N5 datalink3 switching

Computer Networks : 

Data link layer Computer Networks

Overview: 

Overview Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs

Multiple LANs?: 

Multiple LANs? Why different LANs in a single organisation? Autonomy of owners Distance too great for a single LAN Load too high for a single LAN Reliability: a single defective node will cripple onle a single LAN Security: most LANs offer a promiscuous mode (receive all packets) Solution at data link layer: Bridge switch

From 802.x to 802.y: 

From 802.x to 802.y How do bridges work?

Bridges: from 802.x to 802.y: 

Bridges: from 802.x to 802.y Common problems: different data rates maximum frame length security quality of service frame formats

Overview: 

Overview Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs

Transparent bridges: 

Transparent bridges Goal: interconnect LANs with a bridge and everything should work perfectly, instantly No software changes No hardware changes No setting of address switches No downloading of routing tables How? Promiscuous mode receive packet… Source LAN = destination LAN  discard frame Destination LAN unknown  flooding (on LAN level) Destination LAN known  forward frame Learn about LANs? Backward learning: construct routing table Initially empty Receive frame from node A via LAN L  A reachable via L Dynamic topologies: attach timestamp to entries

Transparent bridges: 

Transparent bridges A B Bridge L1 L2 frame L3

Transparent bridges: 

Transparent bridges A B Bridge L1 L2 L3

Transparent bridges: 

Transparent bridges A B Bridge L1 L2 L3

Spanning Tree bridges: 

Spanning Tree bridges Bridges in parallel To increase reliability  loops in topology To increase forwarding capacity Problem Frame forwarding/flooding forever Solution: Spanning tree Bridges

Transparent bridges: 

Transparent bridges Spanning tree bridges: Solution transparent to hosts Bridges construct spanning tree one path from every LAN to every other LAN All forwarding follows spanning tree Spanning tree construction Select root of tree: bridge with lowest serial number (unique by construction; broadcast is used to inform all bridges) Distributed algorithm to add (shortest) paths

Transparent bridges: 

Transparent bridges Spanning tree bridges

Remote bridges: 

Remote bridges Interconnect distant LAN Bridge on each LAN Point-to-point line between bridges Routing? No difference Point-to-point line = LAN without hosts Protocol Select point-to-point protocol Inside frame Complete MAC frame Only LLC frame (without MAC header, trailer)

Overview: 

Overview Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs

Interconnections <> layers: 

Interconnections <> layers Which device in which layer? Frames, packets and headers

Interconnections <> layers: 

Interconnections <> layers Repeater Analog devices to interconnect cable segments Signal on one segment is amplifies and put on other segment Not aware of frames, addresses Hub Frames arriving on one line are sent out on all the other lines A single collision domain Not aware of frames, addresses Physical layer

Interconnections <> layers: 

Interconnections <> layers Bridge Interconnects LANs Each line has its own collision domain Switch Interconnects hosts cut-through switches Forwarding starts when header is received Difference? Data link layer

Overview: 

Overview Design issues Point-to-point links Local area Networks Data Link layer Switching Multiple LANs? From 802.x to 802.y Transparent bridges Spanning Tree bridges Remote bridges Interconnections <> layers Virtual LANs

Virtual LANs: 

Virtual LANs Example of a wiring infrastructure Logical or physical configuration of LANs?

Virtual LANs: 

Virtual LANs LANs should to reflect organizational structure Security Load Broadcasting Solutions? LAN = hub + rewiring Rewiring in software  VLAN Implemented by specially-designed VLAN aware switches/bridges

Virtual LANs: 

Virtual LANs Example VLANs: White & Gray How to forward a (broadcast) packet sent out by A?

Virtual LANs: 

Virtual LANs How to forward a (broadcast) packet sent out by A? Forwarded by bridge/switch on all Gray ports What is the color of an incoming frame?

Virtual LANs: 

Virtual LANs Coloring incoming frames? VLAN color assigned to Port Does not work for bridges MAC address Use layer 3 Could be useful Violates independence of layers Color frames  IEEE 802.1Q Changes in Ethernet header to support VLANs

IEEE 802.1Q: 

IEEE 802.1Q Bridges/switches fill in the VLAN color Used on lines interconnecting bridges/switches Future ethernet cards will be 802.1Q compliant? Max length increased from 1518 to 1522 bytes

IEEE 802.1Q: 

IEEE 802.1Q VLAN fields: VLAN protocol ID (>max length) Pri: hard real-time <> soft real-time <> time-insensitive CFI: Canonical Format indicator: 802.5 frame

IEEE 802.1Q: 

IEEE 802.1Q Configuration of bridges/switches Autoconfiguration If only 802.1Q frames arrive Learning Manual For mixed installations

Computer Networks : 

Data link layer Computer Networks