I was following JNCIE-SP boot camp tasks and I stuck in one task for a while and couldn’t figure out how it goes.
First of all, I draw this simple diagram to elaborate the issue simply as much as I can.
The task is to aggregate the routes which are advertised by DC coming from OSPF and export it to ISIS level 2.
Note: Wide-metric is configured in level 2 to all external routes will have route preference with 18.
The solution was pretty easy as per steps below:
But I got different behavior each time
Sorry for this long discussion!
"Create policy to export this aggregated routes to ISIS, without matching the protocol, because as they mentioned either R3 or R5 will receive the route from other router (R3/R5) from ISIS with route preference 18 which is better than the locally aggregated routes, so it will be redistributed either from routes coming from ISIS or aggregated one."
When the route is learnt from IS-IS and prefered in the routing-table, the policy is trying to export an IS-IS route from the same level & area back into the same IS-IS database, which I suppose is not accepted as only external routes can be exported to the IGP. When looking at 'show isis database detail' the aggregate route does not come back into the LSP for that router.
I understand the solution would be configuring the route preference for the aggregate lower than the IGP preference [<18 in this case], thus always exporting the aggregate route into IS-IS DB from R3 & R5 when present.
This is really what I was thinking about, but what is written and elaborated in boot camp is something different which makes me more confused and stuck on it for long time.