Section 9: Routing

Router

• Forwards traffic between subnets, between an internal and external network, or between two external networks

• Each subnet or external network is going to be its own broadcast domain

• Multilayer switches (Layer 3 Switches) also perform routing functions

Switch Terminology

• Switch - Layer 2 Switch

• Multilayer switch (router) - Layer 3 device

Basic Router Functionality

• Routers forward traffic between networks based on IP addresses

• To connect two networks, routers use WAN connections like fiber, serial, satellite, or VPN links

• When a packet needs to travel between networks, it is forwarded to the router (default gateway)

• Routers use IP addresses (Layer 3) to route packets between networks

  • MAC addresses are used internally, but IP addresses are used at Layer 3 for communication between routers

• Routers repackage data frames (Layer 2) as packets (Layer 3) for transmission over the WAN

• Routers strip off the IP header and converts packets back to data frames for delivery to the destination device on the local network

• Communication between devices on different networks involves routers forwarding packets based on IP addresses and switches delivering data frames based on MAC addresses

Routing becomes more complex when packets are sent over the internet, which is the world’s largest WAN

Routing Table

• Helps determine which route entry is the best fit for the network

• Used to decide where packets need to go inside and outside of networks

• Routing decisions are based on Layer 3 information and map to Layer 2

  • ARP Cache

    • Used by routers to map IP addresses to MAC addresses within a local area network

• Entries in a routing table contain a prefix

  • Longer prefixes indicate more specific networks

    • Means fewer available IP addresses in that range

Routing Information Sources (3)

• Directly Connected

• Static

• Dynamic

Directly-Connected route

Learned by a physical connection between routers

Static Routes

• Configured manually by an administrator

• 0.0.0.0/0

  • Default static route to handle unknown destinations

  • “If you don’t know where to go just go here”

Dynamic Route

• Learned through through dynamic routing protocols

• Learned by exchanging information automatically between routers based on the protocols

• Dynamic Routing Protocols

  • Negotiate based on factors like number of hops and link bandwidth

Methods for preventing routing loops(2)

• Split Horizon

• Poison Reverse

Split Horizon

Prevents a route learned on one interface being advertised back out of the same interface

Poison Reverse

Advertises a route back out the same interface but with a high cost to prevent its use

Dynamic Routing Protocols (2)

• Internal

  • Interior Gateway Protocol (IGP)

    • Operates with an autonomous system

• External

  • Exterior Gateway Protocol (EGP)

    • Operates between autonomous systems on exterior networks

Interior Gateway Protocol (IGP)

• Operates with an autonomous system

Exterior Gateway Protocol (EGP)

• Operates between autonomous systems on exterior networks

Routing Protocol Characteristics

• Router Advertisement Method

  • Distance Vector

    • Sends full routing table to directly connected routers at regular intervals

    • Convergence Time

      • Time for all routers to update routing tables in response to topology changes

      • Slow

    • Hold-down Timer

      • Prevents updates for a specific period of time and speeds up convergence

    • Hop Count

      • Number of routers from the source router through which data must pass to reach the destination network

      • Used as a metric for routing decisions

  • Link State

    • Requires all routers to know about the paths that all other routers can reach in the network

    • Faster convergence time compared to distance vector

    • Considers cost, including link speed, as metric for routing decisions

  • Hybrid

    • Combines aspects of both distance vector and link state protocols

Distance Vector

• Sends full routing table to directly connected routers at regular intervals

• Convergence Time

  • Time for all routers to update routing tables in response to topology changes

  • Slow

• Hold-down Timer

  • Prevents updates for a specific period of time and speeds up convergence

• Hop Count

  • Number of routers from the source router through which data must pass to reach the destination network

  • Used as a metric for routing decisions

Link State

• Requires all routers to know about the paths that all other routers can reach in the network

• Faster convergence time compared to distance vector

• Considers cost, including link speed, as metric for routing decisions

Link State/ Distance Vector Hybrid

• Combines aspects of both distance vector and link state protocols

Routing Protocols (5)

• RIP

• OSPF

• IS-IS

• EIGRP

• BGP

Routing Information Protocol (RIP)

• An interior gateway protocol that is used internal to the networks

• Oldest dynamic routing protocol

• Uses distance vector and hop count

  • 15 maximum hops

• Updates every 30 seconds

• Easy to configure

• Runs over UDP

Open Shortest Path First (OSPF)

• Interior gateway protocol

• Uses link state and cost for routing decisions

  • Cost is based on link speed

• Faster convergence than RIP

Intermediate System to Intermediate System (IS-IS)

• An interior gateway protocol similar to OSPF

• Uses cost based on link speed for routing decisions

• Functions like OSPF but not as popular

Enhanced Interior Gateway Routing Protocol (EIFRP)

• Advanced distance vector protocol

• Hybrid of distance vector and link state

• Uses bandwidth, delay, and cost for routing decisions

• A Cisco-developed upgrade to OSPF, popular in Cisco-only networks

Border Gateway Protocol (BGP)

• An external gateway protocol

• Uses path vector and autonomous system hops for routing decisions

• Backbone protocol of the internet

• Slow convergence time

Route Selection

Determines which path router could take

Believability of a Route

• Administrative Distance (AD)

  • An index of believability used by routers

  • Lower value = more believable

  • Administrative Distance of Routing Protocols

    • Directly connected - 0 (most believable)

    • Static - 1

    • EIGRP - 90

    • OSPF - 110

    • RIP - 120

    • External EIGRP - 170

    • Unknown/Unreachable - 255 (unreachable)

