C9300-24U vs C9300-24UB: Deep Buffer, Scale, and Stack Limits
The C9300-24UB is the higher-scale version of the C9300-24U: it doubles the packet buffer from 16 MB to 32 MB and raises MAC, route, ACL, QoS, multicast, and Flexible NetFlow scale. Both provide 24 one-gigabit UPOE downlinks, the same 208 Gbps switching capacity, modular uplinks, and an 830 W PoE budget with the default 1100 W power supply. The purchasing constraint many comparisons miss is stacking: a C9300-24UB can stack only with other Catalyst 9300 higher-scale models, not with a C9300-24U stack.
Choose the C9300-24UB for a new higher-scale stack or a standalone deployment that can prove it needs the extra hardware scale. Choose the C9300-24U when its scale is sufficient or when the new switch must join an existing standard C9300 stack.
C9300-24U vs C9300-24UB Comparison
| Specification | C9300-24U | C9300-24UB | What changes the decision |
|---|---|---|---|
| Downlink ports | 24 × 10/100/1000 Mbps copper | 24 × 10/100/1000 Mbps copper | Same; neither model has mGig access ports |
| Power over Ethernet | Cisco UPOE, up to 60 W per port | Cisco UPOE, up to 60 W per port | Same per-port capability |
| Default power supply | PWR-C1-1100WAC-P | PWR-C1-1100WAC-P | Same |
| Available PoE with default PSU | 830 W | 830 W | Same total budget |
| Uplink design | Modular; network module ordered separately | Modular; network module ordered separately | Same module family |
| Switching capacity | 208 Gbps | 208 Gbps | Same |
| Switching capacity with stacking | 688 Gbps | 688 Gbps | Same |
| Forwarding rate | 154.76 Mpps | 154.76 Mpps | Same |
| Stack architecture | StackWise-480 | StackWise-480, higher-scale-only stack | Bandwidth is the same; membership is not |
| ASIC family | UADP 2.0 | UADP 2.0 XL | UB provides higher hardware scale |
| Packet buffer | 16 MB | 32 MB | UB doubles the buffer |
| MAC addresses | 32,000 | 64,000 | UB doubles Layer 2 scale |
| IPv4 routes, including ARP and learned routes | 32,000 | 112,000 | UB raises the combined scale by 3.5× |
| IPv6 entries | 16,000 | 56,000 | UB raises IPv6 scale by 3.5× |
| Multicast routing scale | 8,000 | 16,000 | UB doubles multicast scale |
| QoS scale entries | 5,120 | 18,000 | UB raises policy scale |
| ACL scale entries | 5,120 | 18,000 | UB raises policy scale |
| Flexible NetFlow entries | 64,000 | 128,000 | UB doubles flow scale |
| DRAM / flash | 8 GB / 16 GB | 8 GB / 16 GB | Same |
Figures are from Cisco’s current Catalyst 9300 Series data sheet. Hardware table limits are platform maximums and can depend on the feature mix and template in use.
What Does the “B” Change?
The B identifies the deep-buffer, higher-scale hardware variant. It does not add faster access ports, more PoE, or a higher forwarding rate.
The 32 MB buffer gives the C9300-24UB more room to absorb short bursts when traffic arrives faster than an egress port can transmit it. A buffer cannot create bandwidth: if a 10G uplink is persistently oversubscribed, a larger queue delays the drop rather than removing the bottleneck. The useful case is transient congestion where the burst ends before the deeper queue fills.
The larger table capacities are often the more important difference. A routed-access or policy-heavy design can run out of route, ACL, QoS, or flow entries before it runs out of port bandwidth. The C9300-24UB moves those ceilings substantially while keeping the same 24-port physical access layout.
The Stack Compatibility Rule Can Override the Buffer Advantage
Cisco classifies the C9300-24UB, C9300-24UXB, and C9300-48UB as Catalyst 9300 higher-scale models. They must be stacked with other higher-scale models. Cisco does not support mixing a C9300-24UB with a standard C9300 model such as the C9300-24U in one stack.
That makes stack membership a first-pass check:
- Expanding a standard C9300 stack: use a compatible standard C9300 model and matching license level; the C9300-24UB is not a drop-in member.
- Building a new higher-scale stack: the C9300-24UB is viable, but every planned member must remain inside the higher-scale compatibility group.
- Running standalone: the stack restriction does not block the design, so buffer and table requirements become the deciding factors.
Both models use StackWise-480 and can form stacks of up to eight supported members. The shared 480 Gbps number does not mean the two hardware groups can mix.
Ports, UPOE, and Forwarding Performance Are the Same
The C9300-24U and C9300-24UB each provide 24 copper access ports at 10/100/1000 Mbps. The U in both model names denotes Cisco UPOE. It does not denote multigigabit Ethernet.
Each UPOE port can deliver up to 60 W to a supported powered device, subject to the total switch budget. With the default PWR-C1-1100WAC-P, Cisco lists 830 W of available PoE for either model. Twenty-four ports at 60 W would require 1,440 W, so the default single supply cannot run every port at the per-port maximum at once.
