[Solved]Sibling Issue in Cisco Switch

Sibling Issue in Cisco Switch: Based on Cisco documentation, the error message %SYS-4-CHUNKSIBLINGSEXCEED: Number of siblings in a chunk has gone above the threshold refers to a scenario where a memory chunk (part of the IOS memory management system) has too many sub-blocks (“siblings”) allocated.

Sibling Issue in Cisco Switch

This is usually a memory fragmentation or resource allocation issue rather than a physical sibling cable issue.

Understanding the Error

Cause: The number of siblings in a memory chunk has exceeded the defined threshold.
Symptoms: This can occur on Catalyst 2xxx and 3xxx switches running IOS 12.2(40)SE or later. It often appears during high CPU usage or when specific processes (like SNMP) are overwhelmed.

Behavior: The command memory chunk siblings threshold might not work as expected in some IOS versions, sometimes showing as -1 after being configured.

Troubleshooting Commands

To analyze this issue, use the following commands in the Cisco CLI:
show running-config | include threshold: Verify if a manual threshold has been configured.
show memory statistics: Check for overall memory fragmentation or exhaustion.
show processes cpu sorted: Determine if a specific process (e.g., SNMP) is causing the memory spike.
show logging: View the history of the %SYS-4-CHUNKSIBLINGSEXCEED messages.

Potential Solutions/Workarounds

Upgrade IOS: This is often a bug. Check the Cisco Bug Search Tool for the specific PID (Process ID) mentioned in the logs (e.g., CSCsq46616).

Reboot the Switch: A reload can clear the accumulated memory chunks and resolve the error temporarily.

Adjust Threshold (If applicable): If the IOS version allows, you can increase the threshold, though this may not solve the underlying cause, as documented in bug CSCsq46616.
If the error message persists after a reload, upgrading to a recommended, stable IOS version is the standard fix.

If you are seeing a “sibling issue” on a Cisco switch, you are likely encountering the %SYS-4-CHUNKSIBLINGSEXCEED error message.

This error occurs when the number of memory “siblings” (related memory blocks) in a specific chunk exceeds a predefined internal threshold.

meaning of sibling issue

The Cause: This is often related to processes that manage large tables or frequent updates, such as SNMP ENGINE, LSP Tunnel Head Control, or mifs sector bu.
Impact: In most cases, this message is purely informational and does not cause a crash or performance degradation, though it can clutter logs.

Common Fixes & Commands

To address or suppress this message, you can use the following commands:

Increase the Threshold: If the sibling count is just slightly above the limit, you can increase the threshold to stop the message:

memory chunk siblings threshold

Identify the Process: Use the show memory command to see which process is consuming excessive chunks.

Check for Bugs: This is a known issue (e.g., Cisco Bug ID CSCsq46616) on certain IOS versions (12.2(40)SE or later) for Catalyst 2xxx and 3xxx switches.

Check for Bugs: This is a known issue (e.g., Cisco Bug ID CSCsq46616) on certain IOS versions (12.2(40)SE or later) for Catalyst 2xxx and 3xxx switches.

Software Upgrade: If the messages are frequent and the command to change the threshold results in a -1 value in the config, a software upgrade to a stable Cisco IOS XE version is recommended.

Verification Commands

show logging: Check how often the error is occurring and which process is triggering it.
show run | include threshold: Check if a manual threshold has already been set

In a Cisco environment, a sibling issue most commonly refers to a Port-Channel (EtherChannel) misconfiguration. Because the physical ports are “siblings” within one logical interface, an error on one can kill the whole family.

The Example: PAgP “Sibling” Desync

Imagine you have two physical ports (Gi1/0/1 and Gi1/0/2) bundled into Port-channel 10.
The Trigger: A technician swaps a cable on Gi1/0/1, but accidentally plugs it into a different switch or a port with different settings.

The Conflict: The switch realizes that Gi1/0/1 is now receiving different information (e.g., a different Device ID) than its sibling Gi1/0/2.

The Trigger: A technician swaps a cable on Gi1/0/1, but accidentally plugs it into a different switch or a port with different settings.

The Conflict: The switch realizes that Gi1/0/1 is now receiving different information (e.g., a different Device ID) than its sibling Gi1/0/2.
The Error: To prevent a Layer 2 loop, the switch puts the entire Port-channel into err-disable.

The Error Message:

%PM-4-ERR_DISABLE: pagp-flap error detected on Gi1/0/1, putting Gi1/0/1 in err-disable state
%PM-4-ERR_DISABLE: pagp-flap error detected on Gi1/0/2, putting Gi1/0/2 in err-disable state
%PORT_CHANNEL-5-FIBER_SP_ERR: Port-channel10 is down (sibling port Gi1/0/1 is err-disabled)

How to Fix this Sibling Issue

Identify the culprit: Use show etherchannel summary. One port will likely show (s) for suspended or (I) for stand-alone.

Match the settings: Ensure both ports have identical:
Speed/Duplex
VLAN Trunking (Allowed VLANs)
STP PortFast settings
Reset the group: You must bounce the logical interface to clear the error on the siblings:

interface Port-channel 10
shutdown
no shutdown

Another Common Case: ASIC Failure

If ports 1 through 8 all go into err-disable simultaneously without a Port-channel configured, you are likely looking at a hardware sibling failure. These ports share a single ASIC (chip).

If that chip overheats or fails, all “sibling” ports in that hardware block will die together.
You can verify the hardware mapping on newer Catalyst switches using:
show platform software fed switch active port summary

If you are seeing the %SYS-4-CHUNKSIBLINGSEXCEED error, focusing on a software upgrade and investigating memory-intensive processes (like Netflow) is the primary path to resolution.