Introduction
Overload bit is used as a mechanism to route around an IS IS router. This could be used in a maintenance or during convergence time.
Overload Bit
- Originally the overload bit was there as a flow control mechanism to notify other ISIS routers not route to a particular router experiencing load issues.
- Setting the overload bit can be performed:
- On Startup - this option can set a manual timer to wait or wait for BGP to converge based on receipt o keepalive messages.
- Manually - set the overload bit manually without a timer to expire.
- Checking the overload bit can be done using the command show isis database
- When this feature is enabled, directly connected interfaces are reachable.
- Additional feature of Overload Bit are to suppress only certain prefixes either external or interlevel.
- External are any redistributed routes that will have the OL set.
- Interlevel are any routes advertised between ISIS Levels.
Relevant Commands
set-overload-bit
Manually sets the over load bit. Router is not be used to forward traffic.
R1(config-router)# set-overload-bit
set-overload-bit suppress
Depending on how the overload bit is set, as long as it is set just certain prefixes can be suppressed.
R1(config-router)# set-overload-bit [..] suppress (interlevel | external)
set-overload-bit on-startup
After a reboot set the overload bit and wait defined timer.
R1(config-router)# set-overload-bit on-startup (seconds)
set-overload-bit on-startup wait-for-bgp
After a reboot set the overload bit and wait for bgp to finish converging. If BGP keepalives are not received from all the BGP neighbors, and wait-for-bgp is configured, IS-IS will disable the overload bit after 10 minutes.
R1(config-router)# set-overload-bit on-startup wait-for-bgp
show isis database
A way to see if a node has the overload bit set.
show isis database
Additional Resources
Uses of the overload Bit with IS-IS - Cisco
RFC3277 - Intermediate System to Intermediate System (IS-IS) Transient Blackhole Avoidance - IETF