Routing¶

Routing is how traffic moves between different networks (Layer 3), as opposed to switching, which moves traffic within a single network/VLAN (Layer 2). See VLANs & Inter-VLAN Routing for the SVI setup this section builds on.

The Routing Table¶

Every router (and Layer 3 switch) keeps a routing table: a list of known destination networks and how to reach them.

show ip route

Sample output:

Codes: C - connected, S - static, O - OSPF, D - EIGRP

C    10.0.10.0/24 is directly connected, Vlan10
C    10.0.20.0/24 is directly connected, Vlan20
S    0.0.0.0/0 [1/0] via 10.0.99.1

C — directly connected network (no config needed beyond an ip address on the interface)
S — static route (manually configured)
O / D — learned dynamically via OSPF / EIGRP

Longest prefix match: when multiple routes could match a destination, the router always picks the most specific (longest) matching prefix.

Static Routes¶

A static route is a manually configured path to a network. Simple, predictable, and appropriate for small networks or a single upstream path.

ip route <destination-network> <subnet-mask> <next-hop-ip>

Example — route to a remote subnet via a next-hop router:

ip route 10.0.30.0 255.255.255.0 10.0.99.1

Default Route (Gateway of Last Resort)¶

Sends any traffic that doesn't match a more specific route out to the internet/upstream router:

ip route 0.0.0.0 0.0.0.0 10.0.99.1

Verify which default route is active:

show ip route 0.0.0.0

Floating Static Route (Backup Path)¶

Add an administrative distance (AD) higher than the primary route so it's only used if the primary fails:

ip route 0.0.0.0 0.0.0.0 10.0.99.1        ! Primary, AD 1 (default)
ip route 0.0.0.0 0.0.0.0 10.0.98.1 200    ! Backup, AD 200 — only used if primary is down

Lower administrative distance = more trusted / preferred. Static routes default to AD 1; connected routes are AD 0.

Dynamic Routing¶

Dynamic routing protocols let routers automatically discover networks and adapt to topology changes (a link goes down, a new subnet is added) without manual reconfiguration. Worth it once you have more than a couple of routers or need automatic failover.

OSPF (Open Shortest Path First)¶

Industry-standard, link-state protocol. Works across multi-vendor environments (not Cisco-proprietary).

router ospf 1
 network 10.0.10.0 0.0.0.255 area 0
 network 10.0.20.0 0.0.0.255 area 0

router ospf 1 — process ID, locally significant only (doesn't need to match between routers)
network <ip> <wildcard-mask> area <area-id> — enables OSPF on any interface whose IP falls in that range, and advertises the network into the specified area
area 0 — the backbone area; simplest designs put everything in area 0

Verify:

show ip ospf neighbor     ! Confirm adjacencies have formed (state should be FULL)
show ip route ospf        ! Show routes learned via OSPF

EIGRP (Enhanced Interior Gateway Routing Protocol)¶

Cisco-proprietary (though now partially open), advanced distance-vector protocol. Fast convergence, easier to configure than OSPF for simple topologies.

router eigrp 100
 network 10.0.10.0 0.0.0.255
 network 10.0.20.0 0.0.0.255
 no auto-summary

router eigrp 100 — autonomous system number; must match on all routers in the same EIGRP domain (unlike OSPF's process ID)