Spine 1
Leaf 1
Spine 2
Leaf 2
Spine 3
Leaf 3
Spine 4
Leaf 4
FIGURE 3: Spine-and-leaf 2-level network topology
in a 10 Gb/s application.
May/June 2019 I 43
connected to the one MPO output receive a 10 Gb/s
channel from each of the QSFP transceivers A, B, C,
and D. Thus, a fully meshed, 10 Gb/s fabric connection
between the QSFP spine switch ports and the leaf switch
ports is achieved without ever having to break out to
LC connections at the main distribution area (MDA).
To optimize the cabling structure of a spine-and-leaf
at the MDA, for example, use a leaf switch with a 48 x
10 Gb/s SFP+ port line card and a spine switch with 4 x
36 40 Gb/s QSFP+ port line cards. If a leaf switch has
an oversubscription ratio of 3:1, 16 x 10 Gb/s uplink
ports of each leaf switch need to connect to 16 spine
switches. Given that the 40 Gb/s port of the spine
switch is used as four 10 Gb/s ports, each spine switch
needs to connect 4 x 36 x 4 = 576 leaf switches as shown
in Figure 3.
If traditional cabling is used to achieve full fabric mesh
of the spine and leaf switches, a 40 Gb/s QSFP+ port
of each spine switch is broken out into 4 x 10 Gb/s
channels through an MPO-to-LC module in the MDA,
then cross-connected through a patch cable with the
corresponding number of the MPO-to-LC modules that
connect to the 10 Gb/s channels of the leaf switch
as shown on the left side of Figure 4. The traditional
method has not been widely used because the cabling
system is very complex, the cost is relatively high,
and it requires a lot of rack space at the MDA. In this
scenario, the use of a mesh module can be a good
solution to solve these problems. As shown in the
graphic on the right side of Figure 4, in the case
of a network module used in the MDA, the full mesh
of the leaf switches is achieved without having to break
out the 40 Gb/s port of the spine switch into 10 Gb/s
FIGURE 4: Full cross-connection cabling structure comparison of spine-and-leaf network architecture MDA.
Spine 16
Leaf 576
Panels = 576
Housings 4U = 12
MPO Jumper = 2,304
Mesh Modules = 576
Housings 4U = 12
Modules = 2,304
Housings 4U = 48
LC Jumper = 9,216
Mesh Modules = 2,304
Housings 4U = 48