CIS460

1
CIS460 NETWORK ANALYSIS AND DESIGN

• CHAPTER 7 -
• Selecting Bridging, Switching, and Routing
  Protocols

2
Introduction

• In this chapter we are going to look at bridging,
  switching, and routing protocol attributes of
• Network Traffic characteristics
• Bandwidth, memory, and CPU usage
• The approximate number of peer routers or
  switches supported
• The capability to quickly adapt to changes in an
  internetwork
• The capability to authenticate route updates for
  security reasons

3
Making Decisions as Part of the Top-Down Network
Design Process

• Factors involved in making sound decisions
• Goals must be established
• Many options should be explored
• The consequences of the decisions should be
  investigated
• Contingency plans should be made
• Use a decision to match options with goals

4
Making Decisions as Part of the Top-Down Network
Design Process (Contd)

• Table 7-1 shows a decision table
• Once decision is made look at it to determine
• What could go wrong
• Hs it been tried before
• How will customer react
• Contingency plans if customer disapproves
• Can use during both logical and physical design
  phase

5
Selecting Bridging and Switching Methods

• Decision making is simple because of few options
• If includes Ethernet bridges and switches most
  likely use transparent bridging with
  spanning-tree protocol
• Might also need a protocol for connecting
  switches that support virtual LANs
• With Token Ring networks options include
  source-route bridging (SRB), source-route
  transparent (SRT) bridging and source-route
  switching (SRS)

6
Characterizing Bridging and Switching Methods

• Bridges operate at Layers 1 and 2 of OSI
• Determine how to forward a frame based on
  information in Layer 2 header
• Bridge does not look at Layer 3 information
• Bridge segments bandwidth domains so that devices
  do not compete with each other for media access
  control
• Bridge does forward Ethernet collisions or MAC
  frames in a Token Ring network

7
Characterizing Bridging and Switching Methods
(Contd)

• Bridge does not segment broadcast domains. It
  sends broadcast packets out all ports
• Bridges normally connect like networks but can be
  a translation or encapsulating bridge
• A switch is like a bridge only faster
• Switches take advantage of fast integrated
  circuits to offer very low latency
• Switches usually have a higher port density and a
  lower cost per port

8
Characterizing Bridging and Switching Methods
(Contd)

• Bridges do store and forward
• Switches can be store and forward or cut-through
• Cut-through is faster but more prone to letting
  runts or error packets through
• On a network that is prone to errors do not use
  cut-through processing
• Adaptive cut-through switching

9
Transparent Bridging

• Most common Ethernet environments
• A transparent bridge (switch) connects one or
  more LAN segments so that end systems on
  different segments can communicate with each
  other transparently
• Looks at the source address in each frame to
  learn location of network devices
• It develops a switching table (Table 7-2)

10
Transparent Bridging (Contd)

• Receives a packet look sup address in switch
  table
• If no address it sends the frame out every port
  like a broadcast frame
• Send Bridge Protocol Data Unit (BPDU) frames to
  each other to build and maintain the spanning
  tree
• Sends BPDU to a multicast address every two
  seconds

11
Source-Route Bridging

• Developed for Token Ring networks in the 80s by
  IBM
• Uses a source-routing-transparent (SRT) standard
• An SRT bridge can act like a transparent bridge
  or a source-routing bridge depending on whether
  source-routing information is included in a frame
• Not transparent if pure SRB is used

12
Source-Route Bridging (Contd)

• Uses explorer frames
• All-routes explorer - take all possible paths,
  take just one route back
• Single-route explorer - takes just one path and
  response take all paths or just one back
• With single-route explorer frames the
  spanning-tree algorithm can be used to determine
  a single path
• Scalability is impacted by amount of traffic
  when all-routes explorer frames are used

13
Source-Route Switching

• SRS is based on SRT bridging
• SRS forwards a frame that has no routing
  information field
• Learns the MAC addresses of devices on the ring
• Also learns source-routing information for
  devices on the other side of SRB bridges

14
Source-Route Switching (Contd)

• Benefits
• Rings can be segmented without adding new ring
  numbers
• can be incrementally upgraded to transparent
  bridging with minimal disruption or
  reconfiguration
• does not need to learn the MAC addresses of
  devices on the other side of source-route bridges
• can support parallel source routing paths
• can support duplicate MAC addresses

15
Mixed-Media Bridging

• Mixture of Token Ring, FDDI and Ethernet bridging
• Encapsulating bridging is simpler than
  translation bridging but is only appropriate for
  some network topologies
• Encapsulating bridge encapsulates an Ethernet
  frame inside an FDDI or Token ring frame for
  transversal across a backbone network that has no
  end systems

16
Mixed-Media Bridging (Contd)

• Support for end systems on a backbone then need
  to use translation bridging which translates from
  one data-link-layer protocol to another
• Problems
• Incompatible bit ordering
• Embedded MAC addresses
• Incompatible maximum transfer unit (MTU) sizes
• Handling of exclusive Token Ring and FDDI
  functions
• No real standardization

