Configuration Guide Configuring Interfaces
1.3.8 Configuring the Carrier Delay
Working Principle
The carrier delay refers to the acceptable time delay in status change of the Data Carrier Detect (DCD) signal from Down to
Up or from Up to Down. If the DCD status changes within the delay, the system will ignore this change and the upper data
link layer does not need to renegotiate. If the carrier delay is set to a large value, nearly every transient DCD change will be
ignored. On the contrary, if the parameter is set to 0, every DCD signal change however minor will be detected by the system,
resulting in higher instability.
If the DCD carrier interrupts for a long time, set the parameter to a smaller value to accelerate topology convergence
and route summarization. On the contrary, if the period of DCD carrier interruption is smaller than the time of topology
convergence or route summarization, set the parameter to a larger value to avoid topology or route flapping.
Related Configuration
 Configuring the Carrier Delay
The default value of carry-delay for an interface is 2 seconds.
Run the carrier-delay seconds command in interface configuration mode to set carry-delay for an interface. The value of
seconds ranges from 0 to 60 seconds. You can run the no carrier-delay command to restore the default value.
1.3.9 Configuring the 802.1Q VLAN Tag
Working Principle
A virtual LAN (VLAN), namely a logical network partitioned in a physical network, is a layer-2 network of the OSI model. IEEE
issued the 802.1Q protocol standard in 1999 to standardize VLAN implementation.
The VLAN technology allows a network administrator to partition a physical LAN into different broadcast areas (or VLANs)
logically. Each VLAN consists of a group of computer workstations with the same needs. Therefore, they have the same
features as the physical LAN. However, since VLANs are partitioned logically but not physically, the workstations in a VLAN
do not need to be placed in the same physical space. In other words, these workstations do not necessarily belong to the
same physical LAN network segment. The broadcast and unicast traffic on a VLAN cannot be forwarded to other VLANs,
which helps control traffic, reduces equipment investment, simplifies network management, and improves network security.
VLAN is a protocol proposed to solve the Ethernet broadcast problem and security issues. On the basis of an Ethernet frame,
a VLAN header is added and a VLAN ID is used to assign users into smaller work groups to restrict the mutual access at
Layer 2 between different work groups. Each work group is a VLAN. The advantage of a VLAN is to restrict the broadcasting
scope, build virtual work groups, and dynamically manage networks.
To communicate with a host in a VLAN, you can configure the 802.1Q (VLAN protocol) VLAN encapsulation tag on an
Ethernet interface or sub interface. In this way, the Ethernet interface encapsulates the VLAN header when sending a packet
and detaches the VLAN header when receiving a packet.
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