CS Networks and Distributed Systems. Lecture 5: Bridging. Revised 1/14/13
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1 CS 3700 Networks and Distributed Systems Lecture 5: Bridging Revised 1/14/13
2 Just Above the Data Link Layer 2 Application Presentation Session Transport Network Data Link Physical Bridging! How do we connect LANs? Function:! Route packets between LANs Key challenges:! Plug-and-play, self configuration! How to resolve loops
3 Recap 3 Originally, Ethernet was a broadcast technology Terminator Tee Connector Repeater
4 Recap 3 Originally, Ethernet was a broadcast technology Terminator Tee Connector Repeater
5 Recap 3 Originally, Ethernet was a broadcast technology Terminator Tee Connector Repeater
6 Recap 3 Originally, Ethernet was a broadcast technology Terminator Tee Connector Pros: Simplicity! Hardware is stupid and cheap Cons: No scalability! More hosts = more collisions = pandemonium Repeater
7 The Case for Bridging 4 Need a device that can bridge different LANs! Only forward packets to intended recipients! No broadcast! A B C
8 The Case for Bridging 4 Need a device that can bridge different LANs! Only forward packets to intended recipients! No broadcast! Send Packet B! C A B C
9 The Case for Bridging 4 Need a device that can bridge different LANs! Only forward packets to intended recipients! No broadcast! Send Packet B! C A B C
10 The Case for Bridging 4 Need a device that can bridge different LANs! Only forward packets to intended recipients! No broadcast! Send Packet B! C A A B B Bridge C C
11 The Case for Bridging 4 Need a device that can bridge different LANs! Only forward packets to intended recipients! No broadcast! Send Packet B! C A Send Packet B! C A B B Bridge C C
12 The Case for Bridging 4 Need a device that can bridge different LANs! Only forward packets to intended recipients! No broadcast! Send Packet B! C A Send Packet B! C A B B Bridge C C
13 Bridging the LANs 5 Bridging limits the size of collision domains
14 Bridging the LANs 5 Bridging limits the size of collision domains
15 Bridging the LANs 5 Bridging limits the size of collision domains
16 Bridging the LANs 5 Bridging limits the size of collision domains! Vastly improves scalability! Question: could the whole Internet be one bridging domain?
17 Bridging the LANs 5 Bridging limits the size of collision domains! Vastly improves scalability! Question: could the whole Internet be one bridging domain? Tradeoff: bridges are more complex than hubs! Physical layer device vs. data link layer device! Need memory buffers, packet processing hardware, routing tables
18 Bridge Internals 6 Inputs Bridge Outputs Switch Fabric
19 Bridge Internals 6 Inputs Bridge Outputs Switch Fabric Memory buffer
20 Bridge Internals 6 Inputs Bridge Outputs Switch Fabric Makes routing decisions
21 Bridge Internals 6 Inputs Bridge Outputs Switch Fabric
22 Bridge Internals 6 Inputs Bridge Outputs Switch Fabric
23 Bridge Internals 6 Inputs Bridge Outputs Switch Fabric
24 Bridge Internals 6 Inputs Bridge Outputs Switch Fabric Bridges have memory buffers to queue packets
25 Bridge Internals 6 Bridge Inputs Outputs Switch Fabric Bridges have memory buffers to queue packets Bridge is intelligent, only forwards packets to the correct output
26 Bridge Internals 6 Bridge Inputs Outputs Switch Fabric Bridges have memory buffers to queue packets Bridge is intelligent, only forwards packets to the correct output Bridges are high performance, full N x line rate is possible