For a BOM, calculate the endpoint draw port by port and include startup margin. Cisco’s power table shows that a 24-port UPOE design needs the 1100 W primary plus a 715 W secondary supply to cover the full 1,440 W port-rating total. Power-supply redundancy and PoE capacity should be evaluated together; combining supplies for maximum PoE and reserving one for failover are different operating goals.
Both Models Use the Same Modular Uplink Family
The switch chassis does not include a network module by default. The non-X Catalyst 9300 module options include:
| Network module | Interfaces | Typical use |
|---|---|---|
| C9300-NM-4G | 4 × 1G | Legacy 1G fiber uplinks |
| C9300-NM-4M | 4 × multigigabit copper | Multi-rate copper uplink requirements |
| C9300-NM-8X | 8 × 10G/1G | Dense 10G or 1G fiber uplinks |
| C9300-NM-2Y | 2 × 25G/10G/1G | Higher-capacity aggregation |
| C9300-NM-2Q | 2 × 40G | 40G aggregation |
Module descriptions are from the Cisco Catalyst 9300 data sheet. Verify software support, optics, cable type, and the complete module PID before ordering.
Installing C9300-NM-4M does not turn the 24 access ports into mGig ports. It changes only the uplink module. If endpoints require 2.5G, 5G, or 10G copper access, evaluate an mGig model such as C9300-24UX or C9300-24UXB instead.
C9300-24U-A, C9300-24U-E, C9300-24UB-A, and C9300-24UB-E
The A and E suffixes describe the Network Stack license tier, not the buffer hardware.
| Hardware | Network Essentials | Network Advantage |
|---|---|---|
| C9300-24U | C9300-24U-E | C9300-24U-A |
| C9300-24UB | C9300-24UB-E | C9300-24UB-A |
Compare like license tiers when quoting the hardware difference. Cisco’s ordering guide also requires the applicable Catalyst or Cisco DNA software subscription selection for new orders.
Network Essentials fits common access functions; Network Advantage adds advanced capabilities. The required tier comes from the feature set and stack design, not from the decision to buy a deeper buffer.
Which Model Should You Choose?
| Deployment condition | Better fit | Reason |
|---|---|---|
| Add a member to a standard C9300 stack | C9300-24U | C9300-24UB cannot join a standard-model stack |
| New standalone 24-port UPOE access switch with ordinary table scale | C9300-24U | Same ports, PoE, forwarding, and uplink flexibility at standard scale |
| New higher-scale stack with large MAC, route, ACL, QoS, or FNF requirements | C9300-24UB | UADP 2.0 XL hardware raises table scale and buffer depth |
| Short, measured traffic bursts are causing egress drops | C9300-24UB, after verification | 32 MB buffer can absorb more transient congestion |
| Persistent uplink saturation | Neither model alone fixes it | Increase uplink capacity or redesign traffic paths; a deeper buffer is not bandwidth |
| 2.5G/5G/10G access devices | Neither | Both have 1G downlinks; choose a verified mGig SKU |
| Need all 24 ports at 60 W simultaneously | Either with a complete power BOM | Default 830 W is insufficient for 24 × 60 W |
Use this matrix only after confirming the exact existing stack type, observed hardware-table utilization, queue-drop evidence, endpoint speed, and complete power BOM.
Do not buy the C9300-24UB merely because the network carries video, wireless, or multicast. First check actual queue drops, route and policy utilization, uplink load, and stack membership. A standard C9300-24U can carry those applications when its measured scale and bandwidth are sufficient.
Frequently Asked Questions
Is the C9300-24UB buffer 32 MB or 128 MB?
It is 32 MB in Cisco’s current Catalyst 9300 data sheet. The C9300-24U has a 16 MB buffer. Claims that the C9300-24UB has a 128 MB packet buffer do not match the current official platform table.
Can a C9300-24UB stack with a C9300-24U?
No. Cisco requires C9300 higher-scale models—C9300-24UB, C9300-24UXB, and C9300-48UB—to stack only with other higher-scale models. License levels must also be compatible inside the stack.
Are C9300-24U and C9300-24UB multigigabit switches?
No. Both provide 24 copper downlink ports at up to 1 Gbps. The U indicates UPOE. For 2.5G, 5G, or 10G access, evaluate a verified mGig model such as C9300-24UX or C9300-24UXB.
Does the default 1100 W power supply support 60 W on every port?
No. It provides 830 W of available PoE. Twenty-four ports at 60 W total 1,440 W. Cisco’s power table uses the 1100 W primary plus a 715 W secondary supply to cover that full port-rating total.
Is C9300-24UB always better for video or wireless?
No. Its deeper buffer and higher tables help only when those resources are the actual constraint. Persistent congestion still needs more uplink capacity, and an existing standard C9300 stack cannot accept a C9300-24UB member.
References
- Cisco, Catalyst 9300 Series Switches Data Sheet
- Cisco, Catalyst 9300 Series Architecture White Paper
- Cisco, Catalyst 9300 Series Ordering Guide
- Cisco, Managing Catalyst 9300 Switch Stacks
For project quantities, compare current options across the Catalyst 9300 product family. Layer23-Switch can verify the stack group, license tier, uplink module, power supplies, optics, stock, and lead time before you request a Catalyst 9300 BOM review.