17
Mixed-Media Bridging (Contd)

• While FDDI is a common choice for backbone
  networks in campus network designs to avoid
  translating Ethernet and FDDI frames should use
  100-Mbps Ethernet or Gigabit Ethernet on backbone
  segments

18
Switching Protocols for Transporting VLAN
Information

• When VLANs are implemented in a switched network
  the switches need a method to make sure
  intra-VLAN traffic goes to the correct segments
• Accomplished by tagging frames with VLAN
  information
• two tagging methods
• adaptation of the IEEE 802.10 security protocol
• Inter-Switch Link (ISL) protocol

19
IEEE 802.10

• A security specification used as a way of placing
  VLAN identification (VLAN ID) in a frame
• Inserted between the MAC and LLC headers of the
  frame
• The VLAN ID allows switches and routers to
  selectively forward packets to ports with the
  same VLAN ID
• VLAN ID removed from frame when forwarded to
  destination segment

20
Inter-Switch Protocol

• Another method for maintaining VLAN information
  as traffic goes between switches
• Developed to carry VLAN information on a 100-Mbps
  Ethernet switch-to-switch or switch-to-router
  link. Can carry multiple VLANs
• ISL link is call a trunk. A trunk is a physical
  link that carries the traffic of multiple VLANs
  between two switches or between a switch and a
  router. Allows VLANs to extend across switches

21
VLAN Trunk Protocol

• Some networks have a combination of different
  media types
• VLAN trunk protocol (VTP) allows a VLAN to span
  the different technologies by automatically
  configuring a VLAN across a campus network
  regardless of media type
• VTP is a switch-to-switch and switch-to-router
  VLAN management protocol that exchanges VLAN
  configuration changes as they are made to the
  network

22
Selecting Routing Protocols

• A routing protocol lets a router dynamically
  learn how to reach other networks and exchange
  this information with other routers or hosts
• Selecting routing protocols is harder than
  selecting bridging protocols because there are so
  many
• Made easier using a table such as 7-1 to pick the
  best one

23
Characterizing Routing Protocols

• General goal to share network reachability
  information among routers
• Some send complete other only an update
• Differ in scalability and performance
  characteristics
• Many are designed for small networks
• Static environment
• Some are meant for connecting interior campus
  networks

24
Distance-Vector Versus Link-State Routing
Protocols

• Two major classes distance-vector and link-state
• Distance-vector protocols
• IP Routing Information Protocol (RIP) Version 1
  and 2
• IP Interior Gateway Routing Protocol (IGRP)
• Novell NetWare Internetwork Packet Exchange
  Routing Information Protocol (IPX RIP)
• AppleTalk Routing Table Maintenance Protocol
  (RTMP)
• AppleTalk Update-Based Routing Protocol (AURP)
• IP Enhanced IGRP
• IP Border Gateway Protocol (BGP) (path-vector)

25
Distance-Vector Versus Link-State Routing
Protocols (Contd)

• Vector means distance or course. A
  distance-vector includes information on the
  length of the course. Many use hop count
• A hop count specifies the number of routers that
  must be traversed
• Maintains a distance-vector routing table that
  lists know networks and the distance to each.
• Sends table to all neighbors, or an update after
  first transmission

26
Distance-Vector (Contd)

• Split Horizon, Hold-Down, and Poison-Reverse
  Features
• Split-horizon technique - sends only routes that
  are reachable via other ports
• Hold-down timer - new information about a route
  to a suspect network is not believed right away.
  A standard way to avoid loops
• Poison-reverse messages - way of speeding
  convergence and avoiding loops. When a router
  notices a problem it can immediately send a route
  update that specifies the destination is no
  longer reachable

27
Link-State Routing Protocols

• Do not exchange routing tables
• Exchange information about the status of their
  directly connected links using periodic multicast
  messages
• Each router builds its own routing table
• Protocols
• IP Open Shortest Path First (OSFP)
• IP Intermediate System-to-Intermediate System
  (IS-IS)
• NetWare Link Services Protocol (NLSP)

28
Link-State Routing Protocols (Contd)

• Converge more quickly
• Less prone to routing loops
• Require more CPU power and memory
• More expensive to implement and support
• Harder to troubleshoot

29
Routing Protocol Metrics

• Used to determine which path is preferable when
  more than one path is available
• Vary on which metrics are supported
• Distance-vector use hop count
• Newer protocols take into account delay,
  bandwidth, reliability and other factors
• Metrics can effect scalability

30
Hierarchical Versus Non-Hierarchical Routing
Protocols

• Some routing protocols do not support hierarchy
• Normally all routers perform same tasks
• Hierarchical protocols assign different tasks to
  different routers and group routers in areas
• Some routers communicate with local routers in
  the same area and other routers have the hob of
  connecting areas, domains, or autonomous systems

31
Interior Versus Exterior Routing Protocols

• Interior protocols, such as RIP, OSPF, and IGRP
  are used by routers within the same enterprise or
  autonomous
• Exterior such as BGP perform routing between
  multiple autonomous systems.

