Point-to-Point Links with /30 and /31

Bundle lab · $29.99

Build confidence addressing IPv4 point-to-point WAN links on Cisco IOS using /30 and /31 masks. Two routers are connected by two parallel links: one classic /30 and one RFC 3021 /31. No routing protocols or static routes are configured — the goal is deterministic, correct interface addressing and verification of directly connected reachability. A small LAN off R1 with two hosts allows additional verification (host-to-gateway only).

Learning objectives

• Assign correct IPv4 /30 and /31 addresses to router-to-router point-to-point links
• Explain address utilization differences between /30 and /31 on P2P media
• Verify link state and directly connected reachability without any routing configured
• Use show commands to confirm addresses and masks on the intended interfaces

Troubleshooting focus

• Mask mismatch on a point-to-point link (/30 on one side, /31 on the other) prevents ARP/neighbor resolution and pings
• Using the network or broadcast address on a /30 (198.51.100.0 or .3) results in an interface that appears up but cannot pass IP traffic
• Forgetting no shutdown leaves the link down/down even with correct IP addressing
• Assigning both ends the same IP or swapping intended higher/lower address across the pair breaks reachability
• Applying the address on the wrong physical interface (Gi0/2 vs Gi0/0 or Gi0/1) causes unexpected failures