Command:

set-overload-bit


Mode:

Router(config-router)#


Syntax:

set-overload-bit [on-startup {seconds | wait-for-bgp}] [suppress {[interlevel] [external]}]
no set-overload-bit

 

Syntax Description:

on-startup (Optional) Sets the overload bit upon the system starting up. The overload bit remains set for the number of seconds configured or until BGP has converged, depending on the subsequent argument or keyword specified.
number (Optional) When the on-startup keyword is configured, causes the overload bit to be set upon system startup and remain set for this number of seconds.
seconds (Optional) When the on-startup keyword is configured, causes the overload bit to be set upon system startup and remain set until BGP has converged. If BGP does not signal IS-IS that it is converged, IS-IS will turn off the overload bit after 10 minutes.
suppress (Optional) Causes the type of prefix identified by the subsequent keyword or keywords to be suppressed.
interlevel (Optional) When the suppress keyword is configured, prevents the IP prefixes learned from another IS-IS level from being advertised.
external (Optional) When the suppress keyword is configured, prevents the IP prefixes learned from other protocols from being advertised.

 

Command Description:

To configure the router to signal other routers not to use it as an intermediate hop in their shortest path first (SPF) calculations, use the set-overload-bit command in router configuration mode. To remove the designation, use the no form of this command.

The overload bit is not set by default.

This command forces the router to set the overload bit (also known as the hippity bit) in its nonpseudonode link-state packets (LSPs). Normally, the setting of the overload bit is allowed only when a router runs into problems. For example, when a router is experiencing a memory shortage, it might be that the link-state database is not complete, resulting in an incomplete or inaccurate routing table. By setting the overload bit in its LSPs, other routers can ignore the unreliable router in their SPF calculations until the router has recovered from its problems.

The result will be that no paths through this router are seen by other routers in the IS-IS area. However, IP and Connectionless Network Service (CLNS) prefixes directly connected to this router will still be reachable.

This command can be useful when you want to connect a router to an IS-IS network but do not want real traffic flowing through it under any circumstances. Examples situations are as follows:

A test router in the lab, connected to a production network.
A router configured as an LSP flooding server, for example, on a nonbroadcast multiaccess (NBMA) network, in combination with the mesh group feature.
A router that is aggregating virtual circuits (VCs) used only for network management. In this case, the network management stations must be on a network directly connected to the router with the set-overload-bit command configured.

Unless you specify the on-startup keyword, this command sets the overload bit immediately.

In addition to setting the overload bit, you might want to suppress certain types of IP prefix advertisements from LSPs. For example, allowing IP prefix propagation between Level 1 and Level 2 effectively makes a node a transit node for IP traffic, which might be undesirable. The suppress keyword used with the interlevel or external keyword (or both) accomplishes that suppression while the overload bit is set.


Example:

The following example sets the overload bit upon startup and until BGP has converged, and suppresses redistribution between IS-IS levels and suppresses redistribution from external routing protocols while the overload bit is set:

Router(config)# interface Ethernet0
Router(config-if)# ip address 10.1.1.1 255.255.255.0
Router(config-if)# ip router isis

Router(config)# router isis
Router(config-router)# net 49.0001.0000.0000.0001.00
Router(config-router)# set-overload-bit on-startup wait-for-bgp suppress interlevel external
Router(config-router)# router bgp 100


Misconceptions:

None


Related Commands:

None

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