Ch'15 Connecting LANs, Backbone Networks, and Virtual LANs

1
Ch.15 Connecting LANs, Backbone Networks, and
Virtual LANs

• Lecturer Tae-Hyong Kim (D132)
• taehyong_at_kumoh.ac.kr

2
Contents

• Connecting Devices
• Repeaters
• Hubs
• Bridges
• Two-Layer Switch
• Router and Three-Layer Switches
• Backbone Networks
• Virtual LANs

3
Connecting Devices

• Networking Devices
• to form (extend/subdivide) a single network for
  sharing data or resources
• ex. repeater (hub), bridge, ...
• Internetworking Devices
• to connect separate networks for exchanging data
  or resources
• ex. router, gateway, ...

4
Repeaters

• Operate on Physical Layer
• extend the physical length of a network
• Connect segments of a LAN

5
Repeaters

• Forwards every frame no filtering capability
• regenerates the signals and puts the refreshed
  copy
• not an amplifier

6
Repeaters

• do not change the functionality of the network
• cannot identify the address of the signal ? just
  pass

7
Hubs

• Passive Hubs (Dummy Hubs)
• just a connector ? collision point
• do not separate collision domain
• physically star topology, logically bus topology
• Active Hubs (Switching Hub)
• a kind of switch ? separate collision domain
• physically and logically star topology
• Used for multiple levels of hierarchy
• removes the length limitation of 10Base T (100m)

8
Bridges

• Operates in both the physical and the data link
  layers
• divide a large network into smaller segments
• relay frames between two originally separate LANs

9
Bridges

• has a table used in filtering decisions (pass/no
  pass)
• connects segments of a LAN together subdivision
• cannot filter broadcast traffic (ARP or IPX
  traffic) a B/C domain
• Does not change the physical (MAC) address in a
  frame

10
Bridges

• can keep the traffic for each segment separate
• checks the destination address (DL layer)
  regenerates (PH layer)
• useful for controlling congestion and isolating
  problem links
• may provide security through this partitioning of
  traffic

lookup table
11
Bridges

• Types of Bridges
• simple bridge
• Manual management of lookup table
• Inexpensive but time-consuming and potentially
  more trouble
• multiport bridge

12
Bridges

• Transparent bridge (learning bridge)
• The lookup table is empty at first
• Builds and manages the lookup table dynamically
  when encountering each packet

• Source address
• ? build manage the lookup table
• Destination address in not known
• ? pass
• Destination address is known
• ? pass / no pass wrt. table

13
Bridges

• Loop problem of transparent bridges

14
Bridges

• Solution of Loop problem of transparent bridges
• Spanning tree approach ? no loop
• The spanning tree of a graph
• should include all the vertices (nodes) with a
  minimum no. of edges (lines) connecting the
  vertices
• Any vertex can be selected as the root of the
  spanning tree
• the shortest path the sum of the weights from a
  specific vertex to the root

15
Bridges

• Spanning Tree Approach
• Finds a designed bridge for each LAN segment
• to find the least-cost path between the LAN and
  the root bridge
• Create a logical topology overlays the physical
  one
• Process (manual)
• Step.1 Each bridge has a built-in ID
• The bridge with the smallest ID is selected as
  the root bridge (RB)
• Step.2 Find the shortest path (a path with the
  shortest cost) from the RB to every other bridge
  or LAN
• Step.3 Find the shortest tree by combining the
  shortest paths
• Step.4 Mark the ports that are part of the
  spanning tree as forwarding ports (FP), mark the
  ports that are not part of the spanning tree as
  blocking ports (BP)
• FP forwards a frame that it receives BP does
  not.

16
Bridges

• Ex.1 Find the FPs and BPs

17
Bridges

• Ex.1
• solution

18
Bridges

• Ex.1 Final result
• Logical topology of the system
• Dynamic Algorithm
• The bridges send special messages to each other
  to update the spanning tree BPDUs (with
  ARC(accumulated root cost) field)

19
Bridges (cont.)

• Another solution to prevent loop Source Routing
• The source of the packet defines the bridges and
  the LANs through which the packet should go
• Designed for Token Ring LANs ? not commonly used
• Bridges connecting different LANs
• Ex. Ethernet ? Token Ring
• Problems of connecting different LANs
• Frame format
• Payload size
• Data rate
• Address bit order
• ? todays bridges can handle these problems

20
Assignment 14

• Find the FPs and BPs.

