What Is a Router? Definition and Differences from a Switch

A router is a device that forwards IP packets between different networks by consulting its routing table. It operates at Layer 3 (L3) of the OSI model, unlike a switch that remains at L2 within the same network. Routers are essential for the Internet (BGP), WAN connectivity, and inter-VLAN routing.

How It Works

• Receives an IP packet
• Looks up the destination in the routing table
• Decrements the TTL
• Re-encapsulates the packet in Ethernet with the next-hop MAC address
• Forwards it through the appropriate port

Routing Protocols

IGP (Interior Gateway)

• Static: manually configured routes
• RIP: legacy distance-vector protocol
• OSPF: standard link-state protocol
• EIGRP: Cisco proprietary protocol
• IS-IS: used by ISPs and telecommunications operators

EGP (Exterior Gateway)

• BGP: the Internet's inter-AS routing protocol
• Full Internet table: 950,000+ routes in 2026

Router Types

• ISP core: Juniper MX, Cisco ASR, Nokia 7750 — 100-400G
• Metro/backbone: ASR 9000, Nokia 7250
• PE (Provider Edge): customer-facing connectivity
• Branch office: Cisco ISR, FortiGate
• SOHO/SMB: ISP router, Ubiquiti EdgeRouter

Additional Functions

• NAT/PAT: address translation
• IPsec VPN
• Stateful firewall
• QoS
• Multicast routing (PIM)
• MPLS labels
• VRF (multi-tenant)

Differences from an L3 Switch

• L3 switch: optimized for campus networks with fast, ASIC-based hardware routing
• Router: optimized for WAN environments with extensive capabilities (full BGP, MPLS, VPN)
• Performance: L3 switch > router for raw Mpps throughput
• Features: router > L3 switch (BGP communities, RP, Segment Routing)

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