Preventing Network Outages in L2VPN with Storm Control: A Real-World Guide

 

Preface:

In modern service provider and enterprise networks, L2VPN services like VPLS, VPWS and EVPN provide transparent, low latency connectivity across geographically distributed sites. However, these services are prone to Layer 2 traffic storms - broadcast, multicast and unknown unicast floods which can originate from a misbehaving customer edge (CE) device or unintended loops in the network. These storms can quickly propagate through the provider edge (PE) infrastructure, saturate link bandwidth, overload CPU and memory and potentially cause widespread service degradation or outages not just for the offending customer but for multiple tenants sharing the same network. To address this critical vulnerability, storm control is implemented as a protective measure on PE interfaces, especially on customer facing ports. By monitoring and rate limiting specific Layer 2 traffic types, storm control stops the storm at its source. This ensures only a permissible amount of broadcast, multicast or unknown unicast traffic enters the provider network and thus protects the core infrastructure and the integrity and stability of L2VPN services. Proper configuration and tuning of storm control thresholds along with features like MAC limiting, port security and loop detection forms a best practice framework for securing modern Layer 2 services from catastrophic outages.

How storm control settings help rate-limit traffic types and safeguard your LAN and L2VPN services from outages?

Using storm control can prevent problems caused by broadcast storms. You can configure storm control to rate limit broadcast traffic, multicast traffic (on some devices) and unknown unicast traffic to a specified level so the switch will drop packets when that level is exceeded and prevent packets from propagating and degrading the LAN. You can also have the device shut down or temporarily disable an interface when the storm control level is exceeded.

A broadcast storm occurs when broadcast packets cause receiving devices to broadcast packets in response. This causes further responses and creates a knock-on effect that floods the device with packets and causes some clients to lose service or go down completely

 Storm control monitors the incoming traffic for the applicable traffic types and compares it to the level you specify. If the combined level of the applicable traffic exceeds the specified level the switch will drop packets for the controlled traffic types. Instead of dropping packets you can configure storm control to shut down interfaces or temporarily disable interfaces when the storm control level is exceeded.

 

• On ELS systems, storm control is enabled by default on all interfaces at 80% of the available bandwidth. 
• On non-ELS systems, storm control is disabled by default on all interfaces. If you enable storm to control the default is 80% of the available bandwidth.

Let's understand by using real world scenario:

Scenario: Zensar technology has dual-homed CE switch with two PE routers: 

1. Reliance Jio and 2. Bharti Airtel. In a datacenter VPLS service is configured for that. The CE switch is part of a campus network and connects to many end devices. The PE routers are part of a large L2VPN fabric that connects to other sites via an MPLS core.

One day, a technician accidentally misconfigures the CE switch, enabling a loop or turning off spanning tree protocol. As a result:

• The CE begins flooding broadcast traffic continuously.
• This traffic enters the PE router via the CE-PE link and starts being replicated across the VPLS instance to all other customer sites.

 

By accidentally, Strom control is not configured earlier.

Step-by-step Event occurs without Strom Control

• PE starts processing and replicating the excessive broadcast traffic.
• CPU and memory on PE go up.
• Broadcast load propagates across the MPLS core, affecting other PEs and customers.
• Multiple customer services go down.
• NOC gets high severity outage alerts from multiple regions.

How can we prevent this outage using Strom Control

• Storm control is configured on CE facing ports of both PE routers.
• PE monitors broadcast, multicast, unknown unicast traffic rates.
• When broadcast traffic from CE exceeds the configured threshold (e.g. 1% of the port bandwidth), PE:
• Starts dropping excess broadcast packets before they hit the MPLS core.
• Logs the anomaly and optionally sends SNMP traps to NOC.
• Other customers and core infrastructure is not affected.
• NOC investigates and finds the misconfigured CE switch.
• Issue is resolved with zero impact to the core network or other customers.

Steps to perform:

1.        Check OOB access of node is possible or not.
2.  Service validation pre-check requires on affected CE node. This is basically used to check whether packet drops are observed or not policy-maps.
3.     Remove strom control from bridging instances.
4.   Wait for 10 minutes, see any ambiguity generated at service level or not. If yes, then reload the node.
5.   Verify and observe service disruptions.
6.  Load the strom control configuration to the sub interfaces under l2vpn instance.
7.   After configuration changes, commit it and reload node again.
8.    Confirm no service disruptions.
9.    If all above steps fail, need to do handy rollback plan in reverse steps.

Command line logs:

Jio-PE# show policy-map interface GigabitEthernet 0/0/0/1.1512

Jio-PE(config)# no l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB storm-control unknown-unicast pps 300

Jio-PE(config)# no l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB storm-control broadcast pps 300

Jio-PE # reload location all

Jio-PE # show policy-map interface GigabitEthernet 0/0/0/1.1512

Jio-PE (config)# l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB interface GigabitEthernet0/0/0/1.1512 storm-control unknown-unicast pps 300

Jio-PE (config)# l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB interface GigabitEthernet0/0/0/1.1512 storm-control broadcast pps 300

Jio-PE (config) #Show commit config changes diff

Jio-PE (config) # Commit

Jio-PE (config) # exit

Jio-PE# reload

Jio-PE# show policy-map interface GigabitEthernet 0/0/0/1.1512

Jio-PE# show policy-map interface GigabitEthernet 0/0/0/1.1512 | i drop

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Airtel-PE# show policy-map interface GigabitEthernet 0/0/0/1.1512

Airtel-PE(config)# no l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB storm-control unknown-unicast pps 300

Airtel-PE(config)# no l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB storm-control broadcast pps 300

Airtel-PE # reload location all

Airtel-PE # show policy-map interface GigabitEthernet 0/0/0/1.1512

Airtel-PE (config)# l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB interface GigabitEthernet0/0/0/1.1512 storm-control unknown-unicast pps 300

Airtel-PE (config)# l2vpn bridge group 100022311 bridge-domain 100022311 -16VLXM000441CB interface GigabitEthernet0/0/0/1.1512 storm-control broadcast pps 300

Airtel-PE (config) #Show commit config changes diff

Airtel-PE (config) # Commit

Airtel-PE (config) # exit

Airtel-PE# reload

Airtel-PE# show policy-map interface GigabitEthernet 0/0/0/1.1512

Airtel-PE# show policy-map interface GigabitEthernet 0/0/0/1.1512 | i drop 

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Be Attentive:

1 Strom control is a sticky service. Once when we apply a policy map, the sub interface needs to reload the node once, otherwise attempts will wipe out, no effect results at service level.

2. When we reload a node, we need to capture pre-check logs protocol, interface and service level so we can validate it after reloading the node.

Conclusion:

In summary, storm control in data center environments between CE and PE devices is a basic safeguard for L2VPN health and availability. L2VPNs replicate Layer 2 domains across geographically dispersed sites so are especially vulnerable to uncontrolled broadcast, multicast or unknown unicast traffic from misconfigured or faulty CE devices. Without storm control this traffic can become a network wide storm causing congestion, control plane exhaustion and service outages that affect not just the customer but potentially hundreds of tenants on the same provider infrastructure. Storm control is a proactive gatekeeper that enforces traffic thresholds at the network edge and ensures only valid and scaled Layer 2 traffic enters the provider’s core. This isolates faults at their source and allows providers to meet SLAs by keeping the network stable, optimizing resources and reducing the risk of cascading failures. As shown in the example above, with storm control a critical event becomes a manageable one, keeping business running and the provider’s reputation for delivering resilient and secure services intact. So, integrating storm control with other network hygiene practices like MAC limiting, loop detection and interface monitoring is not just recommended but required for robust L2VPN infrastructures.