Switch Interfaces
1.0 Network Fundamentals
1.1 Explain the role and function of network components
1.1.b L2 and L3 switches
1.4 Describe switching concepts
Configuring Speed, Duplex, and Description
-
autonegotiate
-
What speed to use
-
enabled by default
duplex {auto | full | half} and speed {auto | 10 | 100 | 1000}
- configure the speed and duplex settings
(config-int) # description text
- add a text description to the interface
show interfaces status
- lists port #, Name, status, vlan, duplex, speed, and type
a-full and a-100
a- means that the listed speed and duplex values were autonegotiated.
Autonegotiation
IEEE autonegotiation (IEEE standard 802.3u)
-
each node states what it can do, and
-
then each node picks the best options that both nodes support:
-
the fastest speed and the best duplex setting, with full duplex being better than half duplex.
-
disable autonegotiation
-
Configure both the speed and duplex on a switch interface
-
when a node tries to use autonegotiation but hears nothing from the device.
Speed: Use your slowest supported speed (often 10 Mbps).
Duplex: If your speed = 10 or 100, use half duplex; otherwise, use full duplex.
Cisco switches can actually sense the speed used by other nodes, even without IEEE autonegotiation.
-
Cisco switches use this slightly different logic to choose the speed when autonegotiation fails:
-
Speed: Sense the speed (without using autonegotiation), but if that fails, use the IEEE default (slowest supported speed, often 10 Mbps).
-
Duplex: Use the IEEE defaults: If speed = 10 or 100, use half duplex; otherwise, use full duplex.
-
Ethernet interfaces using speeds faster than 1 Gbps always use full duplex.
-
hubs do not react to autonegotiation messages
show interfaces and show interfaces description
Shutdown command is configured
- Line status = administratively down
- Protocol status = down
- Interface status = disabled
Cable, speed mismatch, neighbor device is off, shutdown, or err-disabled
- Line status = down
- Protocol status = down
- Interface status = notconnect
Not expected on LAN switch physical interfaces
- Line status = up
- Protocol status = down
- Interface status = notconnect
Port security has disabled the interface
- Line status = down
- Protocol status = down (err-disabled)
- Interface status = err-disabled
the interface is working
- Line status = up
- Protocol status = up
- Interface status = connected
show interfaces fa0/13 (without the status option)
- lists the speed and duplex for interface Fast Ethernet 0/13
- with nothing implying that the values were learned through autonegotiation.
speed manually set 10 Mbps on one switch and 100 Mbps on the other
- both switches would list the port in a down/down or notconnect state
if the duplex settings do not match
- the switch interface will still be in a connected (up/up) or connected state.
How to identify duplex mismatch problems,
- check the duplex setting on each end of the link to see if the values mismatch.
- watch for incrementing collision and late collision counters
Common Layer 1 problems
- receiving device might receive a frame whose bits have changed values.
- These frames do not pass the error detection logic as implemented in the FCS field in the Ethernet trailer,
- The receiving device discards the frame and counts it as some kind of input error.
- Cisco switches list this error as a CRC error
Runts:
-
Frames that did not meet the minimum frame size requirement
-
(64 bytes, including the 18-byte destination MAC, source MAC, type, and FCS).
-
Can be caused by collisions.
Giants:
-
Frames that exceed the maximum frame size requirement
-
(1518 bytes, including the 18-byte destination MAC, source MAC, type, and FCS)
Input Errors:
- A total of many counters, including runts, giants, no buffer, CRC, frame, overrun, and ignored counts.
CRC:
- Received frames that did not pass the FCS math
- can be caused by collisions
Frame:
-
Received frames that have an illegal format
-
(like ending with a partial byte)
-
can be caused by collisions.
Packets Output:
- Total number of packets (frames) forwarded out the interface.
Output Errors:
- Total number of packets (frames) that the switch port tried to transmit, but for which some problem occurred.
Collisions:
- Counter of all collisions that occur when the interface is transmitting a frame
Late Collisions:
-
The subset of all collisions that happen after the 64th byte
-
(In a properly working Ethernet LAN, collisions should occur within the first 64 bytes
-
Often point to a duplex mismatch
Collisions occur as a normal part of the half-duplex logic imposed by CSMA/CD
a switch interface with an increasing collisions counter might not even have a problem.
- if the CRC errors grow, but the collisions counters do not, the problem might simply be interference on the cable.