Table of Contents Link to heading

• Dynamic Routing Protocols
• Static versus Dynamic Routing
• Autonomous System (AS) or Routing Domain
• Components
• Operation
• Routing Convergence
• Classifying Routing Protocols
  • Interior versus Exterior Gateway Protocols (IGPs/EGPs)
  • Distance-Vector Routing Protocols (DVRPs)
  • Link-State Routing Protocols (LSRPs)
  • Hybrid Routing Protocols (HRPs)
  • Classful versus Classless Routing Protocols
• Multiple Route Sources
  • Administrative Distance (AD)
  • Metric
• Routing Information Protocol (RIP)
• Enhanced Interior Gateway Routing Protocol (EIGRP)
• Comparing Routing Protocols
  • LSRPs Advantages
  • LSRPs Disadvantages
  • LSRPs Support Multiple Areas
  • DVRPs Problems
    • Routing Loop
    • Route Poisoning
  • Summary

Dynamic Routing Protocols Link to heading

Read at 🔗.

Static versus Dynamic Routing Link to heading

Read at 🔗.

Autonomous System (AS) or Routing Domain Link to heading

A collection of routers/networks/areas under a common administration sharing a common routing strategy (e.g., a company’s internal network or an ISP’s network).

Components Link to heading

Each dynamic routing protocol is different but in general the following components are included:

1. Data structures – databases, stored in RAM, are used for operations.
2. Routing protocol messages – various types of messages are used to discover neighbouring routers, exchange routing information, and perform other tasks to learn and maintain accurate information about the network.
3. Algorithm – a well-defined list of steps used for facilitating routing information and for best path determination.

Operation Link to heading

1. A router sends and receives routing messages on its interfaces.
2. A router shares routing messages and routing information with other routers that are using the same routing protocol.
3. Routers exchange routing information to learn about remote networks.
4. When a network topology change is detected by a router, the routing protocol can advertise the changes to other routers.

Routing Convergence Link to heading

A state when all routers have completely understood the current network topology.

• Convergence time is the time it takes routers to share information, calculate best paths, and update their routing table.
• To figure out if a network has converged, examine the routing tables on each and every router.
• A network is not completely operable until it has converged; therefore, most networks require short convergence times.
• Generally, older protocols, such as RIPv1 and IGRP, are slow to converge, whereas modern protocols, such as EIGRP and OSPF, converge more quickly.

Classifying Routing Protocols Link to heading

1. Purpose: IGP or EGP
2. Operation: distance-vector protocol, link-state protocol, or path-vector protocol
3. Behaviour: classful (legacy) or classless protocol

Interior versus Exterior Gateway Protocols (IGPs/EGPs) Link to heading

A routing protocol must be designed either for use within an AS or for use between them.

The Border Gateway Protocol (BGP) is the only EGP used among ISPs, as well as to interconnect ISPs with their larger private clients to exchange routing information.

Distance-Vector Routing Protocols (DVRPs) Link to heading

Routers learn remote networks (distance and vector of routes) from directly connected (immediate) neighbours and these neighbours might have learnt these networks from their other neighbouring routers.

1. Distance - how far it is to the destination network and is based on an accumulated metric value, such as hop count (usually referred to when discussing DVRPs), cost, bandwidth, delay, and more.
2. Vector - the direction of the next-hop router or exit interface to reach the destination.

• Each router has an incomplete and different map of the network topology.
• Routing information is disseminated using local broadcasts.
  • Processed by every device connected its its interface.
  • Resource consumption.
• Periodic, full updates - updates are sent every 30 seconds to neighbouring routers, whether or not something has changed.
  • The entire routing table is sent as an update.

There are four IPv4 DVRPs:

1. RIPv1: First generation, open-standard, albeit legacy, protocol supported by many different router manufacturers.
2. RIPv2: Simple DVRP.
3. IGRP: First generation Cisco proprietary protocol (obsolete and replaced by EIGRP).
4. EIGRP: Advanced version of DVRPs.
   • Also a hybrid routing protocol 🔗.

Link-State Routing Protocols (LSRPs) Link to heading

Whereas DVRPs rely on routing information from immediate neighbours, LSRPs learn the complete network topology - which routers are directly connected to which networks, including what the network number are.

