ISIS OVERLOAD BIT

Understanding the general technical meaning of word “Overload” tells shortage of resources (Memory/CPU etc), getting overloaded with currently handled processes. From here comes meaning of the Overload-bit in ISIS protocol.

When overload-bit is set on a particular router, it signals other routers in the network topology not to use it as transit router while sending any of data traffic.Value of overload-bit is 1 if set and 0 if not set.


Overload-bit is configured for below reasons:

    Configured on routers during start-up to avoid traffic black-holes until routing protocols are fully converged.
    To remove a core router(generally P router) from IGP topology before decommissioning or a maintenance operation like IOS upgrade/SMU (Software Maintenance Upgrade on IOS XR)installation.
    To prevent the Route Reflectors becoming transit router for data plane.
    To verify operation of new installed routers before allowing it forward transit traffic.


Some interesting points:

    If overload-bit is set on a router/IS in Level-1, the other Level-1 Routers/ISs in the topology stop using the overloaded Router/IS to forward Level-1 traffic through it.
    If overload-bit is set on a router/IS in Level-2, the other Level-2 Routers/ISs in the topology stop using the overloaded Router/IS to forward Level-2 traffic. However, the Router/IS can still forward Level-1 traffic.
    If overload-bit set on both L1/L2 router/IS, the Router/IS will block both types of traffic.


Overload-bit Command Syntax:

R1(config)#router isis
R1(config-router)#[no] set-overload-bit on-startup wait-for-bgp max-bgp-wait-time

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To manually remove a device out of IGP topology, we can set the overload-bit as below:

R1(config)#router isis [instance name]
R1(config-router)#set-overload-bit

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To temporarily set the overload bit on after a software reload, configure the below command:

R1(config-router)# set-overload-bit on-startup 10

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This command configures the router to set the overload-bit on in all its IS-IS LSPs sent to other routers during the first 10 seconds following a successful software reload. After 10 seconds the router resets the overload-bit to “0” in all its ISIS LSPs.

We can also set-overload-bit command to prevent route blackholing in support of RFC 3277. Using this option, while a device reboots, ISIS sets the overload bit in its LSPs until BGP has converged.

R1(config-router)# set-overload-bit on-startup wait-for-bgp 1000


Note: The max-bgp-wait-time variable is the maximum time IS-IS will wait for BGP convergence to complete. Once this time has been exceeded without BGP converging, ISIS will resets overload-bit to 0. The default value is 600 seconds (10 minutes), with possible values range: 5 to 86400 seconds.


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