I have 5 Sites, each with local lan and 2 routers (one connects to upstream site over WAN link, one to downstream site over a WAN link) thus: L1-S1R2-W1/2-S2R1-L2-S2R2-W2/3-S3R1-L3-S3R2-W3/4-S4R1-L4-S4R2-W4/5-S5R1-L5-S5R2-W5/1-S1R1-L1
Essentilally forming a ring if you bend the ends of the depiction around and join the L1s together...
Routing is ISIS. Under normal conditions, routes to a LAN subnet directly connected to site S1 (LAN L1) look like this...
S2R1-115/30 via w1/2 (i.e. left)
S2R2-115/40 via L2 (i.e. left)
S3R1-115/60 via w2/3 (i.e. left)
S3R2-115/70 via L3 (i.e. left)
S4R1-115/70 via L4 (i.e. right)
S4R2-115/60 via w4/5 (i.e. right)
S5R1-115/40 via L5 (i.e. right)
S5R2-115/30 via w5/1 (i.e. right)
So, from the point of view of the layout as depicted above, half the routes point "left", and half point "right". So far, so good. Now, if I shut the L1 interface on S1R2, I am expecting all of the routes to the L1 subnet to point "right", as clearly the access to that subnet in the "left" direction is hosed. But, of course, that isn't what happens, and I can't understand why. What I end up with is this:
S1R2-115/150 via w1/2 (i.e. right) - correct
S2R1-115/130 via L2 (i.e. right) - correct
S2R2-115/120 via w2/3 (i.e. right) - correct
S3R1-115/60 via w2/3 (i.e. left) - what?
S3R2-115/70 via L3 (i.e. left) - No!!!
S4R1-115/70 via L4 (i.e. right) - correct
S4R2-115/60 via w4/5 (i.e. right) - correct
S5R1-115/40 via L5 (i.e. right) - correct
S5R2-115/30 via w5/1 (i.e. right) - correct
As you can see, for some reason the routers at S3 simply refuse to re-route the subnet and it remains pointing "left" whereas devices 'more left' of that site are correctly routing it to the right. Distances metrics 'more left' of S3 are as one would expect, but on the S3 routers themselves they are incorrect - as we are incrementing 10 on the local LAN and 20 across the WAN, I'd expect the routing to be "115/100 via L3" for S3R1 and "115/90 via w3/4" for S3R2... Now of course S2 and S3 are cut off from the L1 subnet.
As an aside, if I shut the WAN interface in S1R2 instead of the LAN, the routes to the L1 subnet do all point "right" as expected
I'm not overly experienced with ISIS so I can't fathom why this is happening. can anyone shed any light on the matter?