [email protected]@fbi.h-da.de.

Prof. Dr. R.Nitsch, FH Darmstadt

CCNAv30 – Semester 1 – Module 8 - Ethernet Switching

Reiner Nitsch [email protected]

4.7.2002 Network Layer 2

Prof. Dr. R.Nitsch, FH Darmstadt

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

4.7.2002 Network Layer 3

Prof. Dr. R.Nitsch, FH Darmstadt

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.

4.7.2002 Network Layer 4

Prof. Dr. R.Nitsch, FH Darmstadt

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

initialization listening

learningforwarding

disabled

4.7.2002 Network Layer 5

Prof. Dr. R.Nitsch, FH Darmstadt

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.

4.7.2002 Network Layer 6

Prof. Dr. R.Nitsch, FH Darmstadt

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:

4.7.2002 Network Layer 7

Prof. Dr. R.Nitsch, FH Darmstadt

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?

4.7.2002 Network Layer 8

Prof. Dr. R.Nitsch, FH Darmstadt

So, das war´s erst mal!