Routing Fundamentals

Router forwards traffic between subnets, between interneal network and externetal neteork, or two extenal networks.. each subnet or external netwrok is going to be it’s own broadcast domain.

Multilayer switches alwos perform routing functions. forwards and routes, but remember on the exam switch is layer 2.

switch = layer 2

multilayer switch = router.

device sends an arp request if the switch does not have the ip forward to the default gaateway the default gaatewy will send to the next router on the other network.

Routers & Multilayer Switches

Router

Layer 3 device (Network Layer).
Forwards packets based on IP addresses.
Connects different subnets or networks.
Each router interface/subnet = its own broadcast domain.
Routes between:
- Internal ↔ Internal (different VLANs/subnets)
- Internal ↔ External (LAN to WAN/Internet)
- External ↔ External (e.g., ISP backbone routers)

Default Gateway

Router interface that devices send packets to when destination is outside local subnet.
If switch doesn’t know where to send traffic, it forwards to the default gateway.

Multilayer Switch

Switch that can also perform Layer 3 routing.
Acts like a switch at Layer 2 but has routing functions (static routes, dynamic routing, VLAN routing).
Used heavily in enterprise networks (routing between VLANs without an external router).
Exam tip: If CompTIA just says “switch,” assume Layer 2 only. Only call it a router if they say “multilayer switch” or “Layer 3 switch.”

ARP & Forwarding

Device sends an ARP request to learn the MAC address of another device in the subnet.
If destination IP is outside the subnet, ARP resolves the default gateway’s MAC.
Packet goes to router (gateway). Router checks its routing table and forwards toward the destination network.

✅ Must Know for N10-009

Switch = Layer 2. Router = Layer 3. Multilayer switch = Router.
Each subnet = separate broadcast domain.
Default gateway is how hosts reach other networks.
ARP resolves IP → MAC before frames can be delivered.