Path preference

1.10 Troubleshoot OSPF (v2/v3)

📘CCNP Enterprise – ENARSI (300-410)

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1. What is Path Preference in OSPF?

• Path Preference is how OSPF decides which route to use when multiple paths exist to the same destination.
• OSPF always selects the best path based on cost, but there are other rules as well.

Key point for the exam:

OSPF always prefers the path with the lowest total cost to a destination.

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2. How OSPF Determines the Best Path

OSPF uses several factors to decide which path is preferred:

a) Cost

• OSPF assigns a cost value to each interface.
• Cost is usually based on interface bandwidth:

Cost = Reference Bandwidth / Interface Bandwidth

• Default reference bandwidth: 100 Mbps (Cisco default)
• Example:
  • FastEthernet (100 Mbps) → Cost = 1
  • GigabitEthernet (1 Gbps) → Cost = 1 (if reference bandwidth updated to 1 Gbps)
• Lower cost = more preferred path.

b) OSPF Route Types

If two routes exist with same cost, OSPF prefers based on LSA type:

1. Intra-area (O) → Highest preference
2. Inter-area (O IA) → Next
3. External Type 1 (O E1) → Next
4. External Type 2 (O E2) → Last

Exam tip: Intra-area routes are always preferred over inter-area, even if inter-area has lower cost.

c) Equal-Cost Multipath (ECMP)

• If multiple paths exist with exactly the same cost, OSPF can use all paths simultaneously.
• Cisco supports up to 16 equal-cost paths by default.
• This is load balancing at Layer 3, not just a backup path.

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3. Administrative Distance

• Administrative Distance (AD) determines preference between different routing protocols, not within OSPF itself.
• OSPF default AD: 110.
• Example: If OSPF and EIGRP both know a route, the lower AD route is preferred.
• Internal OSPF paths (O, O IA) have the same AD, so OSPF uses cost-based selection.

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4. OSPF External Routes

• OSPF can receive routes from other routing protocols or redistribute static routes.
• External routes have two types:
  1. E1 → Cost includes internal OSPF cost + external cost
  2. E2 → Cost uses only the external metric (default)
• E1 is preferred over E2 if two external routes to the same network exist.

Exam tip:

Remember: OSPF prefers paths in this order: Intra-area → Inter-area → E1 → E2.

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5. How Path Preference Affects Routing

• OSPF path preference ensures traffic always uses the most efficient route.
• If a better path appears (lower cost), OSPF will recalculate the routing table and prefer the new path.
• If multiple equal-cost paths exist, OSPF load-balances traffic automatically.

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6. Verification Commands

To check path preference and route selection:

1. Show OSPF routes in the routing table

show ip route ospf

• O → Intra-area
• O IA → Inter-area
• O E1 / O E2 → External

2. Check OSPF cost to a destination

show ip ospf database

• Look at LSAs and the advertised cost.

3. Check OSPF neighbor states

show ip ospf neighbor

• Ensures neighbor relationships are stable, which affects path availability.

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7. Summary Table for Exam

Factor	How it affects preference
Cost	Lower cost = preferred path
Route type	O > O IA > O E1 > O E2
ECMP	Equal-cost paths can be used simultaneously (up to 16)
Administrative Distance	Only matters when comparing different protocols
External routes	E1 preferred over E2 if both exist

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✅ Exam Tip:
When a question asks “which OSPF path will be used?”:

1. Check intra-area vs inter-area vs external
2. Check total cost
3. Check if multiple equal-cost paths exist