Computer Networking CEN CN 1
Layer 2 - Bridge Frame Header Data Bridge Trailer Data Link Layer Physical Layer More complex than repeat, it requires hardware and software Bridges have diminished because of the widespread use of switch technology. Operate on frames, PDU= frames and based their decision on the destination address of the frame. CEN CN 2
Layer 2 - Bridge Node Node Server Node Bridge Node Node Server Node Filtering is when the bridge does not forward the frame to the other bridge ports. Forwarding is sending the frame out another port to the appropriate segment. Isolate traffic to local LAN segment to increase the overall performance of a network. CEN CN 3
Transparent Bridge The endpoint devices do not need to know that the bridges exist. Three key functions. Learning. Forwarding or filtering. Spanning-tree protocol. Mac address table MAC Address Associated Port 1 2 1 CEN CN 4
Bridge Operations A frame is received. If the destination is a broadcast or multicast, forward on all ports. If the destination is a unicast and the address is not in the address table, forward on all ports. If the destination is a unicast and the address is in the address table, forward the frame out the associated port, unless the MAC address is associated with the incoming port. CEN CN 5
Transparent Bridge Summary Broadcast & multicast are forwarded by a bridge Layer 2 header & addresses Layer 3 protocol independent Different collision domain One broadcast domain Store-and-forward operation Latency (compare to a single-lan segment) CEN CN 6
Spanning Tree Protocol (STP) Network Redundant Links without STP Physically redundant paths exist. PcA B3 PcC. PcA B2 b1 PcC. Q: PcA sends a single unicast frame to PcC, but PcC is powered off and none of bridges has learned PcC s address. Frame to PcC will loop forever. CEN CN 7
Spanning Tree Protocol (STP) STP blocks some ports so that only one active path exists between any pair of LAN segment (collision domains) to dynamically create a bridge/switched network. Set the bridge port state, forward or block? Benefit Redundant Link in case of failure of one link Only one active link Loop avoided CEN CN 8
Switch Multi-port Bridge Selection Issues: Speed Media or user interface Need for trunking and interswitch links Workgroup segmentation (VLANs) Port density needs Product Category Desktop/Workgroup Level Cisco Catalyst 1900, 2900 XL series Wiring Closet/Backbone Level Cisco Catalyst 5000 series CEN CN 9
Switch - Method Store-and-forward. The switch fully receives all bits in the frame before forwarding the frame. This allows the switch to check the FCS before forwarding the frame. Cut-through. The switch performs the address table lookup as soon as the destination address field in the header is received. The first bit in the frame can be sent out the outbound port before the final bits in the incoming frame are received. Fragment-free. This performs like cut-through, but the switch waits for 64 bytes to be received before forwarding the first bytes of the outgoing frame. According to the Ethernet specifications, collisions should be detected during the first 64B of the frame. CEN CN 10
Traditional LAN vs. VLAN CEN CN 11
VLAN A group of ports or users in the same broadcast domain LAN switches and network management software provide a mechanism to create VLANs Remove Physical Boundary Group users by department, team, or application Routers provide communication between VLANs CEN CN 12
VLAN Frame tagged with VLAN ID Trunking CEN CN 13
Port-based VLAN Types of VLANs Example File Server VLAN1: Port 1,2,7,13 VLAN2: Port 3,4,8 VLAN3: Port 5,6,9 13 1 2 3 4 5 6 7 8 9 10 11 12 A B C D E F G H CEN CN 14
Mac-address Based VLAN Types of VLANs Example MAC address A: 123456789ABC B: 123456789DEF C: 123456789GHI 13 File Server 1 2 3 4 5 6 7 8 9 10 11 12 VLAN1: 123456789ABC 123456789DEF Switch 1 2... Hub A B C D E F CEN CN 15