Editing IS-IS (section)

== Attribute bits in LSPs ==
IS-IS LSPs contain specific information, encoded to Attribute block in LSP header, which is 8 bits long. Here are some of the important ones

* '''P bit''' - Partition repair bit, 8<sup>th</sup> bit, indicates if partitioned L1 area can be repaired (joined together) over L2 area. Modern deployments of IS-IS generally do not support partition repair function, therefore, it is not set.
* '''ATT bit''' - Attached bit, 7<sup>th</sup> - 4<sup>th</sup> bits, indicates if originating router is attached to another area. If these bits are set by L1/L2 router in its L1 LSP, then other routers in L1 area will automatically generate default route to the originator.  Technically, there are 4 ATT bits, each of them responsible for Error, Expense, Delay and Default metrics. This was because when IS-IS was originally developed, it was assumed that routing protocol would support multiple topologies and separately calculate a separate SPF for each metric.   Later, this was deprecated, therefore, modern IS-IS deployments when necessary, set only 4<sup>th</sup> bit (for default metric).
* '''OL bit''' - Overload bit, 3<sup>rd</sup> bit, indicates if the router is overloaded. If this bit is set, then this router is NOT used as transit. However, it will be still reachable.   Overload bit can be set by router under heavy load or intentionally by engineer.  Setting overload bit is an easy way to gracefully offload the router prior to maintenance which requires router reboot. After router reboots and is available, then overload bit can be cleared manually.  Another implementation would be to wait for other dependent protocols (such as BGP) to fully establish neighborship, and only after that become transit. This is because IS-IS converges much faster compared to BGP and if the router becomes transit before BGP has fully converged, this could cause traffic blackholing.  A good example would be PE router, running MPLS VPN with IS-IS and BGP. After PE boots, establishes IS-IS adjacency, establishes BGP neighborship with other routers, overload bit is cleared and this router joins MPLS VPN network.
* '''IS type bits''' - 2<sup>nd</sup> and 1<sup>st</sup> bits, indicate IS type of the originator. It can be L1 only, L2 only and L1/L2. If only first bit is set, then this is L1 only, if only second bit - L2 only, and if both bits are set - then this is L1/L2 router.