This retransmission uses a back-off algorithm by which a station waits for a random amount of time before retransmitting. If a collision occurs, the transmitting stations perform a retransmission. Despite the process of checking the channel before transmitting, it is still possible for two stations to transmit at once, resulting in collisions. If the channel happens to be idle, the station transmits its packets. With CSMA/CD, the transmitting station waits to detect channel traffic before sending the first packet over the channel. There are two variants of CSMA: CSMA with Collision Avoidance (CSMA/CA) and CSMA/CD. CSMA defines when and how to access the channel to transmit data. Carrier Sense Multiple Access (CSMA) is the channel access mechanism used by Ethernet LANs. If hosts from different VLANs need to communicate, the link between the router and the switch must be a trunk link.Įxplanation: The Carrier Sense Multiple Access - Collision Detection (CSMA/CD) contention method verifies that a channel is clear before transmitting, and stops transmitting data when it detects a collision on the channel in use. In the example below, the switch has five VLANs created and some hosts connected to it. Finally, each computer in the VLAN will use the corresponding IP address on the matching subinterface of the router as its default gateway. Then each subinterface is given an IP address from the same subnet as the computers that resideon that VLAN. When a trunk link is extended to a router for the purpose of enabling routing between VLANs, the physical connection that the link connects to is usually subdivided logically into subinterfaces. The other options are incorrect because trunk links do not carry data for a single VLAN, nor are trunks used between switches and hosts (such as workstations and servers). The receiving switch reads the VLAN ID and forwards the frame to the appropriate host in the same VLAN. A trunk link supports all VLANs by default, and frames that are not traveling on the native VLAN are "tagged" with the VLAN ID of the originating port before being sent over the trunk. Similarly, a host in VLAN 20 on SwitchA could also communicate with a host in VLAN 20 on SwitchB. Trunk links do allow a host in VLAN 10 on SwitchA to communicate with a host in VLAN 10 on SwitchB. Trunk links do not allow hosts in different VLANs to communicate, unless there is an additional trunk link connecting to a Layer 3 device, such as a router or a multilayer switch. Helper address and Proxy ARP fields are present in the output of the show ip interface command, not the show ip interface brief command.Įxplanation: Trunk links are used between switches to allow communications between hosts that are in the same VLAN, but connected to different switches. YES? is not a valid field in the output of the show ip interface brief command. Possible values in this field are as follows: Protocol: An indicator of the operational status of the routing protocol for this interface. Method: This field can have one of the following values: Status: Displays the interface status. If the value of this field is "No", it represents an invalid IP address. Explanation: Sample output of the show ip interface brief command is as follows: Router# show ip interface brief Interface IP-Address OK? Method Status Protocol Ethernet0 10.108.00.5 YES NVRAM up up Ethernet1 unassigned YES unset administratively down down Loopback0 10.108.200.5 YES NVRAM up up Serial0 10.108.100.5 YES NVRAM up up Serial1 10.108.40.5 YES NVRAM up up Serial2 10.108.100.5 YES manual up up Serial3 unassigned YES unset administratively down down The following fields are present in the output of the show ip interface brief command: OK?: If the value of this field is "yes", it represents that the IP address is valid.
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