1. A link is an interface on a router, a network segment that connects two routers or a stub network.
2. Link state is information about the state of a link, including the network prefix, prefix length, and cost of the link.

• Each router has a complete and identical map of the network topology by exchanging link-state advertisements (LSAs).
• Routing information is disseminated using multicasts.
  • Processed only by other devices running the LSRP.
  • Resource-friendly.
• Change-triggered, partial updates - LSAs are flooded when a change occurs for the network (e.g. a link going down).
  • Only new changes are sent as an update.

LSRPs work best in situations where:

• The network design is hierarchical, usually occurring in large networks.
• Fast convergence of the network is crucial.
• The administrators have good knowledge of the implemented link-state protocol.

There are two IPv4 LSRPs:

1. OSPF: Popular standards-based routing protocol
   • Given that LSRPs use a shortest path first (SPF) algorithm.
2. IS-IS: Popular among network providers

Hybrid Routing Protocols (HRPs) Link to heading

based on distance vector routing protocols but employ many of the mechanisms and advantages of link state routing protocols.

• Examples of HRPs include RIPv2, EIGRP, and BGP.
• HRPs are less CPU- and-memory-intensive by acting like a DVRP when it comes to processing routing updates.
• HRPs are more network- and router-friendly as by acting like a LSRP. They only send out incremental, reliable updates via multicast messages.
• VLSM and CIDR are supported.

Classful versus Classless Routing Protocols Link to heading

A discontiguous network is when subnets from the same classful major network address are separated by a different classful network address.

Multiple Route Sources Link to heading

Administrative Distance (AD) Link to heading

• Read at 🔗.

Metric Link to heading

or routing metric, a measurable value that is assigned by the routing protocol to different routes based on the overall cost of that route from source to destination.

• Only when there are multiple routes to the same destination from a single routing protocol will the metric come into play.
• In such case, the route having the lowest metric will be installed into the routing table.
• Since different routing protocols use different metrics, the metric used by one routing protocol is not comparable to the metric used by another.
  • RIP uses hop count as its metric.
    • Hop count - the number of routers a packet must traverse to get to its destination.
    • RIP will choose the route with the least number of hops, even if the link is much slower that all other links, hence the possibility of high resource usage.
  • OSPF determines a cost based on cumulative bandwidth from source to the destination. Each router adds theirs and passes it on.
    • Bandwidth - the data capacity of a link. A link with high bandwidth is able to carry large amount of information at a time and therefore that link is also considered “fast”.
    • OSPF will choose the fastest route (having the highest bandwidth) despite hop count.
  • EIGRP determines a cost based on an accumulated metric value, including bandwidth, delay, load, reliability, and maximum transmission unit (MTU). These, however, can further be influenced by weights.

Routing Information Protocol (RIP) Link to heading

a distance-vector routing protocol that uses hop count as the metric for path selection.

• Uses the Bellman-Ford algorithm as its routing algorithm.

  • One router begins by sharing routes with its neighbours.
  • Those neighbours will share the learnt routes to other neighbours.
  • Eventually, all routers know of all routes on the network.
  • This is known as routing by rumour.

• A hop count greater than 15 hops is considered too far. That 15^th hop router will not propagate the routing update to the next router; thus, RIP is only suitable in small network having fewer than 15 routers.

• Administrative distance is 120.

• Routing updates are propagated periodically every 30 seconds, encapsulated into a UDP segment with both source and destination port numbers set to UDP port 520.

RIPng is the IPv6-enabled version of RIP, based on RIPv2.

• It still has a 15-hop limitation and the administrative distance is 120.

Enhanced Interior Gateway Routing Protocol (EIGRP) Link to heading

a hybrid routing protocol that uses bandwidth, delay, load, and reliability to create a composite metrics for path selection.

• Uses the Diffusing Update Algorithm (DUAL) algorithm as its routing algorithm to achieve rapid convergence (the fastest IGP).

  • Each router maintains all available backup routes received from neighbours for destinations (not only the best paths) so that it can quickly adapt to alternate routes.
  • If no appropriate route or backup route exists in the local routing table, EIGRP queries its neighbours to discover an alternate route.
  • If a primary route fails, the router can use the alternate route identified.
  • The switchover/transition to the alternate route is immediate and does not involve interaction with other routers.
  • DUAL guarantees loop-free operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronise at the same time.

• Uses IP protocol number 88.

