Local Area Networks & Hubs, Switches, Bridges

1. Local Area Networks&Hubs, Switches, Bridges Prof. A. Sahoo KReSIT

2. Ethernet Frame, MAC address • IP • IP Unicast address • IP Broadcast address • IP Multicast address • 802.3/Ethernet address type • MAC Unicast address • MAC Broadcast address • MAC Multicast Address

3. IP addresses and MAC addresses • An IP address is a 32-bit Network Layer (L3) address on the OSI model. It is configured on each IP host. • A MAC address is a 48-bit Data Link Layer (L2) address on the OSI model. It is typically “burned in” to the network interface card or equivalent, and is a combination of the manufacturer ID and the board ID (serial number). • An IP packet, with source and destination IP addresses, is encapsulated in an Ethernet frame, with source and destination MAC addresses.The Ethernet frame is then transmitted on the LAN segment. • On a LAN segment, hosts communicate with one another using MAC addresses, even though applications use IP addresses.–Therefore, each IP host must resolve the destination IP address to the destination MAC address before sending an IP packet.–This is done using the Address Resolution Protocol (ARP).

4. How ARP works • Host X needs to send an IP packet to host Y but only knows Y’s IP address. • sends an ARP Request message containing Y’s IP address, which is broadcast to all the hosts on the LAN segment. • –Remember that hosts communicate with each other using MAC addresses. • –This broadcast is a MAC broadcast, which means that the destination MAC address is a L2 broadcast address (all 48 address bits are ones). • –The source MAC address of this ARP Request message is X’s MAC address. • All hosts on the LAN segment receive the ARP Request message, but only Y recognizes the request as pertaining to its IP address. • The ARP Request message contains X’s MAC and IP addresses. • –All hosts make an entry with this mapping in their respective ARP caches. • Y sends a unicast ARP Reply message containing its MAC and IP addresses directly to X. • X now knows Y’s MAC and IP addresses, and makes a corresponding entry in its ARP cache. • Entries in ARP caches are designed to time out, typically after a few minutes. When this happens, the ARP process is repeated.

5. HUB • HUB • A hub is a L1 (physical layer) multi-port repeater. • –It receives a signal on one port, regenerates it, and transmits it out all ports. • –All devices connected to a hub receive any transmission on that hub, regardless of the intended recipient. • Two or more devices on a hub cannot transmit at the same time. • Because of these characteristics, a hub (or a group of hubs connected together) is known as a collision domain.

6. Bridges • Bridge • A bridge is more than just a repeater ( more Intelligent). It is a L2 (data link layer) bridge, which means that it is “aware” of L2 MAC addresses and functionnality • A switch keeps track of which devices are connected to which ports by maintaining a table of the MAC-address-to-switch-port mapping. • Transmissions on a switch are sent only to the intended recipients, determined by the destination MAC address. • Broadcasts are sent to all recipients, as they are intended to be. • For this reason, a switch (or a group of switches connected together) is known as a broadcast domain. • Bridge is a device to connect more than one LAN

7. why LANs need to be connected 1. connect 2 existing LANs (CS, math) -- different organizations want to be connected 2. LAN too big; need to split it, but stay connected -- too many stations or traffic for one LAN 3. connect geographically separate LANs. -- eg, 2 offices in different towns need connecting ( Remote Bridges, with PPP connection between bridges) 4. reduce collisions -- help increase efficiency 5. security --help restrict traffic to one LAN

8. Learning Bridges • Intelligent filtering • Necessary when the extended LAN grows large • Bridges are used to interconnect different LAN ( see next slide) • Internal table with Host --> Port no mapping • Table builds up dynamically • Bridges see inside MAC frame to know the source destination address and records along with incoming port no. • Entries are time stamped • Broadcasted in all ports when no entry is found

9. Spanning Tree Algorithm • Works fine until there is no loop • Sometime added on purpose for fault tolerance • In this case there may be infinite loop with a MAC frame • Solved by building a spanning tree ( subgraph encompassing all vertices,having only one path between the vertices, no bridges) • Perlman algorithm (Peterson, page 193) • Problems: bandwidth wastage, only one path is being used. • How to handle--- source route bridge is one way. Not very widely deployed

10. Spanning Tree Formation • Each bridge has id. • Every body sends config mesg: • Initially each bridge thinks itself a root, send configuration message having : ( enters the same in its table) • It’s own id as root • Distance as zero • A received ( at any bridge x) config message replaces ( considered better) entry for best root in x, if • It identifies a root with smaller id or • Equal id with smaller distance or • Root id and distance is same but the sending bridge has smaller id.

11. Spanning Tree Formation • A node stops send its own config mesg as root if it has already decided that it is not root. Then it only forwards config messages from other node by adding a distance 1 to it.

12. Limitation of Bridges based solution • Scalability • Spanning tree algorithm not scalable, the configuration messages flood entire extended LAN. • Extended LAN becomes single L2 broadcast domain. • Separation of traffic: • Logical workgroups but geographically separated, want to be on a same LAN without the above problems • Could be connected with a router ( with different IP subnet) • Speed and latency problem

13. Virtual LAN W X Vlan 200 • Operation • Extended LAN in partitioned into virtual groups • Each group given an id ( by administrator; marketing group, admin group) • Each port of the switch belongs to atleast one vlan id • A host belongs to one VLAN • In the adjacent diagram packets from vlan100 is never sent to vlan200 • Configuration can be manual. There is proposal for semi automatic group membership • Dynamic topology: • Membership can be changed without changing physical topology. Z can belong to vlan100. Only B2 needs a new entry. Vlan 100 B1 B2 Vlan 200 Vlan 100 Y Z

14. Virtual LAN • Tags • Ethernet frames need to be tagged • Protocol: 802.1 Q

15. 802.1 Q • The tag is inserted after Ethernet header either by host or Ingress switch • Priority bits are to implement 802.1 p • 8 priority classes • Implementation not specified

16. configuration • The VLANs must be configured independently on each switch, using any of the following methods. • manually via the CLI or web interface. • with a VLAN management tool provided by the vendor. • automatically with a standard protocol like GVRP (GARP VLAN Registration Protocol), which works in conjunction with 802.1Q. • automatically with a proprietary protocol like Cisco’s VTP (Virtual Trunking Protocol), which works in conjunction with Cisco’s proprietary ISL (Inter-Switch Link) trunking protocol.