27 Bridges 7 Original form of Ethernet switch Connect multiple IEEE 802 LANs at layer 2 Goals! Reduce the collision domain! Complete transparency Plug-and-play, self-configuring No hardware of software changes on hosts/hubs Should not impact existing LAN operations
28 Bridges 7 Original form of Ethernet switch Connect multiple IEEE 802 LANs at layer 2 1. Forwarding of frames 2. Learning of (MAC) Addresses 3. Spanning Tree Algorithm (to handle loops) Goals! Reduce the collision domain! Complete transparency Plug-and-play, self-configuring No hardware of software changes on hosts/hubs Should not impact existing LAN operations
29 Frame Forwarding Tables 8 Each bridge maintains a forwarding table MAC Address Port Age 00:00:00:00:00:AA 1 1 minute 00:00:00:00:00:BB 2 7 minutes 00:00:00:00:00:CC 3 2 seconds
30 Frame Forwarding Tables 8 Each bridge maintains a forwarding table MAC Address Port Age 00:00:00:00:00:AA 1 1 minute 00:00:00:00:00:BB 2 7 minutes 00:00:00:00:00:CC 3 2 seconds
31 Frame Forwarding Tables 8 Each bridge maintains a forwarding table MAC Address Port Age 00:00:00:00:00:AA 1 1 minute 00:00:00:00:00:BB 2 7 minutes 00:00:00:00:00:CC 3 2 seconds
32 Frame Forwarding Tables 8 Each bridge maintains a forwarding table MAC Address Port Age 00:00:00:00:00:AA 1 1 minute 00:00:00:00:00:BB 2 7 minutes 00:00:00:00:00:CC 3 2 seconds 00:00:00:00:00:DD 1 3 minutes
33 Frame Forwarding Tables 8 Each bridge maintains a forwarding table MAC Address Port Age 00:00:00:00:00:AA 1 1 minute 00:00:00:00:00:BB 2 7 minutes 00:00:00:00:00:CC 3 2 seconds 00:00:00:00:00:DD 1 3 minutes
34 Frame Forwarding in Action 9 Port 1 Port 4 Port 2 Port 3 Assume a frame arrives on port 1
35 Frame Forwarding in Action 9 Port 1 Port 4 Port 2 Port 3 Assume a frame arrives on port 1
36 Frame Forwarding in Action 9 Port 1 Port 4 Port 2 Port 3 Assume a frame arrives on port 1 If the destination MAC address is in the forwarding table, send the frame on the correct output port
37 Frame Forwarding in Action 9 Port 1 Port 4 Port 2 Port 3 Assume a frame arrives on port 1 If the destination MAC address is in the forwarding table, send the frame on the correct output port
38 Frame Forwarding in Action 9 Port 1 Port 4 Port 2 Port 3 Assume a frame arrives on port 1 If the destination MAC address is in the forwarding table, send the frame on the correct output port If the destination MAC isn t in the forwarding table, broadcast the frame on all ports except 1
39 Frame Forwarding in Action 9 Port 1 Port 4 Port 2 Port 3 Assume a frame arrives on port 1 If the destination MAC address is in the forwarding table, send the frame on the correct output port If the destination MAC isn t in the forwarding table, broadcast the frame on all ports except 1
40 Learning Addresses 10 Manual configuration is possible, but! Time consuming! Error Prone! Not adaptable (hosts may get added or removed) 00:00:00:00:00:AA Port 1 Port 2 00:00:00:00:00:BB
41 Learning Addresses 10 Manual configuration is possible, but! Time consuming! Error Prone! Not adaptable (hosts may get added or removed) Instead, learn addresses using a simple heuristic! Look at the source of frames that arrive on each port 00:00:00:00:00:AA Port 1 Port 2 00:00:00:00:00:BB
42 Learning Addresses 10 Manual configuration is possible, but! Time consuming! Error Prone! Not adaptable (hosts may get added or removed) Instead, learn addresses using a simple heuristic! Look at the source of frames that arrive on each port 00:00:00:00:00:AA Port 1 Port 2 00:00:00:00:00:BB