• Routing updates are sent out to unicast and multicast address 224.0.0.10 via Reliable Transport Protocol (RTP) (designed to work on top of L3).

  • It does not have an associated port number.

Other EIGRP’s features include:

1. Bounded triggered updates: Unlike RIP, only when there is a change in the routing table will routing information be propagated (and only neighbours that require such updates will receive).
   • It uses less bandwidth, especially in large networks with many routes, thus reducing the amount of load the routing protocol places on the network.
2. Load balancing: EIGRP supports unequal-cost load balancing 🔗, which allows better distributions of traffic flow in the networks.
3. Hello keepalive mechanism: A small Hello message is periodically exchanged to maintain adjacencies with neighbouring routers. This means a very low usage of network resources during normal operation, instead of the periodic updates.
4. Multiple network layer protocols support: EIGRP uses Protocol Dependent Modules (PDM), which means that it is the only protocol to include support for protocols other than IPv4 and IPv6, such as legacy IPX and AppleTalk.

Comparing Routing Protocols Link to heading

LSRPs Advantages Link to heading

1. Topological map creation: LSRPs create a topological map, or shortest path first (SPF) tree of the network topology. Because LSRPs exchange link states, the SPF algorithm can build an SPF tree of the network. Using the SPF tree, each router can independently determine the shortest path to every network.
2. Fast convergence: When receiving an link-state packet (LSP), LSRPs immediately flood the LSP out all interfaces except for the ingress interface.
   • In contrast, DVRPs need to process each routing update and update its routing table before flooding the routing update out other interfaces.
3. Event-driven updates: After the initial flooding of LSPs, LSRPs only send out an LSP when there is a change in the topology (not periodically). The LSP contains only the information regarding the affected link.
   • In contrast, DVRPs propagate updates periodically and each update contains information about the entire table.
4. Hierarchical design: LSRPs use the concept of areas. Multiple areas create a hierarchical design to networks, allowing for better route summarisation and the isolation of routing issues within an area.

LSRPs Disadvantages Link to heading

1. Memory requirements: LSRPs require additional memory to create and maintain the link-state database and SPF tree.
2. Processing requirements: LSRPs can also require more CPU processing than DVRPs. The SPF algorithm requires more CPU time than DVRP’s algorithm (Bellman-Ford) because LSRPs build a complete map of the topology.
3. Bandwidth requirements: The flooding of LSPs can adversely affect the available bandwidth on a network. This should only occur during initial startup of routers, but can also be an issue on unstable networks.

LSRPs Support Multiple Areas Link to heading

• Modern LSRPs are designed to minimise the effects on memory, CPU, and bandwidth.
• The use and configuration of multiple areas can reduce the size of the link-state databases.
• Multiple areas can also limit the amount of link-state information flooding in a routing domain and send LSPs only to those routers that need them.
• When there is a change in the topology, only those routers in the affected area receive the LSP and run the SPF algorithm.
• This can help isolate an unstable link to a specific area in the routing domain.

DVRPs Problems Link to heading

Routing Loop Link to heading

a misunderstanding regarding how a destination should be reached.

• To verify whether a routing loop exists, use the ping tool to test connectivity to the destination network.
  • If the message reply reads “TTL expired in transit” for the destination, it indicates that a routing loop exists.
• A possible solution is counting to infinity, which means assigning a hop count limit to restrict how far a packet can travel.

Route Poisoning Link to heading

When a router detects that one of its connected routes has failed, the router will poison the route by assigning an infinite metric to it.

• In RIP, the route is assigned a hop count of 16 (15, by default, is the maximum), thus making it an unreachable network.
• In order to give the routers enough time to propagate the poisoned route and to ensure that no routing loops occur while propagation of the poisoned route occurs, the routers implement a hold-down mechanism.
• During this period, the routers will freeze the poisoned route in their routing tables for the period of the hold-down timer, which is typically three times the interval of the routing broadcast update.

Summary Link to heading

• Years ago, running a DVRP, such as RIP, was sufficient for small to medium networks, given the overhead involved with LSRPs.
• Nonetheless, with the advancement of hardware, DVRPs are not commonly used today, even in smaller networks.
• The most common dynamic IGP is OSPF, with EIGRP a distant second.
• In SOHO networks, static routes are the most common routing mechanism used.