Conditional ATT Bit

Load isis.inter.area.cfg

#IOS-XE
config replace flash:isis.inter.area.cfg

#IOS-XR
configure
load bootflash:isis.inter.area.cfg
commit replace
y

Configure R4 so that it only sets the attached bit for area 49.1234 when the prefix 200.200.200.200/32 is present in the routing table.

Answer

#R4
ip prefix-list LO200 permit 200.200.200.200/32
!
route-map ISIS_ATT
 match ip add prefix LO200
!
router isis
 set-attached-bit route-map ISIS_ATT

Explanation

The ISIS command set-attached-bit provides a way to control when the router sets the ATT bit in its L1 LSP.

Currently the prefix 200.200.200.200/32 is not present in the RIB, so R4 clears the ATT bit from its LSP. We can verify this on any L1 router in the area.

If we inject 200.200.200.200/32 on any router, for example XR2, R4 should begin setting the ATT bit again.

#XR2
int lo200
 ip add 200.200.200.200/32
!
router isis 1
 int lo200
  passive
  add ipv4 

On R4, the route-map now evaluates to “True,” so the attached bit is set again: