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Interconnection: Switching and Bridging. CS 4251: Computer Networking II Nick Feamster Spring 2008. In This Lecture. How hosts find each other on a subnet Address Resolution Protocol (ARP) Broadcast Interconnecting subnets Switches: Forwarding and filtering Self-learning bridges

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Interconnection switching and bridging

Interconnection: Switching and Bridging

CS 4251: Computer Networking IINick FeamsterSpring 2008

In this lecture
In This Lecture

  • How hosts find each other on a subnet

    • Address Resolution Protocol (ARP)

    • Broadcast

  • Interconnecting subnets

    • Switches: Forwarding and filtering

    • Self-learning bridges

    • Spanning tree protocols

  • Switches vs. Hubs

  • Swtiches vs. Routers

  • Can Ethernet scale to a million nodes?

    • VLANs

    • Other alternatives

Bootstrapping networks of interfaces



Bootstrapping: Networks of Interfaces

  • LAN/Physical/MAC address

    • Flat structure

    • Unique to physical interface (no two alike)…how?



link layer protocol




  • Frames can be sent to a specific MAC address or to the broadcast MAC address

What are the advantages to separating network layer from MAC layer?

Arp ip addresses to mac addresses
ARP: IP Addresses to MAC addresses

  • Query is IP address, response is MAC address

  • Query is sent to LAN’s broadcast MAC address

  • Each host or router has an ARP table

    • Checks IP address of query against its IP address

    • Replies with ARP address if there is a match

Potential problems with this approach?

  • Caching on hosts is really important

    • Try arp –a to see an ARP table

Life of a packet on a subnet
Life of a Packet: On a Subnet

  • Packet destined for outgoing IP address arrivesat network interface

    • Packet must be encapsulated into a frame with the destination MAC address

  • Frame is sent on LAN segment to all hosts

  • Hosts check destination MAC address against MAC address that was destination IP address of the packet

Interconnecting lans
Interconnecting LANs

  • Receive & broadcast (“hub”)

  • Learning switches

  • Spanning tree (RSTP, MSTP, etc.) protocols

Interconnecting lans with hubs
Interconnecting LANs with Hubs

  • All packets seen everywhere

    • Lots of flooding, chances for collision

  • Can’t interconnect LANs with heterogeneous media (e.g., Ethernets of different speeds)





Problems with hubs no isolation
Problems with Hubs: No Isolation

  • Scalability

  • Latency

    • Avoiding collisions requires backoff

    • Possible for a single host to hog the medium

  • Failures

    • One misconfigured device can cause problems for every other device on the LAN

Improving on hubs switches
Improving on Hubs: Switches

  • Link-layer

    • Stores and forwards Ethernet frames

    • Examines frame header and selectively forwards frame based on MAC dest address

    • When frame is to be forwarded on segment, uses CSMA/CD to access segment

  • Transparent

    • Hosts are unaware of presence of switches

  • Plug-and-play, self-learning

    • Switches do not need to be configured

Switch traffic isolation
Switch: Traffic Isolation

  • Switch breaks subnet into LAN segments

  • Switch filters packets

    • Same-LAN-segment frames not usually forwarded onto other LAN segments

    • Segments become separate collision domains


collision domain




collision domain

collision domain

Filtering and forwarding







Filtering and Forwarding

  • Occurs through switch table

  • Suppose a packet arrives destined for node with MAC address x from interface A

    • If MAC addressnot in table, flood (act like a hub)

    • If MAC addressmaps to A, do nothing (i.e. drop the packet - packet destined for same LAN segment)

    • If MAC address maps to another interface, forward

  • How does this table get configured?

Advantages vs hubs
Advantages vs. Hubs

  • Better scaling

    • Separate collision domains allow longer distances

  • Better privacy

    • Hosts can “snoop” the traffic traversing their segment

    • … but not all the rest of the traffic

  • Heterogeneity

    • Joins segments using different technologies

Disadvantages vs hubs
Disadvantages vs. Hubs

  • Delay in forwarding frames

    • Bridge/switch must receive and parse the frame

    • … and perform a look-up to decide where to forward

    • Storing and forwarding the packet introduces delay

    • Solution:cut-through switching (faster, but more errors are possible).

  • Need to learn where to forward frames

    • Bridge/switch needs to construct a forwarding table

    • Ideally, without intervention from network administrators (which creates overhead)

    • Solution:self-learning

Motivation for self learning
Motivation For Self-Learning

  • Switches forward frames selectively

    • Forward frames only on segments that need them

  • Switch table

    • Maps destination MAC address to outgoing interface

    • Goal: construct the switch table automatically






Self learning bridges
(Self)-Learning Bridges

  • Switch is initially empty

  • For eachincoming frame, store:

    • The incoming interface from which the frame arrived

    • The time at which that frame arrived

    • Delete the entry if no frames with a particular source address arrive within a certain time


Switch learns how to reach A.




