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What are breakouts and how do you use them?

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Updated:February 1, 2021

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    (590.6 KB)
    View with Adobe Reader on a variety of devices
Updated:February 1, 2021
 

 

Network connectivity using breakout mode is becoming increasingly important as new high-speed ports become available on switches, routers, and other communication equipment. Breakouts allow these new ports to interface with lower-speed ports.

Introduction

Breakouts enable connectivity between network devices with different speed ports, while fully utilizing port bandwidth.

Breakout mode on network equipment (switches, routers, and servers) opens up new ways for network operators to keep up with the pace of bandwidth demand. By adding high speed ports that support breakout, operators can increase face plate port density and enable upgrade to higher data rates incrementally.

What is breakout mode?

Breakout mode is when a high-speed, channelized port on a network element is broken down into multiple low-speed ports, each going to multiple network elements. For example, a switch with 40G ports can be connected to 10G server ports.

40G to 10G breakout

Figure 1.               

40G to 10G breakout

Is breakout always possible?

Breakout always involves the connection of a channelized port to multiple unchannelized ports. Channelized ports are always implemented in multilane form factors, such as QSFP+, QSFP28, and QSFP56-DD. Typically, unchannelized ports are implemented in single-channel form factors, including SFP+, SFP28, and future SFP56. Some port types, such as QSFP28, can be on either side of the breakout, depending on the situation.

Today, channelized ports include 40G, 100G, and 400G as shown in Table 1 and unchannelized ports include 10G, 25G and 50G as shown in Table 2:

Table 1.           Channelized ports

Rate

Technology

Lane speed (Gbps)

Number of lanes

40G

QSFP+

10

4x 10G

100G

QSFP28

25

4x 25G

400G

QSFP56-DD

50

8x 50G

Unchannelized ports include 10G, 25G, and 50G.

Table 2.           Unchannelized ports

Rate

Technology

Lane speed (Gbps)

Number of lanes

10G

SFP+

10

1x 10G

25G

SFP28

25

1x 25G

50G

SFP56

50

1x 50G

What types of breakout options are available?

Breakout is physically implemented with cables or transceivers. Some of the options include breakout DACs, breakout AOCs, and transceivers.

Breakout DACs

Direct Attached Cables (DACs) are fixed length and made of multiple copper twinax cables. They are generally 1 m to 5 m long in passive configuration and 7 m to 10 m long in active configuration and include the modules that plug into the equipment ports as shown in Figure 2.

Breakout DAC

Figure 2.               

Breakout DAC

Breakout AOCs

Active Optical Cables (AOCs) are fixed length and made of multiple optical cables generally 1 m to 25 m long and include the modules that plug into the equipment ports as shown in Figure 3.

Breakout AOC

Figure 3.               

Breakout AOC

Transceivers

Transceivers could use either Single-Mode Fiber (SMF) or Multimode Fiber (MMF) when paired with a breakout cable as shown in Figure 4.

Transceivers for breakout

Figure 4.               

Transceivers for breakout

Examples of transceivers used in breakout applications are as follows.

For MMF applications:

     40GBASE-SR4 to 4x10GBASE-SR

     100GBASE-SR4 to 4x25GBASE-SR

     400GBASE-SR8 to 8x50GBASE-SR

For SMF applications:

     400GBASE-DR4 to 4x100GBASE-DR

Comparing downlink density for switches with and without breakout capability

While breakout is not a new concept, it is becoming increasingly important as equipment manufacturers look to dramatically increase bandwidth and faceplace density while still having access to low-speed ports. For example, consider the advantages of a 1RU 19-inch fixed port switch, which has a faceplate consumed by 36x QSFP ports (Figure 5) that can be used as either uplinks or downlinks. The QSFP ports can be QSFP+ (40G), QSFP28 (100G), or QSFP-DD (400G). Compare this with an example of a same-size switch that has 48x single-lane downlink ports, (Figure 6) which cannot be broken out, and 6x QSFP uplink ports. The data rates of the downlink ports would match the lanes of the QSFP ports. For example, 10G SFP+ for QSFP+, 25G SFP28 for QSFP28, and 50G SFP56 for QSFP-DD.

