IS IS Attach Bit Control


The Attach Bit is set by L1L2 routers to notify L1 routers that they can reach the rest of the network i.e. default routing.

Attach Bit

  • Inter-area routing is accomplished by directing all traffic to the nearest L1/L2 router.
  • L1 routers discover L1/L2 router with the attached bit set in L1 LSPs.
  • The Attach Bit is set anytime a router seen multiple areas.
  • When multiple L1/L2 router loses connectivity to the rest of the network, advertising the Attach Bit can cause black-holing of traffic.
  • The Attach Bit Control feature allow you to use route-maps policies to control the advertisement of attach bit.
  • There is another related command IS-IS-Ignore-Attach-Bit which is a hidden command that ignores it.

Set Attach Bit

  • Route-map selects a condition to set the attach bit.

Additional Resources

Using ISIS Att Bit Control - Cisco PDF
Using IS-IS ATT-Bit Control Feature - Cisco


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