14 848 VIRTUAL LANS LECTURE 16 FALL 2018

14 -848: VIRTUAL LANS LECTURE 16 * FALL 2018 * KESDEN

SOCRATIVE • https: //api. socrative. com/rc/Nfu 6 Lp

WHERE ARE WE? • Virtual LANs (VLANs) • Note: We’d normally have covered this before moving up to the transport layer, but slightly “inverted” the schedule to give you more time on Project #2. • Application • Transport • Network • Link • Physical

REVIEW: LOCAL AREA NETWORKS (LANS) • Originally described a network that was local in space, i. e. a small network • These almost universally used one shared communication fabric • As distinguished from Wide Area Network (WAN), which often described (and still described) networks over larger distances, often times connected by leased lines, e. g. service from a telco. • For decades LAN has been used to describe a network that has one shared communications fabric • Regardless of size

REVIEW: HOW BIG CAN A LAN BE? • Originally, the size of a LAN was very limited by utilization • Broadcasting + limited network time means they top out at ~30% utilization • Modern switches limit broadcasting and enable more stations. • But, there is still a collision domain, so scaling still has limits • There are other reasons for multiple LANs • Administrative domains • Security concerns • Different technologies

REVIEW: NETS AND SUBNETS: ALTERNATIVES TO LARGE LANS • Benefits: • Hierarchical addresses enable greater scale • Technology specific link-layer and protocols hidden underneath enabling the joining of different technologies • Different administrative domains can provide own services, firewall policies, etc • Costs: • Big one is IP address space fragmentation • Administrative costs

VLANS PROVIDE BIG BENEFITS • Large flat network • Limit traffic to VLAN • Limits scope of broadcasts and thereby impact upon congestion • Limits scope of transmissions, and thereby who can learn ID of hosts and about their traffic • Can cross geographic boundaries, extending VLANs beyond traditional LAN limits

VLANS: BIG PICTURE BENEFIT • Build out one large switched network • Configure it to act like any number of LANs • But without any of the geographic limitations

VLANS: PORT-BASED • Static VLAN: VLAN=Group of Ports • Port = switches’ wire connection • Two VLANs configured on a 16 -port switch • How do the VLANs communicate with each other? Courtesy Bill Nace, 14 -740

VLANS: TRUNKED SWITCHES • Trunked connection: port belongs to all VLANs ➙ all frames at that port are forwarded to all VLANs • But, how does the receiving side know which VLAN a particular frame belongs to? Courtesy Bill Nace, 14 -740

802. 1 Q TAGGED ETHERNET • VLAN identifier added to Ethernet frame • 4 -byte VLAN tag • Includes 12 -bit VLAN identifier • Sending switch adds tag, receiving switch parses and removes tag Courtesy Bill Nace, 14 -740