3.2 Layer 3

What this section tests

Section 3.2 is about path selection. A router receives a packet, checks a table or a policy, and chooses a next hop. The exam wants you to explain why the router chose that path.

For each topic, name the control point and the proof command:

• Routing concepts: the control point is the protocol logic. Proof: the metric and the route in show ip route.
• OSPF: the control point is the link-state database and the area design. Proof: show ip ospf neighbor.
• eBGP: the control point is the TCP session and the path attributes. Proof: show bgp ipv4 unicast summary.
• Policy-Based Routing: the control point is the route map on the inbound interface. Proof: show route-map.

Study order

1. EIGRP vs OSPF routing concepts
2. OSPFv2 and OSPFv3 simple environments
3. eBGP between directly connected neighbors
4. Policy-based routing

Pass check

You are ready for section 3.2 when you can do these things:

• Compare EIGRP and OSPF metric and load-balancing behavior without notes.
• Build a multi-area OSPF network and prove neighbors and summaries.
• Bring up an eBGP session and explain each neighbor state.
• Send one subnet over a second path with a route map and prove it with counters.