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Key Elements of Ethernet Networks:

CSMA/CD – Only used with half-duplex (hubs).
Full-duplex switches do not use CSMA/CD.
Ethernet signals are sent to every host in the LAN.

Carrier Sense:

All devices must listen before transmitting.
If signal detected (CS), wait before transmitting.
If no tfc detected, can transmit.
While transmitting – must listen for collisions.

Multi-access:

If 2 devices transmit at the same time = collision.

Collision Detection:

Other devices (not involved in collision), and transmitting devices, detect collision.

Jam Signal and Random Backoff:

Transmitting devices send a jamming signal and each backoff a random time.
After the delay the device goes back to “listening before transmiting”.

Ethernet Communications:

Three types of transmissions:
Unicast. 1 sender and 1 receiver.
Predominant form of transmission.
Broadcast: a frame is sent from 1 host to ALL hosts.
E.G. – ARP, RIP, DHCP
Multicast. a frame is sent from 1 host to a specific group.
Multicast clients must be members of a logical group.
E.G. – collaborative business meeting, routing protocols.

Ethernet Frame:

MAC Address = Burned In Address (BIA).
A 2-part 48-bit value expressed as 12 hexadecimal digits.
All Ethernet devices use the MAC to determine if a msg should be passed to the upper layers for processing.
Equals both an OUI and Vendor Assigned #.
Organizational Unique Identifier.
1st 2 bits of OUI have special meanings for destinations.
Broadcast/multicast bit: 0=unicast, 1=broad/multicast.
Locally administered bit: 1 = modifiable locally.

Duplex Settings:

Half Duplex: 2-way, but one at a time. (walkie-talkies).
Implements CSMA/CD – hubs.
Performance issues due to waiting.
Full Duplex: 2-way, simultaneously.
The collision detect circuit is disabled.
Shared hub-based Ethernet ~50 – 60%
Fast Ethernet = 100% x 2 (both directions).

Switch Port Settings:

A Cisco switch port can be configured 3 duplex settings.
Auto ? autonegotiation.
Full.
Half.
For 10/100/1000 ports the default is auto.
For 100BASE-FX ports, the default is full.
When port speed is set to Gb – operates only in full-duplex.
Auto-negotiation can produce unpredictable results.
If it fails, the switch sets the port to half-duplex.
Half-duplex on 1 end and full on the other causes late collisions. (seen)
Manually set the duplex to match the attached device.

Auto-MDIX (media dependent interface):

Older IOS versions required the correct cable types (cross-over, straight-through).
Now the mdix auto interface command in the CLI enables the auto-MDIX.
Enabled by default on IOS 12.2(18)SE or later.
Disabled on IOS between 12.1(14)EA1 and 12.2(18)SE.

Switch MAC Address Tables:

Used to direct tfc through switch fabric to correct port.
Fabric = ICs (integrated circuits) and prgming that allows the data paths through the switch to be controlled.
Switches must learn which nodes exist on each of its ports.
It does this by recording the MAC address of the nodes connected to each of its ports.
Typically, ports used to connect 2 switches have multiple MAC addresses recorded in the MAC table.
The following describes this process:
1. Switch rcvs a BDXT frame from PC1 on Port 1.
2. Switch enters the source MAC addr and the port that rcved the frame.
3. Because the dest addr is a BDXT, the switch floods the frame to all ports, except the src.
4. The dest device replies with a unicast frame to PC1.
5. The switch enters the source MAC of PC2 and the posrt that rcved the frame.  The dest addr of the frame and its port is found in the table.
6. The switch can now forward btwn src and dest devices without flooding.

Design considerations for Ethernet:

Collision Domains.

The area where frames originate and collide.
When a host connected to a switch creates a dedicated connection == collision domain == microsegment.

Broadcast Domains:

Switches do not filter BDXT frames.
A collection of connected switches = BDXT domain.
Only a L3 entity (router or VLAN) stops L2 BDXT.
Routers and VLANs segment collision and BDXT domains.
L2 BDXT = 48 ones or 12 “F’s” separated every 4 by a period.
The BDXT domain at L2 = MAC BDXT domain.
Incoming BDXT is forwarded to all ports, except src.
Each device recognizes the BDXT and processes it.
When two switches are connected, the BDXT domain is increased.

Network Latency:

The time a frame takes to travel fm src to dest.
Latency has at least 3 sources.
1. The time it takes the NICs to send/rcv ~ 1ms @ 10Mbps.
2. Propagation delay through the cable ~ 0.556ms per 100m.
3. Latency is added for each device in the path.
Switches are faster than routers (Application Specific Integrated Circuit (ASIC)s for many tasks).
Latency may also be due to oversubscribed switch fabric. (ref Ch1)
Entry-level switches do not have enough throughput to manage full bandwidth on all ports simultaneously.

Network Congestion:

A primary reason for segmenting a LAN is to isolate tfc and to achieve better use of BW/user.
Without segmentation, a LAN quickly becomes clogged with tfc and collisions.
Most common causes of congestion:
Increasingly powerful network technologies (PCs etc).
Increasing volume of network traffic.
High-bandwidth applications.  Software apps are becoming richer and requiring more BW.

