Interconnecting Cisco Networking Devices

Broadcast and collusion domains
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If a switch receives a broadcast frame, the switch floods it out every active interface, just as it does for an unknown destination MAC address. All devices that receive this broadcast make up the broadcast domain. As more switches are connected together, the size of the broadcast domain increases.
Collision domains create a similar problem. The more devices participating in a collision domain, the more collisions occur.
Hubs create large collision domains. Switches, however, use a feature called microsegmentation to reduce the size of collision domains to a single switch port.
Microsegmentation – division of a network into smaller segments, usually with the intention of increasing aggregation bandwidth to network devices.

Ethernet Communication
PDU – Frame.
Preamble – pattern of alternating 1 and 0 bits used to synchronize timing.
SFD – marks the end if the timing information and starts of the frame.
Destination MAC
Source MAC
Length / Type
type value indicates which protocol will receive the data.
the length indicates the number of bytes of data that follows this field.
Encapsulated Data – contains the packet of information being sent. Each frame must be between 65 and 1518 bytes.
FCS – frame check sequence.
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Формат MAC адреса
МАС адрес состоит из 12 шестнадцатеричных значений.
3 первых байта – код производителя устройства
3 последних байта – уникальный идентификатор устройства
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Передача данных в «плоской» сети
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Broadcast and collusion domains
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How switch works.
When a host connects to a switch port, the switch creates a dedicated connection. When two connected hosts communicate with each other, the switch consults the switching table and establishes a virtual connection, or microsegment, between the ports.
Switches can support either symmetric or asymmetric switching.
Symmetric – switches that have ports of all the same speeds
Asymmetric – connections between ports of different speeds
Switches can handle traffic on the second and third layers of OSI.
Multilayer switching combines hardware-based switching and hardware-based routing in the same device
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Types of switching.
When switching was first introcuded, a switch could support one of two major methods to forward a frame from one port to another. The two methods are store and forward and cut-through switching.
Store and forward
entire frame is read and stored in memory before being sent to the destination device
switch checks the integrity of the bits in the frame by recalculating the cyclic redundancy check (CRC) value
this method keeps damaged frames from being switched to other network segments
the highest amount of latency
Cut-through
fast-forward
fragment-free
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Types of switching.
Fast-forward
the fastest method of switching
switch forwards the frames out the destination port as soon as it reads the destination MAC address
the lowest latency
forwards collision fragments and damaged frames
Fragment-free
switch reads the first 64 bytes of the frame before it begins to forward it out the destination port
Smaller frames are usually the result of a collision and are called “runts”
Checking the first 64 bytes ensures that the switch does not forward collision fragments
latency is in the middle of other methods
fast-forward switching would be the preferred method
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Types of switching.
Adaptive Cut-through
These switches begin by forwarding traffic using the fast-forward method to achieve the lowest latency possible. Even though the switch does not check for errors before forwarding the frame, it recognized the errors and stores an error counter in memory. It compares the number of errors found to a predefined threshold value.
If the number of errors exceeds the threshold value, the switch has forwarded an unacceptable number of errors. In this situation, the switch modifies itself to perform store and forward switching. If the number of errors drops back below the threshold, the switch reverts back to fast-forward mode.
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CAM table
Switches only place the source MAC address of a frame in the CAM. Additionally, the CAM stores which port and VLAN the frame was received from.
By default, dynamically learned MAC addresses are stored for 300 seconds in the CAM. After 300 seconds, if no activity is received from that MAC address, its entry is removed from the CAM. MAC address entries can also be statically entered into the CAM.
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Заполнение CAM на коммутаторах
При пустой CAM таблице коммутатор записывает MAC адрес отправителя и порт коммутатора, на который пришел Ethernet кадр.
Коммутатор проверяет нет ли записей относительно MAC адреса получателя в CAM таблице.
Если информация есть – кадр отправляется через определенный порт.
Если информации нет – кадр отправляется из всех портов, кроме порта на который был получен кадр.
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Frame processing.
When a Layer 2 switch receives a frame on a port, it places that frame in one of the port’s ingress queues. When the switch decides which port that frame should sent out of, it places the frame in that port’s egress queue. If the destination MAC address in the frame is not in the MAC address table, the frame is placed in the egress queue of all ports and is flooded throughout the network.
Before a Layer 2 switch can take a frame from one port’s ingress queue to another port’s egress queue, it must consult two tables:
Content Addressable Memory (CAM), which is Cisco’s term for the MAC address table. It can also be referred to as the Layer 2 Forwarding Table.
Ternary Content Addressable Memory (TCAM), which contains access lists that can filter frames by MAC address, and QoS access lists to prioritize traffic. In multi-layer switches, the TCAM also contains access lists to filter frames based on IP address or TCP/UDP port.
Both the CAM and TCAM are stored in RAM
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Frame processing.
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