Hey there,
I have a small question I want to understand.
In ISIS protocol, L1 route can be converted to L2 route when L1/L2 router from a neighbour-ship with L2 router of another area, but why L2 route can not be converted to L1 route and instead send Attach bit to generate default route.
In the attach diagram, we can see that Router 5 can not receive loopback address of R1 because R4 is receiving it as a L2 route and it has L1 neighbour-ship with R5, where L2 route can not be converted to L1 route. so R4 add ATT bit when prorating it's own loopback to R5 and further R5 generate default route towards R4.