LAN Segmentation:

LANs are segmented into smaller collision and BDXT domains using routers and switches.
Bridges have only a few ports, whereas switches have many. (Bridges are old technology).
Routers (breaks BDXT domains up).
All hosts on a switch are in the same BDXT domain.
Only routers create BDXT domains.
Creating smaller BDXT domains with a router reduces BDXT tfc and provides more available BW.
Each router interface = BDXT domain.

LAN Design Considerations:

Controlling Network Latency.

Switches can introduce latency when oversubscribed.
E.G. a CL switch w/ 48 GbE ports ~ 96 Gbps throughput.
A L3 device needs to examine L3 addresses which creates a longer processing time.
Removing bottlenecks.
Bottlenecks on a network are places where high network congestion resultis in slow performance (aggregate)

Switch Forwarding Methods:

Store-and-Forward Switching:

When a switch receives a frame, it stores it in buffers.
The switch checks the dest and performs the CRC.
If the CRC is OK the frame is forwarded to its dest.
Bad frames are discarded.
This reduces the BW consumed by corrupt data.
This is the sole forwarding method used on current models of Cisco switches.

Note: 1518 bytes in an normal Ethernet Frame….

Cut-Through switching:

Forwards data before the transmission is complete.
Buffers just enough to read the dest MAC address (6 bytes).
The switch does not perform any error checking.
Faster than S&F, but forwards corrupt frames.
Two type of cut-through switching:
Fast-forward: typical cut-through method of switching.
Fragment-free: reads first 64 bytes before forwarding.
Most errors and collisions occur during the first 64 bytes (smallest _______

Symmetric and Asymmetric Switching:

Symmetric switching – all ports have the same BW.
Asymmetric switching – enables more BW to be dedicated to a server switch port to prevent a bottleneck.
Buffering is required on a asymmetric switch.

Memory Buffering:

Buffering is used to store frames before forwarding them or when a dest port is busy.
There are 2 methods of buffering:
Port-based Memory Buffering.
Frames are stored in queues linked to incoming ports.
A frame is sent when all frames ahead of it have been sent.
A single frame can delay all frames in memory because of a busy dest port even if other frames have open dest ports.
Shared Memory Buffering:
Deposits all frames into a common memory bufrfer.
The amt of bufrfer by a port is dynamically allocated.
Frames in the buffer are linked dynamically to the dest port.
Keeps a map of frame to port links.
Map links are cleared after the frame has been sent.
Permits larger frames to be transmitted with fewer dropped.
Important to asymmetric switching, where frames are being exchanged btwn different rate ports.

L2 and L3 Switching:

A L2 switch is completely transparent to network protocols and user applications.
A L3 switch (eg 3560) functions similarly to an L2 switch, (2960), but it can also use IP info.
A L3 switch can also learn which IP addresses are associated with its interfaces.
L3 switches are also capable of L3 ruting, reducing the need for dedicated routers.  Because of specialized switching HW, they can route extremely fast (faster then a router).

L3 Switch and router Comparison:

L3 switches do not completely replace the need for routers.
Routers perform additional L3 services that switches are not capable of performing.
Remote access connections to networks and devices.
Support of WICs, ? WAN connectivity.

Switch Configuration:

Navigating CLI Modes:

Interface Config Mode.
Ports on a switch start with 1 NOT 0.
E.g. fa0/1 – fa0/24 or Gi0/1 – Gi0/24.

GUI-based alternatives:

Cisco Network Assistant.
CiscoView Application.
Cisco Device Manager.
SNMP Management.

The Command History Buffer:

Save time retyping commands by using the history buffer.
By default, keeps last 10 commands (max 256).
show history to view all.
terminal history size # to change buffer size.
terminal no history to disable it for current session only.

Boot Sequence:

load the boot loader SW (prgm in NVRAM).
low-level CPU and register initialization (boot loader).
POST.
Initializes the flash file system.
Loads OS image and boots the switch.
The OS initializes interfaces etc from the config file.
Recovering from a System Crash.
The boot loader also provides a facility to access the files stored on Flash before the oS is loaded. Used for:
Password Recovery.
IOS Restoration.

Configure a Switch:

Management Interface Considerations.
An AL switch is like PC.
To manage a switch remotely, you need to assign it an IP address, SN mask and default gateway.
This IP is assigned to a virtual interface called a VALAN, and the VLAN is assigned to a specific port or ports.
The default is to control the switch through VLAN 1.
Best practice – change mgmt VLAN to anything else (eg 99) [1-4096]).
Configure Management Interface.

S1(config)# interface vlan 99.
S1(config-if)# ip address 172.17.99.11 255.255.255.0.
S1(config-if)#no shutdown.
S1(config-if)# interface fa0/18 (assign a port to the VLAN).
S1(config-if)#switchport mode access.
S1(config-if)#switchport access vlan 99.

You must use no shutdown for this L3 VLAN interface.
“interface VLAN x” refers to the L3 interface.
Only the mgmt VLAN has a VLAN interface associated with it.
L2 switches only permit a single VLAN interface to be active.
Configure a Default Gateway (like on a PC).
A switch only uses this if managed from remote networks.

R1(config)#ip default-gateway 172.17.99.1.

Verify configuration.
S1#show running-config.
S1#show ip interface brief, (not so brief, has all port info).

Configure Duplex and Speed:

S1(config)#int fastethernet.
S1(config-if)#duplex _____.
S1(config-if)#speed _____.

Tags: study, vtp, 802.1q, guides, switch, cisco