IP Routing Fundamentals
This domain tests the forwarding logic behind Nokia IP networks: IPv4/IPv6 addressing, longest-prefix lookup, route preference, ARP/ND, static and dynamic routing, and the separation between control plane and data plane. Questions usually ask you to predict the chosen route or next hop rather than merely define a protocol.
Core rules
- Longest prefix match is evaluated before route preference. A more specific route wins even if a less specific route has a lower preference.
- Nokia SR OS default preferences are direct 0, static 5, OSPF 10, IS-IS 15, and BGP 170. Lower preference is preferred.
- A route is usable only if its next hop resolves. Connected routes populate ARP for IPv4 and neighbor discovery for IPv6.
- Each router decrements TTL; fragmentation and MTU handling affect whether large packets arrive.
Worked example
A router has 10.0.0.0/8 via 192.0.2.1 as a static route and 10.1.0.0/16 via 192.0.2.2 learned by OSPF. A packet to 10.1.2.3 matches both. The /16 is longer, so the OSPF next hop 192.0.2.2 wins even though static has preference 5 versus OSPF 10. If both routes were /16, the static route would win. If 192.0.2.2 were unresolved, the OSPF route would not be installed as active.
Traps that cost marks
- Assuming preference beats prefix length.
- Miscounting subnet boundaries, especially
/31point-to-point and/32host routes. - Forgetting that IPv6 uses ND, not ARP.
- Ignoring MTU or reverse-path forwarding checks when a route looks correct.
Key points
- Longest prefix first, then preference.
- Know SR OS default preferences.
- Next-hop resolution is required for route installation.
- Direct routes are preferred over all dynamic routes.
- Check MTU and RPF, not just the routing table.