43 Learning Addresses 10 Manual configuration is possible, but! Time consuming! Error Prone! Not adaptable (hosts may get added or removed) Instead, learn addresses using a simple heuristic! Look at the source of frames that arrive on each port 00:00:00:00:00:AA MAC Address Port Age 00:00:00:00:00:AA 1 0 minutes Port 1 Port 2 00:00:00:00:00:BB
44 Learning Addresses 10 Manual configuration is possible, but! Time consuming! Error Prone! Not adaptable (hosts may get added or removed) Instead, learn addresses using a simple heuristic! Look at the source of frames that arrive on each port 00:00:00:00:00:AA MAC Address Port Age 00:00:00:00:00:AA 1 0 minutes 00:00:00:00:00:BB 2 0 minutes Port 1 Port 2 00:00:00:00:00:BB
45 Learning Addresses 10 Manual configuration is possible, but! Time consuming! Error Prone! Not adaptable (hosts may get added or removed) Instead, learn addresses using a simple Delete heuristic old entries after a timeout! Look at the source of frames that arrive on each port 00:00:00:00:00:AA MAC Address Port Age 00:00:00:00:00:AA 1 0 minutes 00:00:00:00:00:BB 2 0 minutes Port 1 Port 2 00:00:00:00:00:BB
46 Complicated Learning Example 11 Bridge 1 Bridge 2 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
47 Complicated Learning Example 11 <Src=AA, Dest=FF> Bridge 1 Bridge 2 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
48 Complicated Learning Example 11 <Src=AA, Dest=FF> Bridge 1 Bridge 2 AA 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
49 Complicated Learning Example 11 <Src=AA, Dest=FF> Bridge 1 Bridge 2 AA 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
50 Complicated Learning Example 11 <Src=AA, Dest=FF> Bridge 1 Bridge 2 AA 1 AA 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
51 Complicated Learning Example 11 <Src=AA, Dest=FF> Bridge 1 Bridge 2 AA 1 AA 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
52 Complicated Learning Example 11 <Src=AA, Dest=FF> Bridge 1 Bridge 2 AA 1 AA 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
53 Complicated Learning Example 11 <Src=AA, Dest=FF> <Src=CC, Dest=AA> Bridge 1 Bridge 2 AA 1 AA 1 CC 2 CC 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
54 Complicated Learning Example 11 <Src=AA, Dest=FF> <Src=CC, Dest=AA> Bridge 1 Bridge 2 AA 1 AA 1 CC 2 CC 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
55 Complicated Learning Example 11 <Src=AA, Dest=FF> <Src=CC, Dest=AA> Bridge 1 Bridge 2 AA 1 AA 1 CC 2 CC 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
56 Complicated Learning Example 11 <Src=AA, Dest=FF> <Src=CC, Dest=AA> <Src=EE, Dest=CC> Bridge 1 Bridge 2 AA 1 AA 1 CC 2 CC 1 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
57 Complicated Learning Example 11 <Src=AA, Dest=FF> <Src=CC, Dest=AA> <Src=EE, Dest=CC> Bridge 1 Bridge 2 AA 1 AA 1 CC 2 CC 1 EE 2 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
58 Complicated Learning Example 11 <Src=AA, Dest=FF> <Src=CC, Dest=AA> <Src=EE, Dest=CC> Bridge 1 Bridge 2 AA 1 AA 1 CC 2 CC 1 EE 2 EE 2 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
59 Complicated Learning Example 11 <Src=AA, Dest=FF> <Src=CC, Dest=AA> <Src=EE, Dest=CC> Bridge 1 Bridge 2 AA 1 AA 1 CC 2 CC 1 EE 2 EE 2 Port 1 Port 2 Port 1 Port 2 AA BB CC DD EE FF
60 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 Port 1 Port 1 AA BB
61 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 Port 1 Port 1 AA BB