Metrics for Route Selection

• Factors

  • Hop count

    • Least number of hops gives less distance to cover

  • Believability

    • Lowest number is the most believable

  • Reliability

  • Bandwidth

    • Lowest cost of bandwidth means the highest amount of bandwidth to use

  • Delay

    • Want the least amount of delay

  • Costs

  • Other metrics

• Each protocol uses a different metric based on its programming to determine best routes

• Lower numbers are better

Address Translation

• Allows private IP addresses to be translated into public IP addresses for routing over public networks like the internet

  • IPv4 faced address exhaustion, prompting the development of address translation

Address Translation Types (2)

• NAT

• PAT

Network Address Translation (NAT)

• Conserves limited IPv4 addresses by translating private IPs into public IPs

• Dynamic NAT

  • Automatically assigns IP addresses from a pool of IPs for one-to-one translation

• Static NAT

  • Manually assigns private IPs to public IPs for one-to-one translation

  • Used as a security feature

Port Address Translation (PAT)

• Allows multiple devices to share a single public IP address by using different port numbers to differentiate traffic

• Many-to-one translation

NAT IP Address Terminology

• Inside Local

  • Private IP address referencing an inside device

• Inside Global

  • Public IP address referencing an inside device

• Outside Local

  • Private IP address referencing an outside device

• Outside Global

  • Public IP address referencing an outside device

Comparison of NAT and PAT

• NAT translates private IPs to public IPs for individual devices

• PAT uses port numbers to differentiate between multiple devices sharing a single public IP

Routing Redundancy Protocol

• A network protocol that prevents disruptions in communication by automatically rerouting data traffic in case of path or device failure

First Hop Redundancy Protocol (FHRP)

• A group of protocols ensuring network reliability by providing automatic failover to a backup router if the primary router fails

  Components

  • Vir

FHRP Benefits

• Benefits

  • Reliability

    • Ensures communications remain up if a router fails

  • Load Balancing

    • Distributes network traffic across multiple routers to prevent overload

  • Seamless Transitions

    • Quick and seamless transitions from sending data to one router to another

FHRP Components

• Components

  • Virtual IP

    • Represents one or more devices

  • Subinterface

    • Allows a single physical interface to be divided into multiple logical interfaces, improving network management and security

FHRP Protocols (3)

• HSRP

• VRRP

• GLBP

Hot Standby Router Protocol (HSRP)

• Establishes a fault-tolerant default gateway for devices on a local network segment

• Enables two or more routers to work together

  • Active router

    • Handles all the networks routing responsibilities

  • Standby router

    • Designed to wait to take over when the active router fails

• Preempting

  • Allows a higher priority router to take over as the active router

Virtual Router Redundancy Protocol (VRRP)

• Functions similar to HSRP but is an open standard

  • Not tied to a specific vendor

• Enables multiple routers to act as a single virtual router

  • Active router and backup routers

• Provides a simple and automatic election scheme

Gateway Load Balancing Protocol (GLBP)

• Adds load balancing capabilities, allowing multiple routers to simultaneously forward packets to distribute traffic load

• Assigns different virtual MAC addresses to each group member for load balancing

• Automatically redirects traffic to other routers in the group if any fail

Importance of Routing Redundancy Protocols

Critical for network designs to ensure continuous network availability, reliability and efficiency as networks are relied upon for global communication and entertainment

Multicast Routing

• Sending traffic to a class D IP address (multicast group) to deliver messages to multiple recipients efficiently

• Objective

  • Send traffic out once and have all interested devices receive it, while others ignore it

Methods of Multicast Routing (2)

• IGMP

• PIM

Internet Group Management Protocol (IGMP)

• Used by clients and routers to let the routers know which interfaces have multicast receivers

• Allows clients to join multicast groups and receive messages

• More about clients and servers together

• IGMPv1

  • Caused unnecessary traffic due to periodic group queries

• IGMPv2

  • Improved by allowing clients to send leave messages

• IGMPv3

  • Added support for source specific multicast

Protocol Independent Multicast (PIM)

• Enables multicast traffic routing between multicast-enabled routers

• Forms multicast distribution trees

• Focuses more on routing

• PIM Dense Mode (PIM-DM)

  • Uses flood and prune behavior, causing network performance issues

    • Floods traffic initially, then prunes non-optimal routes

    • High network performance impact due to periodic flooding

  • Not commonly used in modern networks

• PIM Sparse Mode (PIM-SM)

  • Uses shared distribution tree initially, then switches to optimal tree (Shortest Path Tree/SPT)

  • Lower impact on network performance

  • Preferred in modern networks for efficiency

Generic Routing Encapsulation (GRE)

• A tunneling protocol used to encapsulate a wide variety of network layer protocols inside a virtual point-to-point link over an Internet Protocol network

• GRE tunnels operate at Layer 3(Network Layer) of the OSI model

• Serves as a universal translator

  • Allows different protocols to communicate and traverse over a shared network infrastructure

  • Useful for connecting similar network topologies over a different intermediate network

GRE Use Cases

• Connecting branch offices securely and efficiently over the internet without expensive dedicated leased lines

• Encapsulating protocols for tunneling without the additional overhead of encryption techniques inside a VPN, making it ideal for connecting heterogenous networks

GRE Comparison with VPN

• GRE

  • Favored for:

    • Simplicity

    • Efficiency

    • Lightweight compared to a full site-to-site VPN

    • Versatility

      • Allow different network protocols to co-exist and collaborate in heterogeneous network environments

  • Preferred when the main objective is to encapsulate protocols for tunneling without additional overhead created by encryption techniques

  • GRE tunnels are set up and configured on network routers

• VPN

  • Could provide more robust security features, but with additional overhead due to encryption