Cut through switching
Cut-Through Switching

  • Buffering a frame takes time

    • Suppose L is the length of the frame

    • And R is the transmission rate of the links

    • Then, receiving the frame takes L/R time units

  • Buffering delay can be a high fraction of total delay, especially over short distances




Cut through switching1
Cut-Through Switching

  • Start transmitting as soon as possible

    • Inspect the frame header and do the look-up

    • If outgoing link is idle, start forwarding the frame

  • Overlapping transmissions

    • Transmit the head of the packet via the outgoing link

    • … while still receiving the tail via the incoming link

    • Analogy: different folks crossing different intersections




Limitations on topology
Limitations on Topology

  • Switches sometimes need to broadcast frames

    • Unfamiliar destination: Act like a hub

    • Sending to broadcast

  • Flooding can lead to forwarding loops and broadcast storms

    • E.g., if the network contains a cycle of switches

    • Either accidentally, or by design for higher reliability

Worse yet, packets can be duplicated and proliferated!

Solution spanning trees
Solution: Spanning Trees

  • Ensure the topology has no loops

    • Avoid using some of the links when flooding

    • … to avoid forming a loop

  • Spanning tree

    • Sub-graph that covers all vertices but contains no cycles

    • Links not in the spanning treedo not forward frames

Constructing a spanning tree
Constructing a Spanning Tree

  • Elect a root

    • The switch with the smallest identifier

  • Each switch identifies if its interfaceis on the shortest path from the root

    • And it exclude from the tree if not

    • Also exclude from tree if same distance,but higher identifier

  • Message Format: (Y, d, X) i.e.,

  • (Root-Distance-Node)

    • From node X

    • Claiming Y as root

    • Distance is d


One hop

Three hops

Steps in spanning tree algorithm
Steps in Spanning Tree Algorithm

  • Initially, every switch announces itself as the root

    • Example: switch X announces (X, 0, X)

  • Switches update their view of the root

    • Upon receiving a message, check the root id

    • If the new id is smaller, start viewing that switch as root

  • Switches compute their distancefrom the root

    • Add 1 to the distance received from a neighbor

    • Identify interfaces not on a shortest path to the root and excludethose ports from the spanning tree

Example from switch 4 s viewpoint
Example From Switch # 4’s Viewpoint


  • Switch #4 thinks it is the root

    • Sends (4, 0, 4) message to 2 and 7

  • Switch #4 hears from #2

    • Receives (2, 0, 2) message from 2

    • … and thinks that #2 is the root (lower id)

    • And realizes it is just one hop away

  • Switch #4 hears from #7

    • Receives (2, 1, 7) from 7

      • (i.e., 2 is the root, it’s 1 hop from “me”, I’m node 7.)

    • And realizes this is a longer path

      • because it adds 1 to the path from 7 – 2, wh/is already 1 hop, so 4-7-2 = 2 hops

    • So, prefers its own one-hop path (4-2 =1hop)

    • And removes 4-7 link from the tree







Robust spanning tree algorithm
Robust Spanning Tree Algorithm

  • Algorithm must react to failures

    • Failure of the root node

      • Need to elect a new root, with the next lowest identifier

    • Failure of other switches and links

      • Need to recompute the spanning tree

  • Root switch continues sending messages

    • Periodically reannouncing itself as the root (1, 0, 1)

    • Other switches continue forwarding messages

  • Detecting failures through timeout

  • Switch waits to hear from others

    • Eventually times out and claims to be the root

Extension virtual lans
Extension: Virtual LANs

  • Partition a single switched LAN into several virtual ones

    • Switched LANs do not scale well to large networks

    • Spanning tree algorithm has linear scaling behavior

    • Some frames are broadcast

  • Group users/hosts based on organizational structure, rather than physical location

    • Improve privacy and isolation

    • Exploit locality

  • Avoid physical rewiring

  • More in Lec. 12 (Plus, Network Layers as Link Layers)

Switches vs routers
Switches vs. Routers


  • Switches are automatically configuring

  • Forwarding tends to be quite fast, since packets only need to be processed through layer 2


  • Router-level topologies are not restricted to a spanning tree

    • Can even have multipath routing

Scaling ethernet
Scaling Ethernet

  • Main limitation: Broadcast

    • Spanning tree protocol messages

    • ARP queries

  • High-level proposal: Distributed directory service

    • Each switch implements a directory service

    • Hosts register at each bridge

    • Directory is replicated

    • Queries answered locally

  • …are there other ways to do this?