Upgrading the network from 10G to 25G is a straightforward migration option. Hence Cisco offers a gradual migration path with the support of dual-rate optics, where the same 25G optics can operate at both 10G and 25G speeds. With this approach, distribution layer devices can be upgraded to 25G while the access layer still operates at 10G, and the access layer switches can be upgraded over a period of time.
Use case 1: Speed transition with similar cable distances
As access bandwidth increases, campus backbones are transitioning from 10G and 40G speeds to 25G and 100G speeds, and customers are demanding newer optics to support a cabling distance similar to that of existing optics. Cisco’s innovative SFP-10/25G-CSR-S modules support traditional link lengths of up to 300 or 400 m over OM3/OM4 (depending on the fiber quality), now at 25G speeds (Figure 5).
Figure5. Speed transition with similar cable distances
Use case2: Speed migration with dual-rate optics
25G switches and optics provide 2.5 times more bandwidth and are not significantly more expensive than 10G Ethernet solutions. Cisco’s 25G portfolio provides full backward compatibility with 10G with dual-rate optics. These optics will automatically negotiate with the far-end device to the highest speed supported. For example, if the far-end device is capable only of 10 Gbps, the two devices would settle at 10 Gbps speed, meaning that you don’t need to upgrade your infrastructure to 25G right away. Instead you can upgrade the network as part of regular refresh cycles and still have it ready for 25G capabilities (Figure 6).
Figure6. Speed migration with dual-rate optics
Use case3: Speed transition with similar oversubscription ratios
Oversubscription in enterprise campus networks isn’t new. For a long time, the rule of thumb for oversubscription was about 20:1 for access ports and 4:1 for distribution to core. However, these numbers do not really hold up in modern network design. Hence, to retain the recommended oversubscription ratio there is a need to upgrade the uplinks (switch to switch). As natural successors to 10G and 40G technologies, 25G and 100G will help retain the same oversubscription ratios (Figure7).
Figure7. Speed transition with similar oversubscription ratio
Use case4: Access speed transition to 5G
The next generation of Wi-Fi is driving the need for higher speeds, including 2.5G, 5GBASE-T, and 10GBASE-T, in the campus. Most larger enterprise campus networks implement multitier architecture, and these are generally oversubscribed. Network expansions at access or edge have always demanded refreshing the campus core switching capacity. IT needs to reevaluate two major bottleneck points in campus networks, those being the distribution layer that aggregates 10G physical connections and the core switching capacity to maintain 4:1 oversubscription ratios. While mitigating campus core scale and performance challenges and protecting oversubscription ratios, the Cisco Catalyst 9500 Series Switches allow easy and seamless upgrade from existing 10G- or 40G-based infrastructure to 40G- or 100G-based infrastructure (Figure 8).
Figure8. Access speed transition to 5G
Use case5: Load sharing vs. higher speeds
Speed upgrades have outpaced the refresh cycles for cabling, and until now there were two options: (1) rip and replace your existing cabling to support higher speeds, or (2) add additional links to satisfy bandwidth requirements. Neither of these options is ideal. There are significant cost implications, and adding additional uplinks compounds the challenges of achieving effective load sharing that depends on the hashing technologies supported by switches. Moving to faster Ethernet technologies can overcome those concerns as well as add more bandwidth.
Figure9. Load sharing vs. higher speed topologies
Info from https://www.cisco.com/c/dam/en/us/products/collateral/switches/catalyst-9000/nb-09-25ge-100ge-wp-cte-en.pdf