62 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 AA 1 AA 1 Port 1 Port 1 AA BB
63 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 AA 1 AA 1 Port 1 Port 1 AA BB
64 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 AA 1 AA 1 Port 1 Port 1 AA BB
65 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 AA 12 AA 12 Port 1 Port 1 AA BB
66 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 AA 12 AA 12 Port 1 Port 1 AA BB
67 The Danger of Loops 12 <Src=AA, Dest=DD> CC DD Port 2 Port 2 AA 21 AA 21 Port 1 Port 1 AA BB
68 The Danger of Loops 12 <Src=AA, Dest=DD> This continues to infinity! How do we stop this? CC DD Port 2 Port 2 AA 21 AA 21 Port 1 Port 1 AA BB
69 The Danger of Loops 12 <Src=AA, Dest=DD> This continues to infinity! How do we stop this? Remove loops from the topology! Without physically unplugging cables uses an algorithm to build and maintain a spanning tree for routing CC Port 2 Port 2 AA 21 AA 21 Port 1 Port 1 DD AA BB
70 Spanning Tree Definition
71 Spanning Tree Definition
72 Spanning Tree Definition
73 Spanning Tree Definition
74 Spanning Tree Definition
75 Spanning Tree Definition 13 A subset of edges in a graph that:! Span all nodes! Do not create any cycles This structure is a tree
76 Spanning Tree Definition 13 A subset of edges in a graph that:! Span all nodes! Do not create any cycles This structure is a tree
77 Spanning Tree Poem 14 Algorhyme I think that I shall never see a graph more lovely than a tree. A tree whose crucial property is loop-free connectivity. A tree that must be sure to span so packet can reach every LAN. First, the root must be selected. By ID, it is elected. Least-cost paths from root are traced. In the tree, these paths are placed. A mesh is made by folks like me, then bridges find a spanning tree. Radia Perlman
78 802.1 Spanning Tree Approach Elect a bridge to be the root of the tree 2. Every bridge finds shortest path to the root 3. Union of these paths becomes the spanning tree
79 802.1 Spanning Tree Approach Elect a bridge to be the root of the tree 2. Every bridge finds shortest path to the root 3. Union of these paths becomes the spanning tree Bridges exchange Configuration Bridge Protocol Data Units (BPDUs) to build the tree! Used to elect the root bridge! Calculate shortest paths! Locate the next hop closest to the root, and its port! Select ports to be included in the spanning trees
80 Definitions 16 Bridge ID (BID) = <Random Number> Root Bridge: bridge with the lowest BID in the tree Path Cost: cost (in hops) from a transmitting bridge to the root Each port on a bridge has a unique Port ID Root Port: port that forwards to the root on each bridge Designated Bridge: the bridge on a LAN that provides the minimal cost path to the root! The designated bridge on each LAN is unique
81 Determining the Root 17 Initially, all hosts assume they are the root Bridges broadcast BPDUs: Root ID Path Cost to Root Bridge ID Based on received BPDUs, each switch chooses:! A new root (smallest known Root ID)! A new root port (what interface goes towards the root)! A new designated bridge (who is the next hop to root)
82 Comparing BPDUs 18 BPDU1 BPDU2 R1 Cost1 B1 R2 Cost2 B2 if R1 < R2: use BPDU1 else if R1 == R2 and Cost1 < Cost2: use BPDU1 else if R1 == R2 and Cost1 == Cost 2 and B1 < B2: BPDU1 else: use BPDU2 use
83 Comparing BPDUs 18 BPDU1 BPDU2 R1 Cost1 B1 R2 Cost2 B2 if R1 < R2: use BPDU1 else if R1 == R2 and Cost1 < Cost2: use BPDU1 else if R1 == R2 and Cost1 == Cost 2 and B1 < B2: BPDU1 else: use BPDU2 use