Switch that supports high-density QSFP downlinks with breakout

Figure 5.               

Switch that supports high-density QSFP downlinks with breakout

Switch with single-lane downlinks

Figure 6.               

Switch with single-lane downlinks

The benefit of a QSFP-only switch over a switch with single-lane downlinks depends on the type of QSFP port, and the acceptable oversubscription ratio.

Table 3 shows that the switches that support breakout in all ports provide twice the density of downlink ports as switches that have single-lane downlink ports.*

Table 3.           Single-lane downlink vs. QSFP+- or QSFP28-only switch

Switch Type

Uplink rate

# Uplinks

Downlink rate

# Downlinks

Single-lane downlinks

40G (QSFP+)

6

10G

48

100G (QSFP28)

6

25G

48

QSFP-only (supports breakout)

40G (QSFP+)

12

10G

96

100G (QSFP28)

12

25G

96

* Oversubscription = 200%

Table 4 shows a similar comparison between 400G QSFP-DD switches. In this case, the switch that supports breakout in all ports provides triple the downlink port density of the single-lane type.*

Table 4.           Single-lane downlink vs. QSFP-DD-only switch

Switch Type

Uplink rate

# Uplinks

Downlink rate

# Downlinks

Single-lane downlinks

400G (QSFP-DD)

6

50G

48

QSFP-only (supports breakout)

400G (QSFP-DD)

18

50G

144

* Oversubscription = 100%

Breakout advantages and disadvantages

Advantages of breakout:

     Higher density. For example, a 36-port QSFP-DD breakout switch can provide double or triple the density of a switch with single-lane downlink ports.

     Access to lower speed interfaces. For example, the QSFP-4X10G-LR-S transceiver enables a switch with only QSFP ports to connect 4x 10G LR interfaces per port.

Disadvantages of breakout:

     More difficult replacement strategy. When one of the ports on a breakout transceiver, AOC or DAC, goes bad, it requires replacement of the whole transceiver or cable.

     Not as customizable. In switches with single-lane downlinks, each port is individually configured. For example, an individual port could be 10G, 25G, or 50G and could accept any type of transceiver, AOC or DAC. A QSFP-only port in breakout mode requires a group-wise approach, where all interfaces of a transceiver or cable are the same type.

Other considerations for breakout

New connector

The new CS connector (Figure 7) provides breakout capability at the transceiver (Figure 8) instead of requiring a breakout cable or breakout cartridge in a patch panel. Traditional assemblies use MPO (multifiber push on) connectors on one end and multiple LC duplex connectors at the other end.

New CS connector

Figure 7.               

New CS connector

Module with CS connector

Figure 8.               

Module with CS connector

Breakout and redundancy

Breakout enables redundant connectivity. Consider the example in Figure 9, which shows QSFP28 100G transceivers in switch and server ports, where the 4x25G lanes of each transceiver are broken out into 50G (2x25G) connections. The breakout capability provides redundant 50G connectivity between switches and servers.

Breakout and redundancy

Figure 9.               

Breakout and redundancy

Conclusion

Breakout mode for communication applications is becoming more important as solutions demand increased density and lower cost, as well as other factors, such as interconnect and replacement strategies that need to be considered for the overall solution.

Cisco has a variety of breakout cables and transceivers. For more information see: https://www.cisco.com/go/optics.

For breakout capability of specific Cisco® transceivers, see Cisco Optics-to-Optics Interoperability Matrix: https://tmgmatrix.cisco.com/iop.

The Cisco Optics-to-Device Compatibility Matrix is a great tool for determining if a transceiver supports breakout across a wide variety of Cisco platforms and software releases: https://tmgmatrix.cisco.com/.

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