VLANs Port-based VLAN: switch ports grouped (by switch management software) so that single physical switch …… Switch(es) supporting VLAN capabilities can.

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Presentation transcript:

VLANs Port-based VLAN: switch ports grouped (by switch management software) so that single physical switch …… Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure. Virtual Local Area Network … Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) 15 … Electrical Engineering (VLAN ports 1-8) … … Computer Science (VLAN ports 9-16) … operates as multiple virtual switches

Port-based VLAN … Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) 15 … traffic isolation: frames to/from ports 1-8 can only reach ports 1- 8 – can also define VLAN based on MAC addresses of endpoints, rather than switch port  dynamic membership: ports can be dynamically assigned among VLANs router  forwarding between VLANS: done via routing (just as with separate switches)  in practice vendors sell combined switches plus routers

VLANS spanning multiple switches trunk port: carries frames between VLANS defined over multiple physical switches – frames forwarded within VLAN between switches can’t be vanilla frames (must carry VLAN ID info) – 802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports … Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) 15 … Ports 2,3,5 belong to EE VLAN Ports 4,6,7,8 belong to CS VLAN

Type 2-byte Tag Protocol Identifier (value: 81-00) Tag Control Information (12 bit VLAN ID field, 3 bit priority field like IP TOS) Recomputed CRC 802.1Q VLAN frame format frame 802.1Q frame

IP header IPsec header Secure payload IP header IPsec header Secure payload IP header IPsec header Secure payload IP header payload IP header payload headquarters branch office salesperson in hotel Public Internet laptop w/ IPsec Router w/ IPv4 and IPsec Router w/ IPv4 and IPsec Virtual Private Network (VPN)

Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: – no Media Access Control – no need for explicit MAC addressing – e.g., dialup link, ISDN line popular point-to-point DLC protocols: – PPP (point-to-point protocol) – HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!

PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame – carry network layer data of any network layer protocol (not just IP) at same time – ability to demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address

PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!

PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (e.g., PPP-LCP, IP, IPCP, etc)

PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error detection

Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern – Q: is received data or flag? Sender: adds (“stuffs”) extra byte after each data byte Receiver: – two bytes in a row: discard first byte, continue data reception – single : flag byte

Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

NRZI Encoded Flag Makes Synchronization Easy! HDLC: High-Level Data Link Control HDLC is a bit-oriented protocol Bit-Stuffing: Insert a zero after five consecutive ones. So six ones in a row means it must be a flag.