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How does a router choose the best path when multiple routing protocols provide a route to the same destination?

 When a router receives multiple routes to the same destination from different routing protocols, it follows a systematic process to select the best path to install in its routing table and use for forwarding traffic.

How does a router choose the best path when multiple routing protocols provide a route to the same destination?

Step-by-step: How a Router Chooses the Best Path

1. Administrative Distance (AD) Check

  • The router first compares the Administrative Distance of each route.

  • The route with the lowest AD (most trusted source) wins.

Routing SourceDefault AD
Connected interface0
Static route1
EIGRP (internal)90
OSPF110
RIP120
External EIGRP170

Example: If OSPF and RIP both have routes to the same network, the router picks the OSPF route (AD 110 < 120).

2. Metric Comparison Within the Same Protocol

  • If multiple routes to the destination come from the same routing protocol and have the same AD, the router uses the routing metric to choose the best path.

  • Metrics vary by protocol:

    • OSPF: Cost (based on bandwidth)

    • RIP: Hop count

    • EIGRP: Composite metric (bandwidth, delay, reliability, load)

  • Lower metric means better path.

3. Equal-Cost Multi-Path (ECMP)

  • If multiple routes have equal AD and equal metrics, the router may install multiple routes (load balancing).

  • This is called ECMP, which allows traffic to be spread across multiple best paths.

4. Tie-Breakers (Protocol-Specific)

  • If routes have the same AD and metric, some protocols have additional tie-breakers, such as:

    • Router ID (lowest wins)

    • Path age

    • IP address of next hop

Summary Flowchart:

vbnet

Multiple routes received to same destination
            ↓
Compare Administrative Distance (AD)
            ↓
Choose route with lowest AD
            ↓
If AD equal → Compare protocol metric
            ↓
Choose route with lowest metric
            ↓
If metric equal → Use ECMP or protocol-specific tie-breakers
            ↓
Install best path(s) in routing table

Example

A router learns about network 192.168.1.0/24 from:

  • Static route (AD 1, metric N/A)

  • OSPF route (AD 110, cost 20)

  • RIP route (AD 120, hop count 3)

Router chooses the static route because it has the lowest AD (1), even though the OSPF route may have a better metric.

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