32
Classful Versus Classless Routing Protocols

• A classful routing protocol always considers the
  IP network class
• Address summarization is automatic by major
  network number and discontiguous subnets are not
  visible to each other
• Classless protocols transmit prefix-length or
  subnet mask information with IP network
  addresses. The IP address can be mapped so that
  discontinuous subnets and VLSM are supported

33
Dynamic Versus Static and Default Routing

• Static routes are often used to connect to a stub
  network
• A stub network is a part of an internetwork that
  can only be reached by one path
• Internal routers can simply be configured with a
  default route that points to the ISP

34
Scalability Constraints for Routing Protocols

• Consider customers goals for scaling the network
  to a larger size
• There are a number of questions that relate to
  scalability that should be answered
• They can be answered by watching routing protocol
  behavior with a protocol analyzer and by studying
  the relevant specifications

35
Routing Protocols Convergence

• Convergence is the time it takes for routers to
  arrive at a consistent understanding of the
  internetwork topology after a change takes place
• Understand the frequency of changes, links that
  fail often, etc
• Convergence time is a critical design constraint

36
Routing Protocols Convergence (Contd)

• Convergence starts when a router notices a link
  has failed
• If a serial link fails it can start immediately.
  If it uses keepalive frames it starts convergence
  after it has been unable to send two or three
  keepalive frames
• If use hello packets and the hello timer is
  shorter than the keep alive timer then routing
  protocol it can start convergence sooner

37
IP Routing

• Most common protocols are RIP, IGRP, Enhanced
  IGRP, OSPF, and BGP

38
Routing Information Protocol

• The first standard routing protocol developed for
  TCP/IP environments
• It is a distance-vector protocol that features
  simplicity and ease-of-troubleshooting
• Uses a hop count to measure the distance to a
  destination. Cannot be more than 15 hops
• RIPv2 developed to address some of the
  scalability and performance problems with Version
  1

39
Interior Gateway Routing Protocol

• Meet needs of customers requiring a robust and
  scalable interior routing protocol
• Uses composite metric based on bandwidth,
  delay, reliability, and load
• Load balances over equal-metric paths and
  non-equal-metric paths. (3 to 1)
• Has a better algorithm for advertising and
  selecting a default rout than RIP

40
Enhanced Interior Gateway Routing Protocol

• Meet the needs of enterprise customers with
  large, complex, multiprotocol internetworks
• Goal is to offer quick convergence on large
  networks. Diffusing update algorithm (DUAL)
  guarantees a loop-free topology
• The router develops a topology table that
  contains all destinations advertised by
  neighboring routers. It can scale to thousands
  of nodes

41
Open Shortest Path First

• Open standard supported by many vendors
• converges quickly
• authenticates protocol exchanges to meet security
  goals
• supports discontiguous subnets and VLSM
• sends multicast frames vice broadcast frames
• does not use a log of bandwidth
• can be designed in hierarchical areas

42
Open Shortest Path First (Contd)

• Propagates only changes
• accumulate link-state information to calculate
  the shortest path to a destination
• all routers run the same algorithm in parallel
• Allows sets of networks to be grouped into areas
• A contiguous backbone area, called Area ) is
  required
• Assign network numbers in blocks that can be
  summarized

43
Border Gate Protocol

• iBGP used at large companies to route between
  domains
• EBGP is often used to multihome an enterprises
  connection to the Internet
• Main goal is to allow routers to exchange
  information on paths to destination networks

44
Apple Talk Routing

• Three options
• Routing Table Maintenance Protocol (RTMP)
• AppleTalk Update-Based Routing Protocol (AURP)
• Enhanced IGRP for AppleTalk
• RTMP is most common because it is easiest to
  configure and is supported by most vendors

45
Routing Table Maintenance Protocol

• Routing table sent every 10 seconds using split
  horizon
• Works closely with Zone Information Protocol
  (ZIP)
• Checks routing table updates and sends ZIP query

46
Using Multiple Routing and Bridging Protocols

• Important to realize you do not have to use the
  same routing and bridging protocols throughout
  the internetwork
• To merge old networks with new networks it is
  often necessary to run more than one routing or
  bridging protocol
• Solutions include source-route transparent
  bridging, external routes in OSPF and RIP2

47
Redistribution between Routing Protocols

• Redistribution allows a router to run more than
  one routing protocol and share routes among
  routing protocols
• Network administrator must configure
  redistribution by specifying which protocols
  should insert routing information into other
  protocols routing tables
• A router can learn about a destination from more
  than one protocol

48
Integrated Routing and Bridging

• CISCO offers support for IRB which connects VLANs
  and bridged networks to routed networks within
  the same router
• One advantage of IRD is that a bridged IP subnet
  or VLAN can span a router

49
Summary

• Deciding on the right bridging, switching, and
  routing protocols for your customer will help you
  select the best switch and router products for
  the customer