OSPF Fast Convergence Tools

What is Network convergence?
Time required to “Detect the Event”, “Propagate the Event”,” Process the Event” and “Update the Routing and Forwarding Information base” and after that routing the traffic to an alternate path during the outage of primary network path is called network convergence.

OSPF have various tool and techniques for fast convergence as below mentioned:-
• Event Detection
• Event Propagation
• Event Processing
• RIB Update

Read OSPF High Availability Techniques with SSO,NSF and NSR

I will be covering various OSPF Event Detection tools in this post:-
• Carrier Delay
• Bidirectional Forwarding Detection
• Physical Links
• IP Event Dampening
• Fast Hellos

1. Carrier Delay:- When the physical interface states changes, software must notify that change to the routing process. By default carrier delay is 2 seconds. So we can configure the carrier delay to zero for fast link detection by using “carrier-delay msec 0” under interface.
Note:-Sometimes if the flap interval is less than 2 seconds, during that time router doesn’t generate any logs for the same. This could also be the reason that syslog will not see any message.

2. Bidirectional Forwarding Detection

3. Physical links:- If you are using PoS interface, link failure detection is almost < 50ms. Also no need to configure BFD on PoS interface. If you are using Gigabit Ethernet interfaces, use Auto-Negotiation under physical interface as it helps to detect unidirectional failures. By default it is enabled and recommends not disabling it.

4. IP Event Dampening:- By using this tool we can mitigate the flapping links. The concept is same as we normally use in BGP. It simply tracks the flapping interface and applies penalty. Add that flapping interface in down state from routing protocol perspective if it exceeds the penalty threshold.

5. Fast Hellos:- OSPF fast hello packets mainly focus on the stability rather than the convergence. It is very CPU intensive process and sometimes leads to the high CPU problems also. It can be configured under interface by using “ip ospf dead-interval minimal hello-multiplier 3” command. Minimal keyword in the command sets dead interval to 1 second and hello-multiplier 3 means send 3 packets in one second.

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