class19

Hubs, Bridges & Routers: 

Hubs, Bridges & Routers Hub: Active central element in a star topology Twisted Pair: inexpensive, easy to insall Simple repeater in Ethernet LANs “Intelligent hub”: fault isolation, net configuration, statistics Requirements that arise: Hub Station Station Station Two Twisted Pairs User community grows, need to interconnect hubs ? Hubs are for different types of LANs

Hubs, Bridges & Routers: 

Hubs, Bridges & Routers Interconnecting Hubs Repeater: Signal regeneration All traffic appears in both LANs Bridge: MAC address filtering Local traffic stays in own LAN Routers: Internet routing All traffic stays in own LAN ? Higher Scalability

General Bridge Issues: 

Operation at data link level implies capability to work with multiple network layers However, must deal with Difference in MAC formats Difference in data rates; buffering; timers Difference in maximum frame length PHY MAC LLC Network Network PHY MAC LLC 802.3 802.3 802.5 802.5 802.3 802.3 802.3 802.5 802.5 802.5 CSMA/CD Token Ring General Bridge Issues

Bridges of Same Type: 

Bridges of Same Type Common case involves LANs of same type Bridging is done at MAC level

Transparent Bridges: 

Interconnection of IEEE LANs with complete transparency Use table lookup, and discard frame, if source & destination in same LAN forward frame, if source & destination in different LAN use flooding, if destination unknown Use backward learning to build table observe source address of arriving LANs handle topology changes by removing old entries Transparent Bridges

Slide6: 

B1 S1 S2 B2 S3 S4 S5 Port 1 Port 2 Port 1 Port 2 LAN1 LAN2 LAN3

S1→S5: 

B1 S1 S2 B2 S3 S4 S5 Port 1 Port 2 Port 1 Port 2 LAN1 LAN2 LAN3 Address Port S1 1 Address Port S1 1 S1→S5 S1 to S5 S1 to S5 S1 to S5 S1 to S5

S3→S2: 

B1 S1 S2 B2 S3 S4 S5 Port 1 Port 2 Port 1 Port 2 LAN1 LAN2 LAN3 Address Port S1 1 S3 1 Address Port S1 1 S3 1 S3→S2 S3S2 S3S2 S3S2 S3S2 S3S2

S4S3: 

B1 S1 S2 B2 S3 S4 S5 Port 1 Port 2 Port 1 Port 2 LAN1 LAN2 LAN3 S4 S3 Address Port S1 1 S3 2 S4 2 Address Port S1 1 S3 1 S4 2 S4S3 S4S3 S4S3 S4S3

S2S1: 

B1 S1 S2 B2 S3 S4 S5 Port 1 Port 2 Port 1 Port 2 LAN1 LAN2 LAN3 Address Port S1 1 S3 2 S4 2 S2 1 S2S1 S2S1 S2S1

Adaptive Learning: 

Adaptive Learning In a static network, tables eventually store all addresses & learning stops In practice, stations are added & moved all the time Introduce timer (minutes) to age each entry & force it to be relearned periodically If frame arrives on port that differs from frame address & port in table, update immediately

Avoiding Loops: 

Avoiding Loops

Spanning Tree Algorithm: 

Spanning Tree Algorithm Select a root bridge among all the bridges. root bridge = the lowest bridge ID. Determine the root port for each bridge except the root bridge root port = port with the least-cost path to the root bridge Select a designated bridge for each LAN designated bridge = bridge has least-cost path from the LAN to the root bridge. designated port connects the LAN and the designated bridge All root ports and all designated ports are placed into a “forwarding” state. These are the only ports that are allowed to forward frames. The other ports are placed into a “blocking” state.

Slide14: 

LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (1) (1) (1) (2) (2) (2) (2) (3)

Slide15: 

LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (1) (1) (1) (2) (2) (2) (2) (3) Bridge 1 selected as root bridge

Slide16: 

LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (1) (1) (1) (2) (2) (2) (2) (3) Root port selected for every bridge except root port R R R R

Slide17: 

LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (1) (1) (1) (2) (2) (2) (2) (3) Select designated bridge for each LAN R R R R D D D D

Slide18: 

LAN1 LAN2 LAN3 B1 B2 B3 B4 B5 LAN4 (1) (2) (1) (1) (1) (1) (2) (2) (2) (2) (3) All root ports & designated ports put in forwarding state R R R R D D D D

Source Routing Bridges: 

Source Routing Bridges To interconnect IEEE 802.5 token rings Each source station determines route to destination Routing information inserted in frame

Route Discovery: 

Route Discovery To discover route to a destination each station broadcasts a single-route broadcast frame Frame visits every LAN once & eventually reaches destination Destination sends all-routes broadcast frame which generates all routes back to source Source collects routes & picks best

Detailed Route Discovery: 

Detailed Route Discovery Bridges must be configured to form a spanning tree Source sends single-route frame without route designator field Bridges in first LAN add incoming LAN #, its bridge #, outgoing LAN # into frame & forwards frame Each subsequent bridge attaches its bridge # and outgoing LAN # Eventually, one single-route frame arrives at destination When destination receives single-route broadcast frame it responds with all-routes broadcast frame with no route designator field Bridge at first hop inserts incoming LAN #, its bridge #, and outgoing LAN # and forwards to outgoing LAN Subsequent bridges insert their bridge # and outgoing LAN # and forward Before forwarding bridge checks to see if outgoing LAN already in designator field Source eventually receives all routes to destination station

Find routes from S1 to S3: 

Find routes from S1 to S3

Virtual LAN: 

Physical partition Logical partition Bridge or switch VLAN 1 VLAN 2 VLAN 3 S1 7 2 3 4 5 6 1 8 9 Floor n – 1 Floor n Floor n + 1 S2 S3 S4 S5 S6 S7 S8 S9 Virtual LAN

Per-Port VLANs: 

Logical partition Bridge or switch VLAN 1 VLAN 2 VLAN 3 S1 7 2 3 4 5 6 1 8 9 Floor n – 1 Floor n Floor n + 1 S2 S3 S4 S5 S6 S7 S8 S9 Per-Port VLANs Bridge only forwards frames to outgoing ports associated with same VLAN

Tagged VLANs: 

Tagged VLANs More flexible than Port-based VLANs Insert VLAN tag after source MAC address in each frame VLAN protocol ID + tag VLAN-aware bridge forwards frames to outgoing ports according to VLAN ID VLAN ID can be associated with a port statically through configuration or dynamically through bridge learning IEEE 802.1q