Routing & NAT Basics

Introduction to Routers

Routers connect different networks and forward data packets between them. They operate at Layer 3 of the OSI model and determine the best path for data using routing tables and protocols.

NAT (Network Address Translation)

NAT allows multiple devices on a private network to share a single public IP address.

Types of NAT:

• Static NAT: One-to-one mapping between private and public IP.
• Dynamic NAT: Public IP is picked from a pool.
• PAT (Port Address Translation) aka NAT Overload: Many private IPs share one public IP using different ports.

Implementing NAT:

• Configured on the router/firewall.
• Useful for home users and businesses to access the internet without exposing private IPs.

Port Forwarding

SOHO DMZ:

• Places one device "outside" the firewall for unrestricted access, used for gaming, hosting.

Port Triggering:

• Opens ports dynamically when a specific outbound port is triggered.

Port Range Forwarding:

• Forwards a range of ports to a device (e.g., online games or servers).

Tour of a SOHO Router

• Basic UI with tabs like Status, Wireless, NAT, Firewall, Port Forwarding.
• Limited routing features compared to enterprise routers.

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SOHO vs Enterprise Routers

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Static Routing

• Manually configured routes.
• Used in small or stable networks.

Dynamic Routing

• Routers learn routes automatically using protocols.
• Adjusts to changes in real time.

MTU (Maximum Transmission Unit)

• The largest size a packet can be sent without fragmentation.

Distance Vector vs Link State

• Distance Vector: Shares entire routing table with neighbors (e.g., RIP).
• Link State: Shares link state with all routers (e.g., OSPF).

Routing Protocols

IGP (Interior Gateway Protocol)

• Operates within a single AS (Autonomous System).
• Examples: RIP, OSPF, EIGRP.

EGP (Exterior Gateway Protocol)

• Operates between AS.
• Main example: BGP.

OSPF (Open Shortest Path First)

• Link-state protocol.
• Uses areas and cost for routing decisions.

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BGP (Border Gateway Protocol)

• Exterior gateway protocol used on the internet.
• Path vector protocol.

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EIGRP (Enhanced Interior Gateway Routing Protocol)

• Type: Advanced Distance Vector Routing Protocol, Cisco proprietary (not open standard).
• Purpose: Used within an Autonomous System (Interior Gateway Protocol - IGP).
• Key Features:
  • Combines features of both distance vector and link-state protocols.
  • Uses DUAL (Diffusing Update Algorithm) for loop-free and fast convergence.
  • Supports classless routing (CIDR), VLSM (Variable Length Subnet Mask).
  • Sends partial updates only when topology changes occur, reducing bandwidth use.
  • Supports multiple network layer protocols (not just IPv4).
  • Uses metric based on bandwidth, delay, load, reliability, and MTU (default uses bandwidth and delay).
  • Supports unequal cost load balancing (can use multiple routes with different metrics).
• How it works:
  • Neighbors establish adjacencies via Hello packets.
  • DUAL algorithm calculates shortest path and backup routes.
  • Rapid convergence when network changes happen.
• Typical Use Case: Enterprise networks using Cisco devices where faster convergence is needed without the overhead of full link-state protocols like OSPF.

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FHRP (First Hop Redundancy Protocol)

• Purpose: Provides high availability and redundancy for the default gateway in IP networks.
• Why Needed: If a default gateway (router) fails, hosts can lose network connectivity; FHRP protocols allow failover to a backup gateway.
• Common Protocols:
  • HSRP (Hot Standby Router Protocol)
    • Cisco proprietary.
    • Provides a virtual IP address that is shared among a group of routers.
    • One router is active and forwards traffic; another is standby and takes over if active fails.
  • VRRP (Virtual Router Redundancy Protocol)
    • Open standard alternative to HSRP.
    • Similar function: one master router forwards traffic, others are backups.
  • GLBP (Gateway Load Balancing Protocol)
    • Cisco proprietary.
    • Provides load balancing across multiple routers, not just failover.
• How it Works:
  • Routers share a virtual IP and MAC address.
  • Hosts configure the virtual IP as their default gateway.
  • Routers elect an active/master router.
  • Backup routers monitor the active router; if it fails, backup takes over.
• Benefits:
  • Improves network uptime and reliability.
  • Prevents single point of failure at the default gateway.
  • Supports seamless failover without reconfiguring hosts.
• Typical Use Case: Networks requiring high availability of the gateway, such as enterprise LANs, data centers, or critical business environments.

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Summary for Exam

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