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overload (Protocols IS-IS)

Syntax

Hierarchy Level

Description

Configure the local routing device so that it appears to be overloaded. This statement causes the routing device to continue participating in IS-IS routing, but prevents it from being used for transit traffic. Traffic destined to immediately attached subnets continues to transit the routing device.

You can also advertise maximum link metrics in network layer reachability information (NLRI) instead of setting the overload bit.

You configure or disable overload mode in IS-IS with or without a timeout. Without a timeout, overload mode is set until it is explicitly deleted from the configuration. With a timeout, overload mode is set if the time elapsed since the IS-IS instance started is less than the specified timeout.

A timer is started for the difference between the timeout and the time elapsed since the instance started. If the time elapsed after the IS-IS instance is enabled is less than the specified timeout, overload mode is set. When the timer expires, overload mode is cleared. In overload mode, the routing device IS-IS advertisements are originated with the overload bit set. This causes the transit traffic to take paths around the routing device. However, the overloaded routing device’s own links are still accessible.

The value of the overload bit depends on these three scenarios:

  1. When the overload bit has already been set to a given value and the routing process is restarted: Link-state PDUs are regenerated with the overload bit cleared.

  2. When the overload bit is reset to a lesser value while the routing process is running: Link-state PDUs are regenerated with the overload bit cleared.

  3. When the overload bit is reset to a greater value while the routing process is running: Link-state PDUs are regenerated with the overload bit set to the difference between the old and new value.

In overload mode, the routing device advertisement is originated with all the transit routing device links (except stub) set to a metric of 0xFFFF. The stub routing device links are advertised with the actual cost of the interfaces corresponding to the stub. This causes the transit traffic to avoid the overloaded routing device and take paths around the routing device.

To understand the reason for setting the overload bit, consider that BGP converges slowly. It is not very good at detecting that a neighbor is down because it has slow-paced keepalive timers. Once the BGP neighbor is determined to be down, it can take up to 2 minutes for a BGP router to declare the neighbor down. IS-IS is much quicker. IS-IS only takes 10-30 seconds to detect absent peers. It is the slowness of BGP, more precisely the slowness of internal BGP (IBGP), that necessitates the use of the overload bit. IS-IS and BGP routing are mutually dependent on each other. If both do not converge at the same time, traffic is dropped without notification (black holed).

You might want to configure the routing device so that it appears to be overloaded when you are restarting routing on the device. Setting the overload bit for a fixed amount of time right after a restart of the routing protocol process (rpd) ensures that the router does not receive transit traffic while the routing protocols (especially IBGP) are still converging.

Setting the overload bit is useful when performing hardware or software maintenance work on a routing device. After the maintenance work, clear the overload bit to carry on forwarding transit traffic. Manual clearing of the overload bit is not always possible. What is needed is an automated way of clearing the overload bit after some amount of time. Most networks use a time value of 300 seconds. This 5-minute value provides a good balance, allowing time to bring up even large internal IBGP meshes, while still relatively quick.

Another appropriate application for setting for the overload bit is on dedicated devices such as BGP route reflectors, which are intentionally not meant to carry any transit traffic. In this case, you would not use the timer.

You can verify that the overload bit is set by running the show isis database command.

Options

advertise-high-metrics—Advertise maximum link metrics in NLRIs instead of setting the overload bit.

The advertise-high-metric setting is only valid while the routing device is in overload mode. When advertise-high-metric is configured, IS-IS does not set the overload bit. Rather, it sets the metric to 63 or 16,777,214, depending whether wide metrics are enabled. This allows the overloaded routing device to be used for transit as a last resort.

An L1-L2 router in overload mode stops leaking route information between L1 and L2 levels and clears its attached bit. This is also true when advertise-high-metrics is configured.

allow-route-leaking—Enable leaking of route information into the network even if the overload bit is set.

Note:

The allow-route-leaking option does not work if the routing device is in dynamic overload mode. Dynamic overload can occur if the device has exceeded its resource limits, such as the prefix limit.

external–prefixes—Enable this option to stop receiving local traffic destined to external prefixes unless this is the only router in the network that hosts the prefix. External prefixes are prefixes that IS-IS receives from other protocols. The overloaded router advertises these external prefixes with a high metric to stop receiving traffic. After the configured timeout lapses regular metric is restored to resume traffic reception.

internal-prefixes—Enable this option to stop receiving local traffic destined to internal prefixes unless this is the only router in the network that hosts the prefix. Internal prefixes are local IS-IS prefixes that the overloaded router advertises with a high metric to stop receiving traffic. After the configured timeout lapses regular metric is restored to resume traffic reception.

timeout seconds—Number of seconds at which the overloading is reset.

  • Range: 60 through 1800 seconds

  • Default: 0 seconds

Required Privilege Level

routing—To view this statement in the configuration.

routing-control—To add this statement to the configuration.

Release Information

Statement introduced before Junos OS Release 7.4.

external-prefixes and internal-prefixes options introduced in Junos OS Release 18.3R1.