Topology:
Objective: To test the broken LSP failover using LDP protocol.
All router in the topology are running under OSPF area 0.LDP has been enabled between them. Currently LDP synced and LDP neighbor ship has been established between all routers.
As depicted in topology Router R2 have two path to reach to Router R9 loopback 9.9.9.9.
It can reach either via R2-R3-R4-R9 or R2-R5-R6-R7-R8-R9. Since LDP best path selection is totally dependent on IGP( here ospf) , R2 will choose path R2-R3-R4-R9 as best route to reach to 9.9.9.
Now lets broke the LSP between Primary best path R2-R3-R4-R9.We will simply undo ldp between R3-R4 and check if the LDP LSP synchronized to path R2-R5-R6-R7-R8-R9 or not.
LDP neighborship between R3-R4 has been down now.Let’s trace 9.9.9.9 from R2 if LSP path has been redirected towards backup path R2-R5-R6-R7-R8-R9.
R2 not able to reach 9.9.9.9 since the lsp path between R3-R4 is broken.LDP did not switch over to other backup path R2-R5-R6-R7-R8-R9.
LDP is not a routing protocol , it is only used for generating label and its totally dependent on IGP for LDP label distribution and selection the best lsp path.
Thus when lsp path between R3-R4 went down the IGP was still up and R2-R3-R4-R9 was still the best path as per ospf.
LDP establishes MPLS LSPs along the shortest path to a destination as determined by IP forwarding. In a common network design, LDP is used to provide Label Switched Paths throughout the complete network domain covered by an IGP such as Open Shortest Path First (OSPF)or Intermediate System to Intermediate System (IS-IS)
In LDP enabled network LSP path must be established between end to end host , if any one link is not enabled LDP due to misconfiguration or any bug, a black hole exists and mpls forwarding will fail.
This is one of the well-known issue with LDP and IGP which is covered under RFC5443 – LDP IGP synchronization.
The LDP protocol has currently no way to correct the issue. As LDP is not a routing protocol; it cannot re-direct traffic to an alternate IGP path. So service provider comes with a concept where we can use IGP for serving both the purpose, i.e label distribution and well as routing. This will be more scalable and robust which tends to the origin of new concept which is call Segment Routing.
Summary:
- LDP label distribution is totally dependent on IGP.
- In case if any LSP path is broken between source to destination LDP do not automatically divert the path.
- LDP is not capable of choosing the best path to destination, it has to depend on IGP for choosing the best path.
- Once deployed LDP may take time for label and binding distribution forming lsp. This will add latency to the network.