Speed and Duplex
By default, each Cisco switch port uses Ethernet auto-negotiation to determine the speed and
duplex setting. The switches can also set their duplex setting with the duplex
interface subcommand, and their speed with, the speed interface subcommand. The switch port can run in one of two duplex settings at any given time, half duplex or full duplex.
Switches can dynamically detect the speed setting on a particular Ethernet segment by using a few different methods. The speed setting is telling the interface at what rate to send the traffic out of the interface, 10Mb (b – bits) 100Mb, 1000Mb, 10000Mb. Cisco devices can sense the speed using the Fast Link Pulses (FLP) of the auto-negotiation process. However, if auto-negotiation is disabled on either end of the cable, the switch detects the speed anyway based on the incoming electrical signal.
Switches detect duplex settings through auto-negotiation only. If both ends have autonegotiation
enabled, the duplex is negotiated. However, if either device on the cable disables auto-negotiation, the devices without a configured duplex setting must assume a default, which half duplex (HDX) for 10Mb and 100Mb interfaces or full duplex (FDX) for 1000Mb interfaces. To disable auto-negotiation on a Cisco switch port, you simply need to statically configure the speed and the duplex settings.
When a switch port connects to a hub, it needs to operate in HDX mode, because collisions might occur due to the logic used by the hub. HDX works in 1 way communication, only one device can send traffic onto the segment at any one time. HDX can be seen as using a walky talky system, only one person can talk at a time, when one person is finished talking the other person can begin to talk.
Ethernet devices can use FDX only when collisions cannot occur on the attached cable If a hub was attached to the port the port would run in HDX. FDX works in 2 way communication, like when talking over a phone both parties can talk at the same time. There are no collisions on FDX as it uses pins 1 & 2 to send traffic and pins 3 & 6 to receive traffic. From using this type of communication over the ethernet cable you can experience twice the throughput, 100Mb port will transmit 100Mb both ways, which will work out at an overall total of 200Mb throughput on the port.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) algorithm will deal with the inevitable collisions. CSMA/CD minimizes the number of collisions, but when they occur, CSMA/CD defines how the sending stations can recognize the collisions and retransmit the frame.
PPPoE (Point-to-Point Protocol over Ethernet)
PPPoE is widely used for digital subscriber line (DSL) Internet access because the public
telephone network uses ATM for its transport protocol; therefore, Ethernet frames must be
encapsulated in a protocol supported over both Ethernet and ATM. PPP is the natural choice.
In a DSL environment, PPP interface IP addresses are derived from an upstream DHCP server
using IP Configuration Protocol (IPCP). Therefore, IP address negotiation must be enabled on the router’s dialer interface. This can be done under the dialer interface command;
Router(config-if)#ip address negotiated
Because of the 8-byte PPP header, the MTU for PPPoE is usually set to 1492 bytes so that the
entire encapsulated frame fits within the 1500-byte Ethernet frame. A maximum transmission unit (MTU) mismatch prevents a PPPoE connection from coming up. Checking the MTU setting is a good first step when troubleshooting PPPoE connections.
Configuring PPPoE on EdgeRouter by apply the following syntax;
Router(config)# interface fa0/1
Router(config-if)#ip address 192.168.1.1 255.255.255.0
Router(config-if)#ip nat inside
Router(config-if)#pppoe-client dial-pool-number 1
Router(config-if)#ip address negotiated
Router(config-if)#ppp authentication chap
Router(config-if)#ip nat outside
Verify PPPoE connectivity using the command;
Router#show pppoe session