Thanks Alex,
Say, You have LSP-A->Z which goes via link1->link2->link9->link66.
Then to be "symmetric", LSP-Z->A will have to go link66->link9->link2->link1.
Say, the alternate path to avoid link9 is to use link2->link19->link29.
Hence, if You break link 9, the LSPs can still remain "symmetric" by using a longer path link1<->link2<->link19<->link29<->link66 (except for a short time while LSPs resignal).
Let say we extend this example as below:
Let say LSP-Z->A needs 10M, since we are creating symmetric pair, another LSP from Z->A is also created with BW 10M
Z --link1-link2--link9-----------link66-----A
|-----link19---link29--|
1) Both LSP are using top path ( link1,link2,link9,link66)
2) Link 19 is on transit LSR with many other LSP going though it, total BW available from link19->link29 is 20M, but only 5M is available from link29-link19.
3) Link 9, goes down, LSP-Z->A is routed over link1,link2,link19,link29,link66, since LSP Z->A is configured for symmetric pair,return LSP A->Z needs to go over exactly same links ( link66,link29,link19,link2,link1) but there is no enough BW available in this direction for this LSP ( 10M is needed but only 5M is available from link29->link19)
In this case, symmetry is not possible, will north star controller establish single LSP Z->A ( link1,link2,link19,link29,link66) because we have required BW along this path ( 10M) , without establishing return lsp A->Z which can not be established because required 10M is not available ?
Much appreciated!!