NET 226 - Designing Internetwork Solutions

Chapter 7, Selecting Switching and Routing Protocols


This lesson concerns making choices, particularly among switching and routing protocols. Objectives important to this lesson:

  1. A model for decision making
  2. Switching protocols
  3. Routing protocols

Chapter 7

A model for decision making

The chapter begins with a recap of choices made in the last chapter about placement of switches and routers. It continues toward the goal of implementing protocols that are best suited for your design.

The first topic in the chapter could have been placed anywhere in the text. It is about making choices. It involves comparing the products we might choose, and judging them against what we want them to do, as well as what we require that they do. The best method our author knows for making choices between protocols starts with asking ourselves what we want and need to accomplish.

  • What are the goals we need to achieve with our protocols? Do we need scalability, adaptability, or status as industry standards? Should they create the least traffic possible? Must they be compatible with inexpensive, generic, or existing equipment? Must we be able to manage them centrally? A list should be generated and show on the y axis of a a chart, like the one on page 201.
  • What at the possible choices? A list should be generated and shown on the x axis of our chart.
  • For the best choices that meet our goals, what changes are required to implement those choices? What problems must be solved, and risks do those choices present?
  • What do we do if this does not work, or if the risks we anticipate become realities?

The author points out that a decision might be made based on just the first two steps, laying out the chart and scoring each possibility. She warns us that we should carry out the third and fourth steps as well, to avoid service outages, and to avoid problems with staff and customers.

Switching protocols

The text spends a few paragraphs explaining that switches are better than bridges. Most of you have seen switches, but you may never have seen a bridge, so we can move on. On page 202, the text reminds us that a switch is meant to work on layers 1 and 2 of the OSI model. A hub does not notice MAC addresses, so it works only at layer 1. On a tangent, the text discusses other meanings of the word "switch", which is nice but it is not necessary.

The text reviews the essential functions of a network switch:

  • to receive frames
  • to note the MAC address of the sender
  • to associate that MAC address with the switch port that received the frame
  • to record the information in the second and third bullets in an address table
  • to search for an entry in the address table that matches the frame's destination address
  • to send the frame out the associated port if an association was found, or to send the frame out all ports (except the one on which it was received) if no association was found

On page 203, the text begins a discussion of switching protocols.

  • RSTP - enhanced by IEEE 802.1w and 802.1s to have rapid convergence and to aggregate multiple trees from VLANs into one tree
  • PortFast - IEEE 802.1D and PortFast provide that a switch can be a switch edge port, which means that it is the only switch on a segment, so it goes to forwarding mode faster, not having to be elected as the designated switch
  • UplinkFast - If a switch on the access layer connects to two switches on the distribution layer (two uplinks), STP tells the lower switch to use only one of those higher layer switches. If the intended link goes down, it can take a minute for STP to enable the link to the other upper switch. UplinkFast only takes a second.
  • Unidirectional Link Detection (UDLD) and Loopguard - This kind of link occurs when traffic in one of two directions on a link fails. The could be caused by a bad transmitter, a bad receiver, a bad repeater, or a bad cable segment. Only one device on the link will notice a problem. This can cause a backup switch to not hear from its partner, to go into forwarding modem, and to create a switch loop because its partner is still forwarding. UDLD causes the switch that is having trouble hearing to shut down the port. Loopguard causes the port to go into a new state: loop inconsistent state.
  • IEEE 802.1Q - This standard adds VLAN tags to frames created on a VLAN. This is not an encapsulation. The tag goes inside the frame.
  • Dynamic Trunk Protocol - This Cisco protocol allows switches on either end of a trunk connection to negotiate the addition of VLAN tags.

On page 209, the text begins a discussion of routing protocols that continues to the end of the chapter. Routing protocols are typically sorted into two groups: Distance Vector protocols and Link State protocols. The text gives us a list of Distance Vector protocols first:

  • Routing Information Protocol (RIP) versions 1 and 2
  • Interior Gateway Routing Protocol (IGRP)
  • Enhanced IGRP (EIGRP)
  • Border Gateway Protocol (BGP)

Distance Vector protocols are older protocols that are verbose and limited in range. They learn about routes from other routers, add those routes to a table of routes they know from their own connections, and then proceed to broadcast all of those routes to other routers about twice a minute. The problem with this method is that a router can learn about a new route, add itself to that route, and start advertising it with all the others as a new route with one more hop.

What is a hop? The text reveals that the phrase hop count has two meanings, and different protocols may use either one. A hop count can be either:

  • the number or routers that a route crosses
  • the number of links in a route

This makes comparing the length of two routes harder if they are calculated by two protocols that each use a different definition. Back to the problem. If a router adds itself to a route, and adds 1 to the hop count, it will add that to its table and tell every device in screaming range about it, over and over. Another device learns about it, adds itself to that route, adds 1 to the hop count, and guess who hears about it? They all do! Eventually, the hop count on these routes reaches 16, at which point a Distance Vector router refuses to use it, and it becomes worthless. The text explains three methods to remedy this insane behavior, none of which change it enough: split-horizon, hold-down, and poison-reverse. What else can we do?

Routers running Link State protocols have many healthier features. They only advertise routes they are on themselves. They only tell you about new information, or information about routes that have changed. They try to find the shortest path available. Some protocols that are link state protocols:

  • Open Shortest Path First (OSPF)
  • Intermediate System-to-Intermediate System (IS-IS)
  • NetWare Internetwork Packet Exchange (IPX) Links Services Protocol (LSP)

The author probably realizes that no one in their right mind would choose a Distance Vector Protocol were it not for one reason: they are often installed on routers by default, and they are easy to set up.  So the text offers a list of reasons to choose each type of protocol.

Reasons to choose Distance Vector protocols:

  • Your network is flat
  • Your network is a hub and spoke network
  • Your administrators don't know how to do any thing else (Your admins are stupid.)
  • Long convergences will not be a problem. (Did I mention the long convergence times for these protocols?)

Reasons to choose Link State protocols:

  • Your network is and should be hierarchical
  • Your administrators are trained in Link State protocols
  • You want fast convergence

The chapter continues with several other methods of classifying protocols. It is worth reviewing material on static and dynamic routing on page 215. See my notes for this material on my notes to NET 222.

You should review the summary of protocols discussed in the chapter that appears on page 230.

Week 7 Assignment: Chapter 7

  • From Chapter 7:
    • Review Questions 1 - 4 on page 231
  • See the additional material posted this week for the term project and add it to your project requirements.
  • Read Chapter 8