Open Shortest Path First (OSPF)

[Dayias]

Hi i have this network which uses OSPF

Why packets take different routes sometimes?

[Joseph W. Doherty]

Without knowing OSPF link costs, and/or stability of links, cannot say.

One possible reason, OSPF supports ECMP (equal cost multiple path), so individual flows can take different paths though network, if that's the case.

[Giuseppe Larosa · Accepted solution]

Hello @Dayias ,

assuming all red serial links have the same speed , the same bandwidth and the same OSPF cost you can easily see  that there are some Equal cost multi paths in your topology and OSPF supports this. Actually the chosen path would be chosen by CEF based on a calculation involving source IP address and destination IP address.

We see a path via Routers RX - RW - RU - Rs

The there is a path via RX - RV - RU -RS

Another one via RX- RV- R? - RS

Finally RX-RY -R?- RS

(note : I have used R? for the device for which I cannot read the letter.)

by default OSPF supports up to 4 equal cost multi path ECMP.

Hope to help

Giuseppe