This document discusses options for interconnecting local area networks (LANs) in a campus network. It describes bridges, switches, and routers that operate at the data link layer and can be used to connect multiple LAN segments. Switches are preferred within campus networks today as they provide fast connection speeds with low latency and easy administration. The document also discusses using higher speed backbone technologies like Fast Ethernet, FDDI, or ATM to connect LAN segments at higher speeds while still requiring interconnection devices like switches or routers.

1. Session T2:
Campus LAN
Interconnection
An Introduction to Concepts and
Technologies

2. ethernet: one ethernet is one "collision domain"
 cabling rules ("4-repeater", etc.) allow growth of a
single ethernet
 limited distance:~2500m or less, depending on cable
type
 limited number of stations: 1024 (architecturally); 100
(practical, olden days); 20-30? (practical, today)
 when you grow beyond these limits, build another
ethernet and connect the two together
the issue: expanding a local network


3. the issue: expanding a local network...
A
C
D
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C
D
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B
 Appends data
 Intermediate stations repeat
data
 Receiver copies data and
continues to repeat
 Sender generates new token
one token ring is one "token path"
 limited distance: ~2000m or so, depending on cabling
 limited number of stations: 250 or fewer, depending on
traffic

4. why connect or split LANs?
why connect LANs?
 to allow sharing of files, devices, etc.
why split LANs?
 to provide physical security/isolation
 to implement policies (user groups, etc)
 to give greater average bandwidth per user
("segmentation" or "microsegmentation")
so, what are our options for interconnecting the
LAN segments we create?

5. the issue, restated: which LAN frames should be
forwarded from one segment to another?
a LAN frame on an ethernet:
 SNA, IP, IPX, AppleTalk Address
 Token-Ring, Ethernet Address
 Also known as MAC address (Media
Access Control)
?

6. LAN interconnection technologies
Presentation
Session
Transport
Network
Data Link
Physical
Application
OSI
reference
model
hubs/multiplexors
bridges/switches
routers
application gateways
tunneling/encapsulation

7. bridge
bridge operation:
 at layer 1, connects two physical LAN segments
 at layer 2, connected LANs look like a single logical
LAN
e.g., bridge forwards LAN broadcasts
 forwards frames based on layer 2 info (e.g., MAC
address)
thus, independent of higher layer protocols
 easy to implement -- little or no configuration
B
B
B
B
B
B
B
B
B
B
B
B

8. transparent bridge
bridges agree on a single path through the
network
 path is called a "spanning tree"
all LAN traffic follows that single path
 frames forwarded based on MAC address
parallel bridges may exist, but are inactive
("blocking")
B
B
B
B
B
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B

9. source routing bridge
commonly used in token ring networks (not ethernet)
 each ring is given a ring number (unique in the whole bridged
LAN)
 each bridge is given a bridge number (unique between same
pair of rings)
end stations discover routes via a broadcast process
 bridges place path of broadcast in the frame (routing info
field)
 that same path (rings and bridges) is then used for other
frames
frames forwarded based on routing info field in frame
for connection-oriented protocols, broadcast occurs
only when connection is established
parallel active paths are allowed

10. B
B
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B
A
B
B?
B? B?
B? B?
B
B
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B
B
B
A
B
B via Rt 1!
B via Rt 2!
source routing bridge...
B?

11. switch
basically a fast, multiport, layer-two device
 i.e., similar in function/capability to a bridge
 fast, since functions often performed in hardware
 low latency -- good for fast response time
 easy implementation, low cost
each port connects to a separate LAN segment
 shared or dedicated
 dedicated ports may operate in full-duplex mode

12. router
router isolates logical subnetworks for more efficient
network utilization
 layer 2 traffic not typically forwarded unless addressed to
router
 each subnetwork is given an identification--e.g., IP subnet;
IPX network number
end station sends traffic to router; router forwards
toward ultimate destination
router must understand the layer 3 protocol(s) it is to
handle--complexity, configuration
routing protocols allow router to understand network
topology

