- VLANs(Virtual LANs)
- connections called bridges to join lans together
Uses of Bridges
- Reasons why an organization may have multiple LANs
- Many university and corporate departments have their own LANs
- Organization geographically spread wants to be joined
- Split a single LAN into separate LANs for logical organization
- bridges let LANs be joined while keeping their own capacity
- bridges increases reliability
- can reroute nodes that are malfunctioning giving garbage data
- transparent bridges
- create with backward learning algorithm to stop traffic from being sent to unneeded areas and spanning tree algorithm to break loops
Learning Bridges
- example topology
- bridges developed when classic ethernets were in use
- bridge operates in promiscuous mode accepts every frame to its ports
- chooses to forward or discard
- simple way to implement this scheme is to have a big hash table
- initially empty
- backward learning every frame sent tells the bridge about the topology
- arrival time of frames noted in entry, tells the bridge the last time a frame is seen
- bridge scans and purges entries more than a few minutes old
- routing procedure for incoming frame
- if the port for destination is same as source discard frame
- if destination port is different from source forward to destination
- if destination unknown send frame on all ports except the source
- as each frame arrives algorithm must be applied
- cut through switching/wormhole routing
- bridges only look at MAC addresses to reduce latency
Spanning Tree Bridges
- to increase reliability redundant links used between bridges
- this introduces loops in the topology
- so in order to do this, overlay the topology with a spanning tree to reach every bridge, ignore some links, example as follows
- spanning tree is built with distributed algorithm, bridge periodically broadcasts a configuration message on all ports
- tree of shortest paths from root to every bridge constructed, and spanning tree used between bridges
- Radia Perlman poem
Repeaters, Hubs, Bridges, Switches, Routers and Gateways
- These devices operate in different layers which is why even though they perform similar roles, they are all necessary
- repeaters amplify signals
- hubs don't amplify, but allow for multiple lines
- bridges connects two or more LANs has multiple ports
- isolated in its own collision domain, CSMA/CD not needed
- better performance than hubs
- can join different types of LANs such as Ethernet and Token Ring
- joins different max frame lengths
- join different security protocols
- switches are modern bridges by another name
- utilizes twisted pair cables, often used for ethernet
- routers totally different
- frame header and trailer gets stripped off and the packet located in payload field is passed to routing software
- chooses outgoing line with address
- does not see fram address or where packet comes in on
- transport gateways connect two computers using different connection oriented protocols such as
- TCP/IP
- SCTP
- understands formats such as email into sms messages
Virtual LANs
- early days of LAN setup was designed in this way
- issues
- difficult to replace, geographically new workers have to sit at open workstations instead of with their coworkers
- load issue not even
- broadcast traffic to reach correct persons
- vulnerable to broadcast storm when LAN malfunctions and sends random messages
- solution is to set up VLAN(Virtual LAN)
- based on VLAN aware switches
- network admin decides how many VLANs are possible
- association as configuration tables are built into bridges
- The IEEE 802.1Q Standard
- To implement this scheme bridges need to know which VLAN an incoming
- issues
- do we need to throw out existing ethernet cards to accomodate this standard?
- who generates fields?
- what happens to max size frames
- VLAN fields are only used by bridges and switches and not by user machines
- bridge is also allowed to use higher layer protocol to select colors for VLAN creation
- for 802.1Q Ethernet frames, the tag composed of
- 3 bit priority field
- CFI(Canonical Format Indicator)
- indicate order of bits in MAC adress
- VLAN Identifier specifies color of VLAN
- when this comes to VLAN aware switch the identifier is matched to a table to figure out which ports it should send to
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