What is the IS-IS overload bit, and when do you set it?
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It is a flag in a router's LSP that says "do not use me for transit". Other routers still reach the router's own prefixes, but SPF will not route through it. The classic use is setting it at boot until BGP has converged, so a reloaded core router does not attract traffic it cannot yet forward. It is also the clean way to drain a router before maintenance. The OSPF equivalent is advertising maximum metric in the router LSA (RFC 6987).
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On boot
router isis CORE
set-overload-bit on-startup wait-for-bgpwait-for-bgp clears the bit when BGP reports it has converged. set-overload-bit on-startup 300
clears it after a fixed 300 seconds instead.
The black hole it prevents
A provider core router reloads. IS-IS converges in seconds and the router becomes the shortest path to many PE loopbacks. BGP, which carries the full internet table, takes minutes. Without the overload bit, traffic lands on a router that has no route for most destinations and is dropped.
For maintenance
router isis CORE
set-overload-bitTraffic moves away before you touch anything. Remove it when done. Seeing it in the database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL
P3.00-00 0x0000012A 0x3B7C 1137/1199 0/0/1They will ask next
Does the overload bit make the router's own loopback unreachable?
No. Directly attached prefixes stay reachable, so you can still manage the router and BGP sessions to its loopback still work. It only stops transit through it.
Traps
- Confusing the overload bit with the ATT bit. ATT advertises an exit from an L1 area; OL refuses transit.