FS S5500-48T8SP User manual

Loopback Detection
Configuration Guide
Model: S5500-48T8SP

I
Chapter 1 Setting Loopback Detection...............................................................................................1
1.1 Intro of Loopback Detection...................................................................................................1
1.1.1 Format of Loopback Detection Packet......................................................................1
1.2 Loopback Detection Configuration Tasks.............................................................................2
1.3 Setting Loopback Detection...................................................................................................2
1.3.1 Configuring Loopback Detection Globally ................................................................2
1.3.2 Configuring Port Loop Check......................................................................................3
1.3.3 Configuring a Port to Conduct Loopback Detection in Specified VLAN...............3
1.3.4 Configuring the Loopback Detection Interval of Port (Packet transmission
interval, controlled port recovery time)................................................................................3
1.3.5 Configuring Port Control..............................................................................................4
1.3.6 Configuring the Destination MACAddress of Loopback Detection Packet.........4
1.3.7 Configuring Loopback to Exist on a Port by Default ...............................................4
1.3.8 Displaying the Configuration of Global Loopback Detection..................................5
1.3.9 Displaying the Configuration of Port Loopback Detection......................................5
1.4 Configuration Example............................................................................................................5
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Contents
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Chapter 1 Setting Loopback Detection
1.1 Intro of Loopback Detection
The loopback in a network may trigger the repeated transmission of broadcast,
multicast or unicast packets, wasting network resources and even leaving network
breakdown. To avoid the above-mentioned troubles, it is necessary to provide a
detection mechanism to promptly notify users of detecting network connection and
configuration at the occurrence of loopback and to take troubled ports under control.
Loopback detection can check whether loopback happens on a port of a to-be-tested
device by transmitting a detection packet from this port and checking whether this
packet can be received still on this port. When the device finds that loopback exists on
its port, it can transmit alarm promptly to the network management system for
administrators to detect network problems in time; thus, long time of network
disconnection can be prevented. Moreover, loopback detection is capable of having
ports under control. You can opt for port block, port MAC-learning forbidding or
error-disable according to actual requirements to make corresponding ports under
control and lessen the loopback's network influence to the minimum level.
MY COMPANY switches support loopback detection in the following aspects:
Supporting to set loopback detection on the port
Supporting to set the destination MAC address for loopback detection packets
Supporting to conduct loopback detection to at most 10 specified ports
Supporting to set the transmission interval of loopback detection packets and the
recovery time of controlled port
Supporting to control port, including port block, port MAC-learn forbidding, and
error-disable
Supporting to set whether loopback exists on a port by default
1.1.1 Format of Loopback Detection Packet
Field Length/Byte Value
DMAC 6 0x0180C2B0000A (default value,
configurable )
SMAC 6 MAC address of the switch
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TPID 2 0x8100, VLAN tag type
TCI 2 Specific value of the VLAN tag, priority,
VLAN ID
TYPE 2 Protocol type, which ranges from 0 to 9001
CODE 2 Protocol sub-type, which represents
loopback detection and is 0x0001
VERSION 2 0x0000 (currently reserved)
Length 2 0x0008, length of the header of loopback
detection packet
RESERVE 2 Reserved field
SYSMAC 6 MAC address of the switch
SEQUENCE 4 Sequence ID of packet, which is generated
randomly by the system before the packet
is transmitted
DiID 4 Port ID, which is the ID of the global port of
85 Series
End 2 0x0000, end character
1.2 Loopback Detection Configuration Tasks
Configuring Loopback Detection Globally
Configuring Port Loopback Detection
Setting a Port to Perform Loopback Detection toward Specified VLAN
Configuring the Loopback Detection Interval on a Port
Setting a Port under Control
Setting Loopback to Exist on a Port by Default
Displaying the Configuration of Global Loopback Detection
Displaying the Information about the Loopback Detection Port
1.3 Setting Loopback Detection
1.3.1 Configuring Loopback Detection Globally
Enabling or disabling loopback detection globally means enabling or disabling
loopback detection on all physical ports. Global configuration is just like a switch. Only
when this switch is opened can enabled loopback detection on a port take effect.
Command Purpose
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[no] loopback-detection Sets loopback detection globally.
1.3.2 Configuring Port Loop Check
If you want to enable or disable loopback detection on a specified port, you should
first enable loopback detection globally.
Command Purpose
[no] loopback-detection enable Configures port loopback detection.
1.3.3 Configuring a Port to Conduct Loopback Detection in Specified VLAN
If you set loopback detection in a specified VLAN, a port shall transmit multiple
detection packets with specified VLAN tag regularly and the port can transmit up to 10
detection packets with specified VLAN tag.
One point to be noted is that the port must exist in the specified VLAN, or the
configuration takes no effect. If loopback detection happens in VLAN2 to VLAN8, ports
are configured to be in trunk mode, and trunk vlan-allowed is vlans 5-8, the packets with
tags 2-4 transmitted by the switch cannot pass through this port and the configuration
hence takes no effect.
