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Split Detection Behavior in a Virtual Chassis

 

If there is a disruption to a Virtual Chassis configuration for MX Series 5G Universal Routing Platforms or EX9200 Switches due to the failure of a member router or switch or one or more Virtual Chassis port interfaces, the resulting connectivity loss can cause a split in the Virtual Chassis configuration. Split detection identifies the split and can minimize further network disruption.

Best Practice

We recommend that you use the heartbeat connection instead of the split detection feature in an MX Series Virtual Chassis to avoid unnecessary mastership role changes during an adjacency disruption or split, and to provide additional member health information for the mastership election process.

This topic covers:

How Split Detection Works in a Virtual Chassis

Split detection is enabled by default in an EX9200 or MX Series Virtual Chassis. Optionally, you can disable split detection by including the no-split-detection statement at the [edit virtual-chassis] hierarchy level. Disabling split detection can be useful in certain Virtual Chassis configurations.

Note

Using the no-split-detection statement is prohibited when you configure a heartbeat connection, and the software prevents you from configuring both the no-split-detection and heartbeat-address statements at the same time. If you attempt to do so, the software displays an error message and causes the commit operation to fail. We recommend configuring the heartbeat-address statement rather than the no-split-detection statement.

For example, if the backup router or switch fails in a two-member Virtual Chassis configuration and split detection is enabled (the default behavior), the master router or switch takes a line-card role, and the line cards (FPCs) that do not host Virtual Chassis ports go offline. This state effectively halts routing and disables the Virtual Chassis configuration. By contrast, if the backup router or switch fails in a two-member Virtual Chassis configuration and split detection is disabled, the master router or switch retains mastership and maintains all of the Virtual Chassis ports, effectively resulting in a single-member Virtual Chassis consisting of only the master router or switch.

Best Practice

Effect of Split Detection on Virtual Chassis Failure Scenarios

The behavior of a Virtual Chassis during certain failure scenarios depends on whether split detection is enabled or disabled. Table 1 describes the effect of the split detection setting on common failure scenarios in a two-member MX Series Virtual Chassis.

Table 1: Effect of Split Detection on Common Virtual Chassis Failure Scenarios

Type of Failure

Split Detection Setting

Results

Virtual Chassis port interfaces go down

Enabled

  • VC-B takes VC-M role.

  • Previous VC-M takes line-card (VC-L) role. The line-card role isolates the router or switch and removes it from the Virtual Chassis until connectivity is restored.

  • Result is a single-member Virtual Chassis consisting of only a single VC-M. The VC-M continues to maintain subscriber state information and route traffic.

When Virtual Chassis port interfaces are reconnected:

  • VC-M retains VC-M role.

  • VC-L takes VC-B role.

  • Subscribers are not affected.

Virtual Chassis port interfaces go down

Disabled

When Virtual Chassis port interfaces are disconnected:

  • VC-M retains VC-M role, and VC-B also takes VC-M role. The result is a Virtual Chassis with two VC-M routers or switches, each of which maintains subscriber state information.

  • Initially, both VC-M routers or switches have a complete list of subscribers. Because the two routers or switches have the same configuration, the effect on subscribers, traffic patterns, behavior of external applications, and subscriber login and logout operations is unpredictable while the Virtual Chassis port interfaces are disconnected.

When Virtual Chassis port interfaces are reconnected:

  • Original VC-M before the disconnection resumes VC-M role, and original VC-B before the disconnection resumes VC-B role.

  • Subscribers on the VC-M are preserved.

  • Subscribers on the VC-B are purged.

  • The subscribers preserved on the VC-M are unaffected, and all remaining subscribers are able to log back in to the router or switch.

Virtual Chassis backup router or switch (VC-B) goes down

Enabled

  • VC-M takes line-card (VC-L) role, which causes all line cards (FPCs) that do not host Virtual Chassis ports to go offline.

  • Previous VC-B is out of service.

  • The line-card role isolates the master router or switch and removes it from the Virtual Chassis until connectivity is restored. As a result, the Virtual Chassis is left without a master router or switch, which halts interchassis routing and effectively disables the Virtual Chassis configuration.

When the failed router or switch is brought back into service:

  • The mastership election algorithm is run to determine whether the router or switch takes a VC-M or VC-B role. The Virtual Chassis then becomes operational.

  • All subscribers can log back in to the router or switch.

  • Previous subscriber state information is not preserved.

Virtual Chassis backup router or switch (VC-B) goes down

Disabled

  • VC-M retains VC-M role and maintains all Virtual Chassis ports.

  • Previous VC-B is out of service.

  • Result is a single-member Virtual Chassis consisting of only a single VC-M. The VC-M continues to maintain subscriber state information and route traffic.

Virtual Chassis master router or switch (VC-M) goes down

Split detection setting has no effect on behavior

  • VC-B takes over VC-M role regardless of whether split detection is enabled or disabled.

  • Previous VC-M is out of service.

  • Result is a single-member Virtual Chassis consisting of only a single VC-M. The new VC-M continues to maintain subscriber state information and route traffic.

When the original VC-M is brought back into service, or when the original VC-M is replaced with a new router or switch:

  • Original VC-M or its replacement takes VC-B role.

  • Subscribers are not affected.

Active access link between the VC-M and the access node, such as a digital subscriber line access multiplexer (DSLAM), goes down

Split detection setting has no effect on behavior

  • Previous standby access link becomes the active access link between the VC-B and the access node.

  • Traffic is routed through the new active access link.

  • The VC-M continues to maintain subscriber state information and route traffic.