13. R
R
R
Net A
Net C
Net D
Net E
Net B
router...
R

14. choosing technologies--considerations
protocols (IP, IPX, NetBIOS, SNA, Appletalk, ...)
 how do they work?
 do they have a layer 3 structure (are they "routable?")
 how often do they broadcast? how much traffic?
end user response time--delay/latency in the
interconnection device
administration
 configuration of router vs bridge/switch
 network operations--e.g., moves/changes
 network management, troubleshooting, etc.
cost

15. example - distributed backbone with
bridges
B
B
B
hubs
bridges
hubs
Physical
Logical

16. example - distributed backbone with
bridges
pro:
 easy to implement--little configuration
 inexpensive
 administration is easy
con:
 potential bridge congestion, depending on which
bridge used
 bridge management harder since bridges
distributed

17. example - collapsed backbone with
bridges
Ring 001
Ring 002
Backbone Ring
Bridge Bridge
hubs
bridges
Physical
backbone hub
Logical

18. example -- collapsed backbone with
bridges
pro:
 same as distributed bridge design, plus
centralized bridges/backbone hub are easier to
manage
servers can be centralized while still physically
connected to floor LAN segments
con:
 same as distributed bridge design
 riser cable considerations
fiber? copper? distance? port cost on device?

19. example - collapsed backbone router
subnet A
subnet B
hubs
Physical
backbone router
Logical

20. example -- collapsed backbone router
pro:
 conceptually simple
 popular solution
 more powerful device than bridge--faster, more
intelligence
 router limits broadcast traffic between subnets
con:
 more expensive device than bridge
 operation, management much more complex than
bridge
 user moves more complicated to handle--subnets
 broadcast traffic not usually a problem in campus--
different from a WAN link

21. example - collapsed backbone switch
hubs
Physical
switch
Logical
switch

22. example -- collapsed backbone switch
advantages:
 same pros as bridged network -- low cost, easy
implementation and administration
 avoids subnet issues with user moves
 higher performance and lower latency than bridge or
router
 servers can be attached to dedicated switch ports for
higher performance
 being deployed today as front end to router
Trend today is to use switching within a campus, and
routing for lower speed WAN links

23. what about campus backbone
technologies?
generic picture: LANs (ethernet, token ring)
connected with some kind of high speed backbone
2 or 3 popular backbone technologies
the issues of interconnection devices are still the
same as before
 latency; intelligence; administration; cost; etc.......
B
B
B
B
Fast Enet
FDDI
ATM

24. "big pipe" technologies
...i.e., a faster flavor of what you have today
 e.g., fddi, fast ethernet
strengths
 simplicity; scalability; faster speed to attached devices
considerations
 sensitive to wiring installation quality
 upgrades may be required to hub and all stations
 adapter/CPU performance
 some problems cannot be solved with more bandwidth
--- latency! (bigger pipe doesn't change the
interconnection device--still use switches or routers)

25. cell switching (ATM)
ATM: a layer 2 technology based on cell switching
 low latency for high throughput
 multiple traffic types in cells--mixed voice, data, multimedia
scalable from low to high speeds
 25Mbps to ... 155Mbps? 622Mbps? 2.4Gbps?
 individual links can be different speeds
Quality of Service (QoS) allows (will allow) applications
to specify the network service characteristics they need
LAN Emulation allows applications to use ATM without
change

26. ATM
strengths
 mixed traffic (voice/video/data/multimedia)
 high speed; scalable speed
 very low latency
 Quality of Service support
 point to point technology allows broadcast
control (see IBM's MSS Server)
considerations
 cost
 complexity/learning curve

27. campus LAN interconnection
summary
interconnection devices: bridge, switch, router
 switches preferred today within campus
fast; low latency; easy implementation/administration
 routers good for controlling use of low speed WAN links
campus backbone technologies
 big pipes: fast ethernet, fddi
easy to deploy; faster speed to attached devices;
may or may not solve response time/performance
issues
 ATM
supports voice, video, data; gives true traffic control
for new applications; issues are cost, education