Command Purpose
[no] loopback-detection vlan-control vlanlist Configures a port to conduct loopback
detection in specified VLAN.
1.3.4 Configuring the Loopback Detection Interval of Port (Packet transmission
interval, controlled port recovery time)
Command Purpose
[no] loopback-detection hello-time time Configures the transmission interval of port
loopback detection packets.
Because a network is always changeable, loopback detection is a continuous
process. The port will transmit loopback detection packets in a regular time. This regular
time is called as the transmission interval of loopback detection packets. The default
transmission interval of the system is 3 seconds.
Command Purpose
[no] loopback-detection recovery-time time Configures the transmission interval of port
loopback detection packets.
This command above is used to set the automatic recovery time of a port when
loopback disappears. In default settings, if a port has not received the already
transmitted loopback detection packet within 10 seconds, it is regarded that loopback
vanishes. It is recommended to set the recovery time to be triple of the packet
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transmission time; if the transmission time is set to be a very small value, you'd better
set the recovery time to be at least 10 seconds longer than the transmission time.
1.3.5 Configuring Port Control
Command Purpose
[no] loopback-detection control
{block|learning|shutdown} Configures port control.
When a port detects that loopback exists in its network, you can set port control to
manage this port. The control state of a port can be block, nolearn, shutdown or trap.
When any control state is set and loopback exists on a port, the trap alarm message will
be transmitted. It is not configured by default.
When loopback detection is enabled globally, a loopback detection packet is
transmitted from a port, on which loopback detection is enabled, and received again by
this port, the port may get the following four control actions:
Block: When loopback is found, this port is then isolated from other ports. Hence
the packets entering this port cannot be forwarded to other ports. The port is then in
protocol down state and its MAC address table list ages.
Nolearn: means to forbid the port to learn MAC addresses. When loopback is
detected, the port will not conduct MAC address learning any more and at the same
time the MAC address table of this port ages.
shutdown: Means to close the port. When loopback is detected, except that trap
message will be transmitted and the port’s MAC address table ages, the port will be
automatically closed and it cannot forward packets any more until the
err-disable-recover time.
Trap: It means that the port only reports alarm. When loopback is detected, the
port only reports alarm and ages its MAC address table without any further action.
When the port is in block state, it cannot forward incoming packets and at the same
time it transmits loopback detection packets continuously. When loopback disappears,
the port will recover automatically. In default settings, if a port has not received the
already transmitted loopback detection packet within 10 seconds, it is regarded that
loopback vanishes.
In block state, the port protocol is down; in shutdown state, the port’s link is down
directly.
1.3.6 Configuring the Destination MACAddress of Loopback Detection Packet
Command Purpose
[no] loopback-detection dest-mac
Mac-address Configures the destination MAC address of
loopback detection packet.
The default destination MAC address of loopback detection packet is
01-80-C2-00-00-0a. If you have set other destination MAC, it will be used as the
destination MAC address of loopback detection packet.
1.3.7 Configuring Loopback to Exist on a Port by Default
Command Purpose
[no] loopback-detection existence Configures loopback to exist on a port by
default.
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When a port is up and port loopback detection takes effect, the command above is
used to set whether loopback exists on this port. When a port is in shutdown state, this
port is not suitable to set to have loopback, for the port in shutdown state cannot
forward packets. The default settings is that loopback does not exist in a port.
1.3.8 Displaying the Configuration of Global Loopback Detection
Command Purpose
show loopback-detection Displays the configuration of global
loopback detection.
This command is used to display the information about global loopback detection
configuration, including global configuration, whether loopback exists on each port, and
some ports’ configurations.
1.3.9 Displaying the Configuration of Port Loopback Detection
Command Purpose
show loopback-detection interface intf Displays the configuration of port loopback
detection.
This command is mainly used to display port loopback detection, including the port
timer and the information about transmitted and received packets.
1.4 Configuration Example
Figure 1.1 Loopback detection configuration
As shown in figure 1.1, the port of S1 conducts loopback detection to specified
VLANs 1, 2 and 3. The corresponding configurations on all switches are shown below:
Switch S1:
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Configuration of interface GigaEthernet0/1:
switchport trunk vlan-untagged 1-3
switchport mode trunk
loopback-detection enable
loopback-detection control block
loopback-detection vlan-control 1-5
Global Configuration
loopback-detection
vlan 1-3
Switch S2:
Configuration of interface GigaEthernet0/1:
switchport mode trunk
Configuration of interface GigaEthernet0/2:
switchport mode trunk
Configuration of interface GigaEthernet0/3:
switchport mode trunk
Global Configuration
vlan1-3
Switch S3:
Configuration of interface GigaEthernet0/1:
switchport pvid 3
If loopback exists in the network that S3 connects and the PVID of the interface, on
which loopback exists, is 3, the packets will be transmitted to interface g0/1 of S1 and
S1 will block interface g0/1 after finding loopback.
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