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Introduction to Dynamic Routing Protocols

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Introduction to Dynamic Routing Protocols

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents

Introduction to Dynamic Routing Protocols

Routing Protocol Overview: Primary goal is to fill the routing table with the most current, best routes it can find.


  • Routing Protocol: fills the routing table with routing information.
  • Routed Protocol: protocol with Layer 3 characteristics; define logical addressing and routing (Examples are IPX and IP)
  • Routing Type: type of routing protocol (link state or distance vector)

Exterior Routing Protocols: best used to distribute routes between companies and organizations. Example is BGP, or Boarder Gateway Protocol

Interior Routing Protocols: designed to distribute routes inside a single organization

Comparison Points between Interior Routing Protocols

Point of comparison Explanation
Type of Routing Protocol Distance vector, link state, or hybrid.
Full/partial updates Full routing updates: send entire routing tables regularly.

Partial routing updates: only information about the changed routes (requires less overhead).
Convergence Time required for routers to react to changes. This involves removing bad routes and adding new ones.
Metric Numeric value that describes how good a particular route is.
Support for VLSM Either supports Variable-length subnet masking, or doesn't support it.
Classless or classful Classless Routing protocols: transmit subnet mask along with each route in routing protocol.

Classful Routing protocols: do not transmit subnet mask information. Doesn't support VLSM.

Routing with Border Gateway Protocol

ISPs use BGO to exchange routing information between ISPs and their customers. Routing tables often exceed 100,000 routes.

  • Advertises only routing information to specifically defined peers using TCP
  • Autonomous System (AS): group of devices under the control of a single organization
  • AS Number (ASN): assigned to each AS, uniquely identifying each AS on the internet
  • Included in routing tables to prevent loops
  • If Router receive route with its own ASN, ignores route to prevent loops
  • BGP doesn't use metric, allows for robust set of policies with which routes are chosen

Distance Vector Protocols: RIP and IGRP

Advertise routing information (routing updates) out the interfaces of a router. Each entry consists of subnet and metric (identify how good the route is from that router's perspective).

  • Updates is a new route is found
  • Updates if old route with lower metric

Receiving router also adds what interface sent the update, the IP of that interface, adds the sender as the next-hop router, and the subnet mask if it is given (classless) or not (classful).

*Note: Failure to receive updates in a timely manner leads to removal of the previous routes.

Routing Information Protocol Version 1

Uses hop count as a metric: how many devices/interfaces are between itself and the desired subnet.

  • Based on distance vector logic
  • Uses hop count for the metric
  • Sends full updates every 30 seconds
  • Converges slowly (3-5 minutes)
  • Classful routing protocol

Routing Information Protocol Version 2

Defined in RFC 2453, improvements include support for subnet mask (classless routing)

Feature Description
Transmits subnet mask with route Supports VLSM, classless protocol
Provides authentication Clear text and MD5 encryption authenticates source of routing update
Includes next-hop router IP address in update Router advertises a route, but can direct any listeners to different router on the same subnet
Uses external route tags Ability to distribute routes learned externally, and redistributes using RIP.
Multicast routing updates Rather than broadcast, reserved multicast address of is used. Reduces amount of processing required on non-Rip speaking hosts on the same subnet.

Interior Gateway Routing Protocol (IGRP)

Cisco proprietary IP routing protocol, advertises up to five metrics for each route. Most important metrics are:

1. Bandwidth: describes the constrained link speed (weakest link in the route)

2. Delay: cumulative number (route over 10 Fast Ethernets includes 100 Mbps * 10)

"Calculates metric based on mathematical formula"

IGRP chooses what route to take depending on what the highest total bandwidth/least delay is, RIP chooses based on hop count.

- >Hop may be less, but could be 56kbps, whereas an additional hop is taken, but uses T1 speeds.

Feature RIP-1 RIP-2 IGRP
Update timer for full updates 30 seconds 30 seconds 90 seconds
Metric Hop count Hope count Bandwidth and delay (also can include reliability, load, and MTU)
Supports VLSM No Yes No
Infinite metric value 16 16 4,294,967,295
Convergence Slow Slow Slow

Link State Protocols: OSPF and Integrated IS-IS

Advertise only when updates occur, advertise large topological information about the network, discover neighbors before advertising routing information.

  • Uses Dijkstra Shortest Path First (SPF) algorithm to determine best routes
  • Prevent loops because each router essentially has map of entire network
  • Fast convergence – often less than 10 seconds

Open Shortest Path First (OSPF): Most popular link state routing protocol, widely deployed, router does not send information until it discovers other OSPF-speaking devices.

1. Each router discovers its neighbors on each interface. List is stored in a neighbor table.

2. Each uses a reliable protocol to exchange topology information with each neighbor.

3. Each router places learned topology information into topology table.

4. Each router runs the SPF algorithm on its topology database to determine best routes.

5. Each router places the best routes to each subnet in its IP table.

- > Link state protocols require more processing power/memory than distance vector.

- > Uses cost as a metric (cumulative bandwidth for each route)

Integrated IS-IS: OSi defined not only network layer protocol Connectionless Network Protocol (CLNP), but also routing protocol Intermediate System to Intermediate System (IS-IS). Advertise routes between intermediate systems (routers).

- > Integrated IS-IS simply allows IP in addition to the previous CLNP.


Feature OSPF Integrated IS-IS
Period of individual routing update 30 minutes 15 minutes
Metric Cost (cumulative bandwidth) Metric
Support VLSM? Yes Yes
Convergence Fast Fast

Balanced Hybrid Protocols: Enhanced IGRP (EIGRP)

Doesn't use distance vector or link state, categorized as balanced hybrid protocol. EIGRP is Cisco proprietary.

  • Diffusing Update Algorithm (DUAL) used
  • Doesn't transmit as much topology information as link state, but transmits more than distance vector
  • Computations require much less processing power than link state
  • Calculate best routes, as well as alternative routes if main route fails
  • Feasible Successor Route: alternative route that is calculated
  • Guaranteed to be loop-free
  • Allows for very small convergence time (fastest)
  • Uses same metric as IGRP, but scales by multiplying by 256

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