MAC Withdrawal - VPLS/H-VPLS
Overview
The MAC withdrawal mechanism allows for faster convergence by immediately clearing dynamically learned MAC addresses upon detecting a switchover event, rather than waiting for the usual aging process. This process is triggered by specific network events or state changes, such as:
| • | Spoke-PW switchover: A path between two VPLS network elements, typically between MTU-s (Multi-Tenant Units) and Provider Edge (PE) routers, transitions from standby to active or vice versa. |
| • | MLAG switchover: When a link failure or node failure occurs in a MLAG setup, leading to the activation of a previously standby link. |
| • | VPWS Switchover: With VPWS when secondary becomes primary VC or primary becomes active with revertive mode, the mac withdrawal message would be originated from MTU-s. |
The main goal is to ensure that MAC addresses learned from any given path (spoke or mesh) are promptly removed when a switchover occurs, preventing stale entries from affecting the network.
The MAC withdrawal message, defined as the LDP Positive MAC Flush message, is used to remove MAC address tables. This message can be triggered either by MTU-s or PE-rs depending on the network topology and the specific trigger event (such as a switchover or link failure).
Characteristics of MAC Withdrawal - VPLS/H-VPLS
| • | MAC Withdrawal is used to speed up network convergence during topology changes. |
| • | It ensures that dynamically learned MAC addresses are flushed proactively instead of aging out, reducing traffic disruption. |
| • | Triggers for MAC Withdrawal: |
| • | Spoke PW Switchover: When a Spoke Pseudowire (PW) transitions between active and standby. |
| • | MLAG Switchover: When a multi-chassis link aggregation (MLAG) event causes a switchover between active and standby devices. |
| • | Node Failure: When a PE-rs (Provider Edge router) or MTU-s (Multi-Tenant Unit switch) fails or reboots. |
| • | PW Status TLV: The Preferential Forwarding Status bit in an LDP PW Status TLV can signal a switchover. |
| • | Manual Intervention: Administrators can trigger MAC withdrawal manually for network maintenance. |
| • | With VPWS when secondary becomes primary VC mac withdrawal message would be originated from MTU-s. Since revertive mode is supported the original active coming backup would also generate MAC withdrawal message towards the PE-rs. |
Benefits
| • | Proactively removes outdated MAC entries, reducing the time taken for the network to stabilize. |
| • | Ensures quick adaptation to topology changes without requiring manual intervention. |
| • | Reduces packet flooding by clearing invalid MAC table entries. |
| • | Prevents stale forwarding information, ensuring accurate traffic routing. |
| • | Ensures seamless transition when a standby link becomes active. |
| • | Enables compatibility with industry-standard VPLS/H-VPLS deployments. |
| • | Prevents unnecessary flushing across Mesh PWs, maintaining stability. |