S3
Br1
4
4
2
4
S2
Br2
S1
2
S4
2
2
4
Br3
Br4
Br5
2
2
4
S5
21
Switches

• Act as a multiport bridge to connect devices or
  segments in a LAN
• Check the destination address with switching
  table
• Two different buffer fabrics
• A store-and-forward switch has input buffers
• A cut-through switch has output buffers

22
Switches

• Two-Layer Switch
• Bridges vs. 2-Layer (traditional) Switch Bridge
• switching hub multi-port bridge
• Most switches support VLANs (IEEE 802.1Q)
• has the problems of VLANs
• Three-Layer Switch
• The combination of traditional layer 2 switching
  with layer 3 protocol routing in a single
  hardware implementation
• Switch traffic very fast using Layer 3 address
  information
• multilayer switch, switching router, routing
  switch
• usually faster but not as powerful or flexible
  as a router

23
Switches (cont.)

• Four-Layer Switch
• can identify which application protocols (HTTP,
  SMTP, FTP, ...) are included with each packet
• uses this information to hand off the packet the
  appropriate higher-layer software
• makes packet-forwarding decision not only on the
  MAC address and IP address, but also on the
  application to which a packet belongs
• enables to establish priorities for network
  traffic based on application
• ex. assign a high priority to packets belonging
  to vital in-house applications and a low priority
  to packets of HTTP-based Internet traffic
• also provides an effective wire-speed security
  shield for the network by authorized switched
  ports or users

24
Routers

• Operate in the physical, data link, and network
  layers
• Relays packets among multiple interconnected
  networks like stations
• Checks the destination address
• finds what is considers the best route for the
  packet
• passes it to the destination network or the next
  router on the path chosen by a routing protocol
  (e.g. RIP, OSPF, )

25
Routers

• Connects independent LANs or WANs to create an
  internetwork
• Changes the physical addresses in a packet after
  routing
• Break up the broadcast (collision) domains
• Smarter but may have high latency (traditional
  SW-based one)

26
Gateways

• Potentially operates in all five layers
• A protocol converter
• Accept a packet formatted for one protocol and
  convert it to a packet formatted for another
  protocol before forwarding it

27
Gateways (cont.)

• Generally installed within a router
• Understands the protocols used by each network
• Translate from one to another
• Ex. Connection point between intranet and
  internet
• Often equipped with a proxy server or a firewall

28
Contents

• Connecting Devices
• Backbone Networks
• Bus Backbone
• Star Backbone
• Connecting Remote LANs
• Virtual LANs

29
Backbone Networks

• connects several LANs
• No station is directly connected to the backbone
• The stations are part of a LAN
• A LAN itself that uses a LAN protocol such as
  Ethernet
• Each connection to the backbone is itself another
  LAN
• Major architecture of a backbone
• The bus and the star

30
Bus Backbone

• The topology of the backbone bus
• Normally used as a distribution backbone to
  connect different building in an organization
• Ex. Single- or multiple-floor buildings on a
  campus
• Bridge-based backbone for filtering intra-LAN
  traffic

May have another backbone
31
Star Backbone

• The topology of the backbone star (one switch)
• Mostly used as a distribution backbone inside a
  building
• Ex. Usually one LAN serves each floor
• Backbone switch and all hubs or switches are
  often in a rack or chassis

32
Connecting Remote LANs

• A common application of a backbone network
• Useful when a company has several offices with
  LANs and needs to connect them
• The connection can be done through bridges
  (Remote Bridges)
• RB connects LANs and point-to-point networks
• Point-to-point networks a LAN without stations
• Ex. Leased telephone lines (PPP) or ADSL lines

33
Contents

• Connecting Devices
• Backbone Networks
• Virtual LANs
• Motivation
• Membership
• Configuration
• Communication Between Switches
• IEEE Standards
• Advantages

34
Motivation of Virtual LANs

• What happens if we need a virtual connection
  between two stations belonging to two different
  physical LANs?
• ? defines Virtual LAN configuration by software

35
Properties of Virtual LANs

• Provides flexible division of the whole network
• Configuration of the subnetworks can be done by
  software
• Why should we divide a large LAN according to the
  working group?
• For performance and security issues
• VLANs break up the broadcast domain and still
  retain speed
• Intra-LAN traffic is filtered at the switch
  supporting VLANs
• Need a router for communication between segments
• Most Layer-2 switches and all routers support
  VLANs
• Inter-VLAN traffic is increasing ? the router is
  again the bottleneck
• Layer-3 switches are the solution to this problem

36
Properties of Virtual LANs

• A sample configuration example
• A backbone LAN with two switches and three VLANs
• People in the first building and people in the
  second building can be in the same working group

37
Membership Configurations

• Characteristics used to group stations in a VLAN
• Port numbers
• MAC addresses
• IP addresses
• Multicast IP addresses
• Combination
• Configuration
• Manual configuration by the network administrator
• Automatic configuration using criteria
• Ex. Project number
• Semiautomatic configuration
• Initially manual, then automatic configuration

38
Communication between Switches

• Communication between switches
• Motivation in a multiswitched backbone network
• Each switch must know
• Which station belongs to which VLAN?
• The membership of stations connected to other
  switches
• Three solution methods
• Table maintenance records station membership
• Frame tagging an extra header for identifying
  VLAN
• TDM channel the frame arrived from indicates VLAN

39
Standard and Advantages

• IEEE standard
• IEEE 802.1Q
• Advantages
• Cost and Time Reduction
• reduces the migration cost of stations going from
  one group to another
• Creating Virtual Work Group
• VLANs can be used to create virtual work group
• Security
• provides an extra measure of security by grouping