Juniper Updated JN0-650 Exam Questions and Answers by ayesha

In enterprise networks utilizing Junos OS, IBGP (Internal BGP) route flapping is frequently caused by recursive routing failures within the underlying IGP (Interior Gateway Protocol) , such as OSPF or IS-IS.

Recursive Next-Hop Resolution: Unlike EBGP, where the neighbor is typically directly connected, IBGP neighbors are often several hops away. Consequently, an IBGP route ' s protocol next-hop is usually a loopback IP address that requires a recursive lookup to resolve. The router must consult its unicast routing table (typically inet.0 or inet.3) to find a physical path to that next-hop.

The Flapping Mechanism: A " flap " occurs when a route is repeatedly withdrawn and readvertised. If the IGP path used to resolve the BGP next-hop becomes unstable or if the BGP route itself is inadvertently used to resolve its own next-hop (a recursive loop), the BGP next-hop becomes unreachable.

Impact on Routing Table: When the next-hop cannot be resolved, the BGP route is considered invalid and is withdrawn from the routing table. Once the route is withdrawn, the original IGP path to the next-hop often becomes valid again, causing the BGP route to be readvertised . This cycle of withdrawal and readvertisement creates the instability known as flapping.

Why other options are incorrect: Packet misordering (Option A) and community tagging (Option B) may affect performance or policy but do not cause a route to be withdrawn and readvertised in this cyclical manner. While update intervals (Option C) control how quickly changes are sent, they are not the root cause of the reachability failure that triggers the flap itself.

OSPF determines the best path to a destination by calculating the metric (cost) of each link. By default, Junos OS uses a reference bandwidth of 100 Mbps to calculate this cost using the formula:

When the reference bandwidth is left at the default 100 Mbps, any interface with a speed of 100 Mbps or higher (including 1 GbE and 10 GbE) is assigned a cost of 1 because the minimum OSPF cost is 1. This results in equal-cost paths, preventing the router from preferring the faster 10 GbE link.

To ensure 10 GbE interfaces are preferred, you must create a cost differential:

Option A (Reference Bandwidth): By increasing the reference bandwidth to 10G (or higher), the calculation changes. For a 10 GbE link, the cost becomes $10,000 / 10,000 = 1$. For a 1 GbE link, the cost becomes $10,000 / 1,000 = 10$. Since OSPF prefers the path with the lowest cumulative cost, the 10 GbE link is now preferred.

Option D (Manual Metric): You can manually override the automatic cost calculation by assigning a higher metric specifically to the 1 GbE interfaces. If a 1 GbE interface is manually set to a cost of 50 and the 10 GbE interface remains at 1 (or is set to a lower value), the router will prioritize the 10 GbE path.

Option B is incorrect because a higher metric makes a path less preferred. Option C is incorrect because a 1G reference bandwidth would still result in both 1 GbE and 10 GbE interfaces having a cost of 1.