When working with MPLS L2VPN Xconnect services, one of the common challenges is ensuring proper MTU (Maximum Transmission Unit) configuration. MTU plays a critical role in communication by defining the maximum packet size that can be transmitted without fragmentation. Any mismatch in MTU values between endpoints will prevent the tunnel from establishing.
Problem Overview
In this scenario, we have:
- On one side: A physical interface configured with an MTU of 2600. This MTU is inherited by subinterfaces, which are used as source interfaces for the L2VPN Xconnect tunnel.
- On the other side: A physical interface configured with an MTU of 9216.
The tunnel fails to establish due to an MTU mismatch between the two endpoints.
Attempted Fix
To resolve the issue, we adjusted the MTU of the physical and subinterface on the second endpoint to 2600, matching the first endpoint. However, the tunnel still did not come up.
The Root Cause
The reason lies in the additional VLAN tag of 4 bytes added to the packets. This tag increases the required MTU size on the second endpoint's physical and subinterfaces.
Solution
To address this, the MTU on the second endpoint was recalculated as follows:
- Original MTU: 2600
- Additional VLAN tag: +4 bytes
- Adjusted MTU: 2604
Once the MTU was set to 2604 on the second endpoint, the L2VPN Xconnect tunnel came up successfully, and the MTU mismatch issue was resolved.
Key Takeaways
- Always account for additional headers, such as VLAN tags or other encapsulation overhead, when configuring MTU for L2VPN services.
- Ensure consistent MTU values across all physical and logical interfaces participating in the service.
- Use debugging tools to identify the exact point of failure in case of MTU-related issues.
By understanding and applying these principles, you can prevent MTU mismatches and maintain stable MPLS L2VPN Xconnect connections.