Non-Optimal Intra-Area routing
Last updated
Last updated
Load isis.non-optimal.intra-area.cfg
R1 is now connected to both R3 and R4.
Shutdown the R1-R3 link and do a traceroute to 1.1.1.1 from R3. Traffic is from R3 to R1 is flowing to R2, R4 and then R1. Explain the issue and find a way to fix it so that traffic takes the path R3-R4-R1.
The reason for this non-optimal routing is because the R3-R4 link is level-2-only. Routers prefer intra-area routes over inter-area routes. This means an L1 route is preferred over an L2 route. Even though R3 and R4 are in the same area, they only have an L2 adjacency. The route to 1.1.1.1 via R4 is an L2 route, which although a shorter metric, loses against the higher metric L1 route via R2.
When we allow the R3-R4 adjacency to be both L1/L2 (which it would be by default, when you remove the level-2-only circuit-type), R3 now has a L1 path via R4.
In OSPF, this problem manifests itself as whether to put a link in either area 0 or area 1 (for example). There is a trade off between using either one. Either you will use the link for intra-area paths to area 0 or area 1 but never both. However, with ISIS, we can form both an L1 and L2 adjacency which results in no trade off. In OSPF, we now have the recent multi-area interface configuration which allows an interface to participate in two areas at once (ip ospf multi-area).
