In our BGP Best Path Selection series, we have gradually explored how BGP makes its routing decisions step by step.
So far, we’ve covered the major attributes that influence the path selection — starting from the top of the list:
- Weight
- Local Preference
- Locally Originated Route
- AS Path Length
- Origin Type
- Multi-Exit Discriminator (MED)
- eBGP over iBGP
- Lowest IGP Metric to Reach the Next Hop
Now that all the major decision points are done, we have reached the final stage of the BGP path selection process — the tie-breaker rules.
These come into play when all other attributes are identical between two or more paths.
Let’s dive into these last three parameters:
1. BGP Prefers the Oldest Path
When multiple BGP paths have exactly the same attributes, the oldest path is preferred.
Why does BGP do this?
Because BGP values stability.
A route that has been in the routing table for a longer time is considered more stable and reliable than one that just appeared.
By choosing the oldest path, BGP avoids unnecessary route flapping and helps maintain a consistent routing table.
Example Scenario
Imagine two BGP paths from different neighbors are identical in all attributes.
- Path 1 has been in the BGP table for 10 minutes.
- Path 2 was learned just now.
BGP will prefer Path 1, as it’s the older, more stable route.
Additional Note
If route refresh or soft reset happens, it may reset the “age” of the route, so in practical labs, you may not always see this easily unless timers are tuned.
2. BGP Prefers the Path from the Router with the Lowest BGP Router ID
If two or more paths are still tied even after checking the age, BGP moves to the next tiebreaker —
“Prefer the path that comes from the router with the lowest BGP Router ID.”
What is BGP Router ID?
- The Router ID (RID) is a unique 32-bit identifier (looks like an IP address).
- It’s typically chosen automatically when BGP starts:
- Highest IP address on a loopback interface
- If no loopback exists, highest IP address on a physical interface
- You can also manually set it using:
| router bgp 100 bgp router-id 1.1.1.1 |
Example
Suppose two routers, R2 (Router ID 2.2.2.2) and R3 (Router ID 3.3.3.3), advertise the same prefix to R1.
If all path attributes are equal, R1 will prefer the route from R2 because 2.2.2.2 is numerically lower than 3.3.3.3.
Why this rule?
The Router ID ensures a deterministic decision – so every BGP router will make the same choice in a tie, avoiding loops or route flapping.
3. BGP Prefers the Path from the Neighbor with the Lowest IP Address
If everything – including the Router ID – is still identical (for example, two peers on the same router),
BGP finally uses the neighbor IP address as the last tiebreaker.
“Prefer the path coming from the neighbor with the lowest IP address.”
Example
If two BGP neighbors, 10.1.1.2 and 10.1.1.3, are sending the same prefix with identical attributes,
BGP will choose the path received from 10.1.1.2, because it’s numerically smaller.
Why this final rule exists
This is purely for deterministic resolution – BGP never wants to leave a situation unresolved.
So even in the rare case where everything else matches, the lowest neighbor IP ensures the router always makes a consistent choice.
Putting It All Together
When multiple routes are equally good after checking all major attributes,
BGP uses these tie-breaker steps in sequence:
| Priority | Criterion / Attribute | Description | Applies To | Purpose |
| 1 | Oldest Path | Prefers the path that was learned (installed) first among available peers, the most stable route is considered more reliable. | eBGP only | Maintain route stability by sticking to known good paths. |
| 2 | Lowest BGP Router ID | Prefers the path learned from the neighbor with the lowest BGP Router ID – a deterministic choice when paths are otherwise identical. | eBGP / iBGP | Ensure consistent route selection across routers. |
| 3 | Lowest Neighbor IP Address | Prefers the path learned from the neighbor with the lowest IP address – the final fallback when all previous criteria match. | eBGP / iBGP | Provide a consistent, deterministic final tie-breaker. |
Quick Recap of the Entire BGP Path Selection Order
Here’s the full summary from start to finish –
- Highest Weight (Cisco only)
- Highest Local Preference
- Locally Originated Route
- Shortest AS Path
- Lowest Origin Type (IGP < EGP < Incomplete)
- Lowest MED (if from same AS)
- eBGP over iBGP
- Lowest IGP Metric to Next Hop
- Oldest Path
- Lowest Router ID
- Lowest Neighbor IP
Key Takeaways
- The last three attributes are used only when everything else is equal.
- They ensure BGP always reaches a deterministic final decision.
- These rules are rarely seen in production but are important for lab understanding, troubleshooting, and CCNA/CCNP exam preparation.
- “Oldest path” helps maintain stability, while Router ID and Neighbor IP ensure consistency.
Conclusion
With this, we’ve reached the end of our BGP Best Path Selection series.
We started from understanding how BGP chooses the best route – from Weight and Local Preference – all the way to these final tie-breakers.
Now, you have a complete understanding of how BGP evaluates and decides the best path step-by-step.
This knowledge is not just essential for exams but also for real-world network design and troubleshooting.