RETICUX BGP Mastery — Day 020 — IGP Metric to the BGP NEXT_HOP: The Hot-Potato Decision

Learning objective

Use the IGP cost to a BGP next hop as a controlled best-path decision and troubleshoot recursive reachability.

IGP Metric to the BGP NEXT_HOP: The Hot-Potato Decision

---

1. Opening — the decision point that is easy to misread

BGP best-path selection is sequential. A router does not assign a single universal “route score” and then pick the smallest or largest number. It evaluates eligible paths through an ordered decision process. The first meaningful difference can end the comparison; later attributes may never be examined.

That matters operationally because engineers often see two routes and jump directly to AS_PATH length. By the time AS_PATH is reached, several earlier decisions may already have eliminated one candidate. Conversely, when two routes remain equal through MED, later implementation-specific or topology-dependent decisions become decisive.

This day isolates one such decision point so that the result can be proven rather than inferred from a route table alone.

2. Standards behavior versus Cisco behavior

RFC 4271 defines BGP's route selection framework but deliberately leaves implementation-specific selection details to implementations. Cisco IOS XE documents an ordered decision process containing Cisco-local and BGP attributes.

The engineering rule is therefore: use RFC text to understand protocol semantics, and Cisco documentation to verify the actual IOS XE decision order and configuration knobs. Do not copy an algorithm from a different vendor and assume Cisco behaves identically.

This distinction becomes particularly important for router ID, multipath, best-external and route-reflector features.

3. Scenario

R1 has two otherwise equivalent BGP paths to 203.0.113.0/24. One next hop is reached through a low-cost IGP path and the other through a higher-cost internal path. The lab removes earlier differences so the IGP metric to the BGP next hop becomes the deciding factor.

The lab uses documentation-safe addressing and private lab ASNs. No production prefixes, credentials or real operator identifiers are used.

4. Topology

                         AS 65100
                 +---------------------+
                 |       R1 / Core     |
                 |   BGP decision point|
                 +----------+----------+
                            | iBGP
                            |
                         +--+--+
                         | R2  |
                         +--+--+
                            |
                 +----------+----------+
                 |                     |
              eBGP                  eBGP
                 |                     |
              +--+--+               +--+--+
              | ISP-A|               | ISP-B|
              |65110 |               |65120 |
              +-----+               +-----+

Prefix under test: 203.0.113.0/24

5. Prerequisites

• IOS XE 17.18.x target image or equivalent supported IOS XE 17.x lab image.
• Reachable loopbacks/interfaces before BGP policy is tested.
• IPv4 unicast address family enabled.
• Private/documentation-safe ASNs and prefixes.
• show ip bgp, show ip bgp summary, and show ip route available.
• NTP or a stable lab clock is recommended for incident timestamps.

6. Baseline configuration

router ospf 100
 network 10.0.12.0 0.0.0.3 area 0
 network 10.0.13.0 0.0.0.3 area 0
!
router bgp 65100
 address-family ipv4 unicast
  neighbor 10.0.12.2 remote-as 65100
  neighbor 10.0.12.2 activate
  neighbor 10.0.13.2 remote-as 65100
  neighbor 10.0.13.2 activate
 exit-address-family

The configuration above is intentionally scoped to the learning objective. It should not be described as a universal production template.

7. Verification before modification

Verify the BGP paths and the recursive next-hop routes with show ip route <next-hop>. Then inspect the BGP entry. The expected evidence is a different best path without a change to LOCAL_PREF, AS_PATH, ORIGIN or MED.

Record the baseline best path before changing the single variable under test. The evidence must show both the BGP table and the installed IP route when forwarding behavior is part of the objective.

8. Controlled modification

Increase the IGP cost toward one BGP next hop while leaving BGP attributes equal. Confirm the BGP best path changes only after the IGP metric becomes the first differentiator.

Change only the variable under investigation. Do not simultaneously alter LOCAL_PREF, MED, AS_PATH, next-hop reachability and multipath settings; doing so destroys causal clarity.

9. Fault injection

Illustrative lab — not a real incident.

Illustrative lab — not a real incident. Remove the IGP route to the selected next hop. The BGP path should become unusable or cease to be eligible for installation depending on the exact topology. The symptom is a route-selection or installation change that cannot be explained by BGP attributes alone.

The purpose of the fault is to create a recognizable symptom while preserving enough evidence to identify the exact decision point.

10. Troubleshooting

Use this evidence chain:

1. Confirm the affected prefix.
2. Confirm both candidate paths are present.
3. Compare attributes in decision order.
4. Confirm the next hop is recursively reachable.
5. Identify the first attribute where the candidates differ.
6. Confirm whether the result is a best-path decision or a multipath/install decision.
7. Verify the selected route in the RIB.
8. Perform a positive forwarding test.
9. Perform a negative/containment test where safe.
10. Record the smallest proven cause.

Useful IOS XE commands

show ip bgp 203.0.113.0
show ip bgp 203.0.113.0 longer-prefixes
show ip bgp summary
show ip route 203.0.113.0
show ip route <next-hop>
show ip bgp neighbors <peer> advertised-routes
show ip bgp neighbors <peer> routes

Adapt the command set to the actual feature under test. Do not claim output was observed unless the exact lab was executed.

11. Root cause

The proven cause is recursive next-hop reachability and its IGP cost. BGP may retain path information while the RIB/FIB decision depends on the ability to resolve the BGP next hop through the underlying routing topology.

12. Post-fix verification

Re-run the same evidence set used before the change. The comparison should demonstrate the intended decision change without unrelated routing changes.

13. Rollback

Restore the original IGP metric or route. Verify the next hop is reachable, then confirm the expected BGP path and RIB entry return.

14. Production lessons

This is the operational meaning of hot-potato routing: once earlier policy attributes tie, the router can prefer the exit whose BGP next hop is closer according to the internal routing protocol. Troubleshoot BGP and IGP together.

15. Knowledge check

1. Question: What is the first decision point that can distinguish the two candidate paths in this lab?

• Answer: Inspect the ordered attributes and identify the first actual difference; do not assume AS_PATH is always the first useful discriminator.

1. Question: Why must a best-path change be verified in both the BGP table and the IP routing table?

• Answer: A BGP path can be selected while recursive next-hop or installation conditions prevent the expected forwarding entry.

1. Question: What configuration change would make the lab result misleading?

• Answer: Changing multiple selection inputs simultaneously, because the engineer can no longer prove which factor caused the outcome.

Engineering notes

Use a packet path or traceroute only after proving the control-plane decision. A changed IGP metric is not the same thing as a changed BGP policy.

16. Sources

• RFC 4271 — IETF/RFC Editor; standards baseline for BGP behavior relevant to this post.
• Cisco IOS XE 17.18.x BGP configuration guidance — Cisco official configuration/implementation documentation for IOS XE 17.x/17.18.x.
• RFC 4456 where route-reflector behavior or Cluster List is discussed.
• RFC 7911 where ADD-PATH behavior is discussed.

Access date: 12 August 2026