Rapid alternate paths for network destinations

1. A method for forwarding an Internet Protocol (IP) unicast data packet, the method comprising, at a network node:

using, by the network node, destination based shortest path routing algorithms for the network node to determine and pre-store a link to a primary neighbor in a primary path to a border node and a link to an alternate neighbor in an alternate path to the border node, wherein the border node offers the shortest path to an inter-region destination node, wherein the alternate path to the border node is longer than or equal to the primary path to the border node, wherein the alternate neighbor is a loop-free neighbor other than the primary neighbor and whose shortest path to the border node does not go through the network node that determined the alternate path, and wherein the shortest path between the alternate neighbor and the border node satisfies:

Distanceopt( N,ABR )<Distanceopt( N,S )+Distanceopt( S,ABR ),

where Distanceopt(N, ABR) is the shortest path between the alternate neighbor and the border node, Distanceopt(N, S) is the shortest path between the alternate neighbor and the network node, and Distanceopt(S,ABR) is the shortest path between the network node and the border node;

forwarding, by the network node, the IP unicast data packet to the primary path through the primary neighbor, wherein the primary neighbor is one or more primary next-hops directly from the network node to the border node;

switching over from the primary path to the alternate path upon failure of the primary path; and

forwarding the IP unicast data packet to the alternate neighbor whose shortest path to the border node does not go through the network node that determined the alternate path.

2. The method according to claim 1 , wherein the link to the primary neighbor in the primary path and the link to the alternate neighbor in the alternate path are determined with respect to a single border node.

3. The method according to claim 1 , wherein the link to a primary neighbor in a primary path and the link to the alternate neighbor in the alternate path are determined with respect to two or more border nodes.

4. The method according to claim 3 , wherein the two or more border nodes have different primary neighbors.

5. The method according to claim 3 , wherein the two or more border nodes have a same primary neighbor.

6. The method according to claim 5 , wherein the same primary neighbor has two or more alternate neighbors.

7. The method according to claim 6 , the method further comprises selecting one of the two or more alternate neighbors as the loop-free neighbor based on the one of the two or more alternate neighbors being a loop-free node-protecting alternate.

8. The method according to claim 6 , wherein no loop-free node-protection is available, the method further comprises selecting one of the two or more alternate neighbors as the loop-free neighbor based on the one of the two or more alternate neighbors being a loop-free link-protecting alternate.

9. A network node for forwarding an Internet Protocol (IP) unicast data packet, comprising:

a processor configured with destination based shortest path routing algorithms to determine and pre-store a link to a primary neighbor in a primary path to a border node and a link to an alternate neighbor in an alternate path to the border node, wherein the border node offers the shortest path to an inter-region destination node, wherein the alternate path to the border node is longer than or equal to the primary path to the border node, wherein the alternate neighbor is a loop-free neighbor other than the primary neighbor and whose shortest path to the border node does not go through the network node that determined the alternate path, and wherein the shortest path between the alternate neighbor and the border node satisfies:

Distanceopt( N,ABR )<Distanceopt( N,S )+Distanceopt( S,ABR ),

where Distanceopt(N, ABR) is the shortest path between the alternate neighbor and the border node, Distanceopt(N, S) is the shortest path between the alternate neighbor and the network node, and Distanceopt(S,ABR) is the shortest path between the network node and the border node; and

an interface configured to:

forward the IP unicast data packet to the primary path through the primary neighbor, wherein the primary neighbor is one or more primary next-hops directly from the network node to the border node;

switch over from the primary path to the alternate path upon failure of the primary path; and

forward the IP unicast data packet to the alternate neighbor whose shortest path to the border node does not go through the network node that determined the alternate path.

10. The network node according to claim 9 , wherein the link to the primary neighbor in the primary path and the link to the alternate neighbor in the alternate path are determined with respect to a single border node.

11. The network node according to claim 9 , wherein the link to the primary neighbor in the primary path and the link to the alternate neighbor in the alternate path are determined with respect to two or more border nodes.

12. The network node according to claim 11 , wherein the two or more border nodes have different primary neighbors.

13. The network node according to claim 11 , wherein the two or more border nodes have a same primary neighbor.

14. The network node according to claim 13 , wherein the same primary neighbor has two or more alternate neighbors.

15. The network node according to claim 14 , wherein one of the two or more alternate neighbors is selected as the loop-free neighbor based on the one of the two or more alternate neighbors being a loop-free node-protecting alternate.

16. The network node according to claim 14 , wherein no loop-free node-protection is available, and wherein one of the two or more alternate neighbors is selected as the loop-free neighbor based on the one of the two or more alternate neighbors being a loop-free link-protecting alternate.