84 Comparing BPDUs 18 BPDU1 BPDU2 R1 Cost1 B1 R2 Cost2 B2 if R1 < R2: use BPDU1 else if R1 == R2 and Cost1 < Cost2: use BPDU1 else if R1 == R2 and Cost1 == Cost 2 and B1 < B2: BPDU1 else: use BPDU2 use
85 Comparing BPDUs 18 BPDU1 BPDU2 R1 Cost1 B1 R2 Cost2 B2 if R1 < R2: use BPDU1 else if R1 == R2 and Cost1 < Cost2: use BPDU1 else if R1 == R2 and Cost1 == Cost 2 and B1 < B2: BPDU1 else: use BPDU2 use
86 Comparing BPDUs 18 BPDU1 BPDU2 R1 Cost1 B1 R2 Cost2 B2 if R1 < R2: use BPDU1 else if R1 == R2 and Cost1 < Cost2: use BPDU1 else if R1 == R2 and Cost1 == Cost 2 and B1 < B2: BPDU1 else: use BPDU2 use
87 Spanning Tree Construction 19 0: 0/0 12: 12/0 3: 3/0 27: 27/0 41: 41/0 9: 9/0 68: 68/0
88 Spanning Tree Construction 19 0: 0/0 12: 12/0 0/1 3: 3/0 27: 27/0 0/1 41: 41/0 9: 9/0 68: 68/0
89 Spanning Tree Construction 19 0: 0/0 12: 12/0 0/1 3: 3/0 27: 27/0 0/1 41: 41/0 3/1 9: 9/0 68: 68/0
90 Spanning Tree Construction 19 0: 0/0 12: 12/0 0/1 3: 3/0 27: 27/0 0/1 41: 41/0 3/1 9: 9/0 68: 68/0 9/1
91 Spanning Tree Construction 19 0: 0/0 12: 12/0 0/1 3: 3/0 27: 27/0 0/1 41: 41/0 3/1 9: 3/2 9/0 68: 68/0 3/2 9/1
92 Spanning Tree Construction 19 0: 0/0 12: 12/0 0/1 3: 0/2 3/0 27: 27/0 0/1 41: 41/0 3/1 0/2 9: 3/2 9/0 68: 68/0 3/2 9/1
93 Spanning Tree Construction 19 0: 0/0 12: 12/0 0/1 3: 0/2 3/0 27: 27/0 0/1 41: 41/0 3/1 0/2 9: 0/3 3/2 9/0 68: 68/0 0/3 3/2 9/1
94 Spanning Tree Construction 19 0: 0/0 12: 12/0 0/1 3: 0/2 3/0 27: 27/0 0/1 41: 41/0 3/1 0/2 9: 0/3 3/2 9/0 68: 68/0 0/3 3/2 9/1
95 Bridges vs. Switches 20 Bridges make it possible to increase LAN capacity! Reduces the amount of broadcast packets! No loops
96 Bridges vs. Switches 20 Bridges make it possible to increase LAN capacity! Reduces the amount of broadcast packets! No loops Switch is a special case of a bridge! Each port is connected to a single host Either a client machine Or another switch! Links are full duplex! Simplified hardware: no need for CSMA/CD!! Can have different speeds on each port
97 Switching the Internet 21 Capabilities of switches:! Network-wide routing based on MAC addresses! Learn routes to new hosts automatically! Resolve loops
98 Switching the Internet 21 Capabilities of switches:! Network-wide routing based on MAC addresses! Learn routes to new hosts automatically! Resolve loops Could the whole Internet be one switching domain?
99 Switching the Internet 21 Capabilities of switches:! Network-wide routing based on MAC addresses! Learn routes to new hosts automatically! Resolve loops Could the whole Internet be one switching domain? NO
100 Limitations of MAC Routing 22 Inefficient! Flooding packets to locate unknown hosts
101 Limitations of MAC Routing 22 Inefficient! Flooding packets to locate unknown hosts Poor Performance! Spanning tree does not balance load! Hot spots
102 Limitations of MAC Routing 22 Inefficient! Flooding packets to locate unknown hosts Poor Performance! Spanning tree does not balance load! Hot spots Extremely Poor Scalability! Every switch needs every MAC address on the Internet in its routing table!
103 Limitations of MAC Routing 22 Inefficient! Flooding packets to locate unknown hosts Poor Performance! Spanning tree does not balance load! Hot spots Extremely Poor Scalability! Every switch needs every MAC address on the Internet in its routing table! IP addresses these problems (next week )
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