What is deadlock situation in MPLS Traffic Engineering ? What happens when deadlock occurs ? Is there any mechanism to prevent deadlock ? I will explain all the details in this post. Deadlock occurs when LSP needs to move to the other link but due to lack of available bandwidth cannot move to the other links. I will show you the case with the below topology.
LSP Dead Lock
Figure -1 Dead Lock Problem in Distributed MPLS Traffic Engineering
There are two RSVP signaled MPLS TE LSP in the above figure. Red LDP is signaled with 400 Mbps and Blue LSP is signaled with 400 Mbps capacity.
Link capacities are 1000 Mbps , except P1 – P3 , P2 – P3 , they are 500 Mbps.
Reservation with RSVP is done at the control plane.
RSVP doesn’t take actual link utilization in the dataplane. Which means, if you send 600Mbps traffic over 400Mbps signaled LSP, traffic is not dropped. If there is no utilization on the physical link, you wouldn’t see any problem. Control and data plane is not synchronized by default in MPLS Traffic Engineering deployment with RSVP signaling.
On the above figure, let’s say actual traffic utilization reaches to the 800 Mbps on the Red LSP. And usage on Blue LSP is still 400 Mbps. Two of these LSPs combine, 1200Mbps traffic is sent down over 1000Mbps link.
Thus , the traffic over both of the LSPs will be affected. So, RED LSP’s traffic increase, affects Blue LSP as well. In distributed Traffic Engineering, two futures are used to avoid deadlock problem. LSP priorities and the Auto Bandwidth.
With Auto Bandwidth, routers check the actual interface usage, so the data plane traffic and adjust the RSVP control plane accordingly. So, LSP is resized. But without priority, even when resizing the Red LSP to 800 Mbps, Blue LSP is not moved to the bottom path.
Both priority and Auto bandwidth would work in this topology. But what if Blue LSP’s priority is better and it tries to force Red LSP to move to an alternate link. So solution gets very complex.
Thus, the better solution for the deadlock problem is centralized approach. If you would have a centralized node which knows the real time topology information , traffic demand and the active LSPs in the network, centralized nodes would move the LSPs accordingly to an alternate links. Centralized nodes would take the latency, bandwidth and the many other constraints for the LSPs into an account while placing the LSPs.
Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021
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