Hi Alex-
Loopback detection was a mechanism that was put in place to detect loops in the network
caused by Type1a or tyep2 copper cabling. While it is a good mechansim for those special
situations , there seems to be no need for having it enabled on the Gig ports where you
can use features like UDLD to detect a fiber loop or unidirectional fiber.
But in 12.1EA releases , loopback detection is enabled by default on the fiber ports as
well as copper ports. On some fiber ports , neighboring switch may just switch the
keepalive packet back w/o making any change in the packet causing the original switch that
sent out the keepalive to recive its own packet back kicking in this mechanism of
error-disabling the port.
What you can do is you can disable keepalives on the uplink Gig ports of the 2950 and that
should take care of the problem
On 2950
int gig
no keepalives
But make sure you have UDLD enabled. Once you have disabled the keepalives , please make
sure you have low cpu on the 2950s that were showing the problem as well as make sure the
mac-aging timer stays at 300 sec.
show proc cpu
show mac- aging
UDLD should however be enabled on fiber Gig ports to detect spanning tree loop problems.
Keepalive detection mechansim is speciafically there for Type1a and type 2 cabling and has
no significance on fiber ports.
This is also documented in the the release notes of the following Cisco DDTS id #
CSCea46385.
An interface on a Catalyst switch is errordisabled after detecting a loopback.
Mar 7 03:20:40: %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet0/2.
The port is forced to linkdown.
Mar 7 03:20:42: %LINK-5-CHANGED: Interface GigabitEthernet0/2, changed state to
administratively down
Mar 7 03:20:43: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/2,
changed state to down
Conditions:
This might be seen on a Catalyst 2940, 2950, 2950-LRE, 2955, 2970, 3550, 3560 or
3750 switch running 12.1EA or 12.2SE based code.
Workaround:
Disable keepalives by using the "no keepalive" interface command. This will
prevent the port from being errdisabled, but it does not resolve the root cause of
the problem. Please see section below for more information.
Additional Information:
The problem occurs because the keepalive packet is looped back to the port that sent
the keepalive. There is a loop in the network. Although disabling the keepalive
will prevent the interface from being errdisabled, it will not remove the loop.
The problem is aggravated if there are a large number of Topology Change Notifications
on the network. When a switch receives a BPDU with the Topology Change bit set,
the switch will fast age the MAC Address table. When this happens, the number of
flooded packets increases because the MAC Address table is empty.
Keepalives are sent on the Catalyst 2940, 2950, 2950-LRE, 2955, 2970, 3550, 3560
or 3750 switch to prevent loops in the network. The primary reason for the keepalives
is to prevent loops as a result of Type 2 cabling. For more information, see:
http://www.cisco.com/en/US/netsol/ns340/ns394/ns74/ns149/networking_solution
s_white_paper09186a00800b4249.shtml
Keepalives are sent on ALL interfaces by default in 12.1EA based software. Starting
in 12.2SE based releases, keepalives are NO longer sent by default on fiber and uplink
interfaces. Since there is no 12.2SE release for 2950s , you will have to manually disable
the keepalives on uplink Gig ports.
Hope this helps.
thanks
Salman Zahid
