Routing Protocols and Path Selection
Routing Protocols and Path Selection
This domain tests your knowledge of dynamic routing protocols (OSPF, BGP) and how they interact with SD-WAN path selection. You must understand how OSPF areas and BGP attributes (AS-path, local preference, MED, weight) influence route selection, and how SD-WAN overlays can override traditional routing decisions using application-aware policies. Arista’s SD-WAN uses Pathfinder to compute optimal paths based on real-time metrics like latency, jitter, and packet loss.
Traps often involve BGP best-path selection order: questions may present a scenario where multiple attributes conflict, and you must know the sequence (weight > local pref > locally originated > AS-path length > origin > MED > eBGP over iBGP > IGP metric). Another trap is assuming OSPF cost is the sole determinant in SD-WAN; instead, SD-WAN policies can steer traffic regardless of underlying routing metrics. Also, be clear on the difference between control-plane routing and data-plane forwarding in SD-WAN.
Worked example / rule of thumb: When comparing two BGP routes, always check weight first (Cisco-specific, but Arista EOS also supports it), then local preference. If local pref is equal, prefer the route with the shortest AS-path. Remember that SD-WAN path selection can override BGP decisions for specific applications, so traditional routing may not dictate final forwarding.
Key points:
- BGP best-path selection order: weight, local pref, locally originated, AS-path length, origin, MED, eBGP over iBGP, IGP metric.
- OSPF uses cost (bandwidth-based) for intra-area, and type 3 LSAs for inter-area.
- SD-WAN Pathfinder uses real-time metrics (latency, jitter, loss) to select paths.
- SD-WAN policies can override traditional routing for application steering.
- Understand the interaction between overlay and underlay routing.