Introduction to Routers and Switches

1

• Introduction to Routers and Switches

2
What do they look like?
Access routers e.g. ISDN, ADSL
Core ATM switch
Core router e.g. OC48c POS
3
Basic Architectural Componentsof an IP Router
Control Plane
Datapath per-packet processing
4
Per-packet processing in an IP Router

• 1. Accept packet arriving on an incoming link.
• 2. Lookup packet destination address in the
  forwarding table, to identify outgoing port(s).
• 3. Manipulate packet header e.g., decrement TTL,
  update header checksum.
• 4. Send packet to the outgoing port(s).
• 5. Buffer packet in the queue.
• 6. Transmit packet onto outgoing link.

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General Switch Model
Interconnect
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IP Switch Model
2. Interconnect
1. Ingress
3. Egress
Forwarding Table
Forwarding Decision
Forwarding Table
Forwarding Decision
Forwarding Table
Forwarding Decision
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Forwarding Engine
Packet
header
payload
Router
Destination Address
Routing Lookup Data Structure
Outgoing Port
Forwarding Table
Dest-network
Port
65.0.0.0/8
3
128.9.0.0/16
1
149.12.0.0/19
7
8
The Search Operation is not a Direct Lookup
(Outgoing port, label)
(Incoming port, label)
Address
Memory
Data
IP addresses 32 bits long ? 4G entries
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The Search Operation is also not an Exact Match
Search
Exact match search search for a key in a
collection of keys of the same length.
Relatively well studied data structures

• Hashing
• Balanced binary search trees

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Example Forwarding Table
Destination IP Prefix Outgoing Port
65.0.0.0/8 3
128.9.0.0/16 1
142.12.0.0/19 7
Prefix length
IP prefix 0-32 bits
142.12.0.0/19
128.9.0.0/16
65.0.0.0/8
0
232-1
224
65.0.0.0
65.255.255.255
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Prefixes can Overlap
Longest matching prefix
128.9.176.0/24
128.9.16.0/21
128.9.172.0/21
142.12.0.0/19
65.0.0.0/8
128.9.0.0/16
0
232-1
Routing lookup Find the longest matching prefix
(the most specific route) among all prefixes that
match the destination address.
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Difficulty of Longest Prefix Match
128.9.176.0/24
128.9.172.0/21
128.9.16.0/21
142.12.0.0/19
128.9.0.0/16
65.0.0.0/8
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Lookup Rate Required
40B packets (Mpps)
Line-rate (Gbps)
Line
Year
1.94
0.622
OC12c
1998-99
7.81
2.5
OC48c
1999-00
31.25
10.0
OC192c
2000-01
125
40.0
OC768c
2002-03
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Size of the Forwarding Table
Number of Prefixes
95
96
97
98
99
00
Year

• Source http//www.telstra.net/ops/bgptable.html

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Internal Interconnects
1. Multiplexers
2. Tri-State Devices
3. Shared Memory
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InterconnectsTwo basic techniques
Input Queueing
Output Queueing
Usually a non-blocking switch fabric (e.g.
crossbar)
Usually a fast bus
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Shared Memory Bandwidth
5ns SRAM
Shared Memory

• 5ns per memory operation
• Two memory operations per packet
• Therefore, up to 160Gb/s
• In practice, closer to 80Gb/s

1
2
N
200 byte bus
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Input buffered swtich
Internconnect

• Independent routing logic per input
• FSM
• Scheduler logic arbitrates each output
• priority, FIFO, random
• Head-of-line blocking problem

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Input QueueingHead of Line Blocking
Delay
Load
100
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Head of Line Blocking
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(Virtual) Output Buffered Switch
N buffers per input

• How would you build a shared pool?

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Solving HOL with Input QueueingVirtual output
queues
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Input QueueingVirtual Output Queues
Delay
Load
100
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Output scheduling

• n independent arbitration problems?
• static priority, random, round-robin
• simplifications due to routing algorithm?
• general case is max bipartite matching

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Finding a maximum size match
Requests
Inputs
Outputs

• How do we find the maximum size (weight) match?