Routing and Bridging

1
Routing and Bridging

• Packet Delivery Mechanisms

2
Bridges

• Are multi-homed machines
• Forward frames between the different networks in
  which they are members
• Operate at the data link layer
• Forward network frames (physical-level frames)
• Use physical (e.g. MAC) addresses
• Ethernet to FDDI, Ethernet to ATM, and Ethernet
  to 802.11g bridges exist (as well as many others)
• Read, interpret, and forward packets
• Ensure signals represent real data
• Unlike repeaters, which simply boost any signal
• Normally just interconnect two or more LANs
  within an organization

3
Bridges
LAN 1
LAN 2
F
F
F
B
LAN 4
LAN 3

• The bridge B, that simply retransmits packets
  onto all LANs other than the one on which the
  packet was received, is sometimes called a no
  frills bridge

4
Learning Bridges

• Learning bridges are similar in operation to no
  frills bridges, with one exception
• They waste less bandwidth
• Learning bridges learn which networks contain
  which nodes
• Learning bridges keep a cache of frames and their
  destination port
• The first time an address is seen, the frame is
  sent to every port (except the one on which the
  frame was received)
• When a frame is received with a source address
  A, A is placed into the destination cache for
  some (possibly unlimited) amount of time

5
Learning Bridges
LAN 1
LAN 2
F1
1
F1
F2
F1
2
F2
F1
F2
B
F2
3
4
LAN 4
LAN 3
F1 D LAN2-B S LAN1-A
F2 D LAN1-A S LAN2-B
6
Learning Bridges
LAN 1
LAN 2
F2
1
2
F2
B
F2
3
4
LAN 4
LAN 3
F2 D LAN1-A S LAN2-B
7
Routers

• Are multi-homed machines
• Forward packets between the different networks in
  which they are connected
• Operate at the Network (or Transport) layer(s)
• Forward network (e.g. IP) datagrams
• Use network (e.g. IP) addresses
• Read, interpret, process, modify, and forward
  datagrams
• Make on-the-fly changes such as re-fragmentation
  to packet data
• Use complicated algorithms to determine the best
  path on which to forward the packet
• Consider elements such as network congestion

8
Routers

• Usually connect to several networks, including
  backbone networks of inter-connected routers
• These networks may be of different (data link)
  types
• e.g. Ethernet, Token Ring, FDDI
• Each of these networks will normally share a
  common high-level protocol
• e.g. IP

9
Routers

• Routers are concerned with high-level delivery
• Delivery of messages in a specific way,
  independent of the types of networks to which
  they are connected
• There are two types of routing
• Direct delivery
• Indirect delivery

10
Router Pricing

• Routers range in price quite dramatically
• 720Gbps (40 Gbps x 18 slots)
• Supports 10/100 Ethernet to 10Gbps Ethernet
• Around 10,000
• 1280Gbps (40Gbps x 32 slots)
• Supports FDDI, ATM, Ethernet, Token Ring nets
• Supports RIP, OSPF, IS-IS, BGP, MPLS
• Supports IPv4, IPv6, IPX, IPX
• Around 200,000

11
Direct Delivery

• Direct delivery can occur when the machines share
  the same network
• In cases of direct routing, the packet is simply
  placed on the network as a network frame
• Direct delivery uses physical addressing to get
  packets to their intended destination
• In some sense, bridging can be thought of as a
  form of direct delivery

12
Indirect Delivery

• Indirect delivery is used when a direct
  connection with the destination does not exist
• i.e. the holder of the packet and the destination
  are not on the same network
• Indirect delivery can involve dynamic algorithms
  to determine the next hop in the packets route
• Often these algorithms rely on tables
• Indirect delivery is the subject of most routing
  discussions in this course

13
Routing Tables

• As mentioned previously, routing tables contain
  entries used to direct packets
• These entries relate network addresses with the
  network link that should be used to arrive at
  that address
• Network addresses are used since the paths for
  different machines on the same network from any
  given location, differs very little
• In fact, the only difference is at the end of the
  route, where direct routing is used to deliver
  the message on the destination network

14
A High Level View of Routing
ANSNet
U of M MAN
IBM WAN
Computer Science LAN
Administration LAN
Development LAN
Engineering LAN
Dr. Jones
Mr. Fromm
15
A High Level View of Routing
University of Michigan MAN Backbone
ANSNet
Engineering LAN
Computer Science LAN
Dr. Jones
16
A High Level View of Routing
U of M MAN
IBM WAN
17
A High Level View of Routing
IBM WAN Backbone
ANSNet
Administration LAN
Development LAN
Mr. Fromm
18
Default Routes

• Frequently, routing tables may contain default
  routes
• Table entries which match any network address, in
  the event that another table entry was not found
• These table entries are used by routers of wide
  area networks to represent their connection with
  a larger network (e.g. the Internet)

19
IP Routing

• A specific type of routing used in IP networks
• Use IP addresses, which have 2 layers
• The first layer can normally be used alone by
  routers to determine the correct hop
• End-level routers use the network address (N) to
  pass along the packet by sending it as a network
  frame on the correct network (identified by N)

20
IP Routing Algorithm

• Use subnet mask to compute the network address
  (N) for the datagrams destination address (D)
• If N is directly connected to this router,
  forward the packet at a network frame
• Check the table for an entry for N
• If one exists, lookup the port associated with N
  in the table entry (P), forward the datagram to P
• If one does not exist
• If a default route exists (D), forward the
  datagram to D
• Else send an error message (using ICMP)

21
Routing History

• Routing tables were originally managed manually
• As new machines or networks were added,
  administrators added new routing table entries to
  compensate
• The need for dynamic, auto-configurable routing
  tables was obvious by the late 1980s when many
  networks were joining the Internet
• At this time, routers were called gateways

22
Source Routing

• In rare cases, a host may wish to provide its
  own route
• The header of the packet will contain a list of
  routers IP addresses
• Each router will simply pass the packet to the
  next router in the list