Transcript What are the collision domains?

Prof. Dr. R.Nitsch, FH Darmstadt
CCNAv30 – Semester 1 – Module 8 - Ethernet Switching
Reiner Nitsch
 [email protected]
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.
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Prof. Dr. R.Nitsch, FH Darmstadt
Segment 3
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LAN Switch modes
Prof. Dr. R.Nitsch, FH Darmstadt
• 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.
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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:
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Network Layer
Prof. Dr. R.Nitsch, FH Darmstadt
blocking
initialization
disabled
forwarding
listening
learning
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Layer 2 broadcasts
Prof. Dr. R.Nitsch, FH Darmstadt
• 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.
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Sources of Broadcasts
Prof. Dr. R.Nitsch, FH Darmstadt
• 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
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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.
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Network Layer
Prof. Dr. R.Nitsch, FH Darmstadt
What are the collision domains?
What are the broadcast domains?
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Prof. Dr. R.Nitsch, FH Darmstadt
So, das war´s erst mal!
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