Customer reported they are observing traffic on the ae9 link after adjusting the low cost on another aggregated link compared to the cost on ae9
Customer reported observing traffic on the AE9 link, which was configured metric of 800. They mentioned that the metric on the AE4 link is set to 100, and they expect traffic to traverse via the AE4 link when they ping to router D from router A
Checked the metric on all the links connected to the router. Metric configured on ae50 was 800 and on ae101 metric configured was 4
We could observe ospf neighborship was up on all the connected links
labroot@A> show ospf neighbor
Address Interface State ID Pri Dead
209.141.5.53 ae4.0 Full 10.11.0.215 128 37
209.141.5.57 ae9.0 Full 10.11.0.50 128 38
labroot@A> show ospf3 neighbor
ID Interface State Pri Dead
10.11.0.215 ae4.0 Full 128 32
Neighbor-address fe80::2ab8:29ff:fe6c:a458
10.11.0.50 ae9.0 Full 128 33
Neighbor-address fe80::e623:3cff:fe1a:a68
When we ping the IP the router D from A we could observe the traffic is taking path via ae9 according to customer it should have taken path via ae4
labroot@A> show route 103.54.87.26
inet.0: 21 destinations, 21 routes (21 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
103.54.87.26/32 *[OSPF/10] 00:02:31, metric 804
> to 209.141.5.57 via ae9.0
This is the expected behavior in OSPF since the metric is cumulative, meaning we need to add the metrics of all the routers along the path to the target IP
To reach the loopback IP of router D we have 2 paths as below:
Path 1
A--ae9(800)-->C--- ae101(4)-->D -------> It is preferring this path since total metric is 800+4 =804
Path 2
A--ae4(100)-- -->B---ae50--(800)----D-----> Hence it is not taking this path since total metric is 100+800=900
This is expected behavior even though cost of ae9 is more than the cost of ae4 but total cost to reach the targeted IP of router D via ae9 is less compared to the traffic via ae4. Hence traffic is taking the path via ae9