Cost and ECMP

Understand

Automatic interface cost is based on reference bandwidth divided by interface bandwidth, with a minimum cost of 1 on common router implementations. A legacy 100-Mbps reference collapses 100-Mbps, 1-Gbps and 10-Gbps links to cost 1. Configure a consistent reference across the domain when using automatic costs; it does not negotiate through Hellos.

An explicit ip ospf cost overrides automatic calculation. Our baselines use explicit costs, so changing only auto-cost reference-bandwidth will not change them. Equal-cost multipath installs eligible equal routes up to maximum-paths and platform limits. Forwarding often hashes flows; two next hops do not imply a 50/50 byte split or per-packet alternation.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0, R3: area 0.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.9 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R3
router ospf 1
 router-id 10.255.0.3
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.10 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

Apply

Make the direct R1→R3 transit cost 20, equal to the two-link path via R2. Allow two next hops.

R1

configure terminal
router ospf 1
 maximum-paths 2
 exit
interface GigabitEthernet2
 ip ospf cost 20
end

Verify and interpret

R1# show ip route 10.255.0.3
Routing entry for 10.255.0.3/32
  Known via "ospf 1", distance 110, metric 21, type intra area
  * 192.0.2.2, via GigabitEthernet1
  * 192.0.2.6, via GigabitEthernet2

Both paths total 21 including R3’s loopback. A single ping flow may use just one next hop.

For a separate automatic-cost experiment, restore the fresh triangle, remove explicit transit costs on every router, and apply the following reference on every router. Verify each interface’s configured bandwidth and resulting OSPF cost; changing bandwidth metadata does not change physical line speed.

Every router · automatic-cost experiment

configure terminal
router ospf 1
 auto-cost reference-bandwidth 10000
 exit
interface GigabitEthernet1
 no ip ospf cost
 exit
interface GigabitEthernet2
 no ip ospf cost
end

For a 1-Gbps interface with 10000-Mbps reference, expected cost is 10; for 10-Gbps it is 1. Restore the complete explicit-cost baseline before another lesson.

Technical references

OSPFv2 standard · Router configuration guide