CCNA 200-301 IP routing explained
IP Routing is 20% of the v2.0 CCNA. It covers reading the routing table to find the next hop, troubleshooting IPv4 and IPv6 static routes, configuring single-area OSPFv2 and OSPFv3, and interpreting the state of first hop redundancy protocols. It rewards a precise mental model: how a router chooses among routes, and what has to match before OSPF routers become neighbors.
How a router picks a route
Three rules, applied in order:
- Longest prefix match. The most specific route containing the destination wins, regardless of how it was learned.
- Administrative distance (AD) decides between routes to the same prefix from different sources. Lower is more trusted.
- Metric decides between routes to the same prefix from the same protocol.
| Source | Default AD |
|---|---|
| Connected | 0 |
| Static | 1 |
| EBGP | 20 |
| EIGRP | 90 |
| OSPF | 110 |
| RIP | 120 |
A route to 10.1.1.64/26 beats a static route to 10.1.1.0/24 for destination 10.1.1.77, even though the static route has the lower AD, because the /26 is more specific. The routing table entry shows code, prefix/mask, [AD/metric], next hop and interface.
Static routing
| Type | Example purpose |
|---|---|
| Default | ip route 0.0.0.0 0.0.0.0 next-hop, the route of last resort |
| Network | A specific remote subnet |
| Host | A /32 for one device |
| Floating static | A backup route with an AD higher than the primary |
A floating static route stays out of the table until the preferred route disappears. If the primary is OSPF at 110, give the backup an AD above 110, for example ip route 0.0.0.0 0.0.0.0 198.51.100.1 130. For IPv6, ipv6 unicast-routing must be enabled, and a link-local next hop must be paired with an exit interface.
OSPF
Single-area OSPF uses area 0. Two routers become neighbors only if these line up:
- Area ID and subnet/mask on the connecting link
- Hello and dead timers
- Authentication, which the v2.0 objectives exclude
- MTU and network type for full adjacency
- Unique router IDs
On a point-to-point link no election occurs. On a broadcast network such as Ethernet, routers elect a DR and BDR. Highest OSPF interface priority wins, with the highest router ID breaking ties; priority 0 means never. Election is non-preemptive, so a later arrival with a better priority does not displace an existing DR.
The router ID is the manually configured router-id, otherwise the highest IP address on an up loopback, otherwise the highest IP on an up physical interface. Useful commands: show ip ospf neighbor, show ip ospf interface and show ip route ospf.
First hop redundancy
HSRP (Cisco) and VRRP (standard) present a virtual gateway address shared by two or more routers. The router with the highest priority is active in HSRP, or master in VRRP. A recovering router with a higher priority takes over again only if preemption is configured. Read show standby brief to see state, priority and the active peer.
Example questions
Question 1. A router has these routes: 10.1.0.0/16 via OSPF, 10.1.1.64/26 via EIGRP, 10.1.1.0/24 via a static route, and 0.0.0.0/0 via a static route. Which route is used for a packet to 10.1.1.77?
- A. The 10.1.1.64/26 EIGRP route
- B. The 10.1.1.0/24 static route
- C. The 10.1.0.0/16 OSPF route
- D. The 0.0.0.0/0 static route
Show answer
Answer: A
10.1.1.77 falls in 10.1.1.64/26, which covers .64 to .127. The router uses the longest matching prefix before it considers administrative distance, so the /26 EIGRP route wins even though the static /24 has a lower AD. The /16 and the default route are less specific.
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Question 2. A router learns a default route through OSPF and you want a static default route to a second ISP to act only as a backup. Which administrative distance should the static route use?
- A. 1
- B. 90
- C. 110
- D. 130
Show answer
Answer: D
A floating static route needs an AD higher than the primary so that it stays out of the routing table until the OSPF route disappears. OSPF has AD 110, so 130 works. The default static AD of 1 or an AD of 90 would make the static route preferred, and 110 would tie with OSPF.
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Question 3. R1 (OSPF priority 100, router ID 1.1.1.1), R2 (priority 1, router ID 9.9.9.9) and R3 (priority 1, router ID 5.5.5.5) start at the same time on an Ethernet segment. Which routers become DR and BDR?
- A. R1 is DR and R3 is BDR
- B. R1 is DR and R2 is BDR
- C. R2 is DR and R1 is BDR
- D. R3 is DR and R2 is BDR
Show answer
Answer: B
The router with the highest priority becomes DR, so R1 is DR. R2 and R3 tie on priority, and the highest router ID breaks the tie, making R2 (9.9.9.9) the BDR. R3 stays a DROther. The lowest router ID is not preferred.
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Revision tips
Make a habit of saying the three-step route selection rule aloud before answering any routing table question. For OSPF, memorise the neighbor checklist and test it against each scenario one item at a time. For FHRP, if a router that should be active is not, suspect preemption first.
Next, read network services and security.