There are three LSP flags that can potentially be turned on in IS-IS messaging:
- Attached bit
- Partition Repair bit
- Overload bit
Attached bit – When an L1L2 router forms adjacencies or learns routes to other outside areas, that router will turn the Attached bit on and begin flooding LSPs with it. This will be received by other L1 routers in its area, and indicate to those other routers that it’s attached to other areas. This will cause those L1 routers to install a default route to the closest L2 router with the Attached bit set.
Partition Repair bit – this bit indicates if a router is capable of an optional feature that allows healing a partitioned are over a L2 subdomain, Cisco does not support this so it is always off and set to 0
Overload bit – this was meant to inform other routers that for any reason the originating router was unable to store all LSPs in its memory. An effect of this would be other area routers removing this device from being considered in the SPF topology and would converge network traffic around this device so its no longer used in routing.
A useful method for the Overload bit would be to turn it on manually for a network maintenance to allow the network to immediately converge across a different path so that device can be taken out and worked on.