Can Cisco TOR Switch Work with Dell BCF Server? Troubleshooting Uplink Issues

Connecting network devices from different vendors can sometimes present compatibility challenges. Many users find themselves needing to integrate Cisco switches with Dell servers, particularly in Top-of-Rack (TOR) configurations using Dell’s Barefoot-based Control Fabric (BCF) servers. A common question arises: Can a Cisco TOR switch seamlessly work with a Dell BCF server? This article addresses this question by exploring potential connectivity issues and offering troubleshooting steps when establishing an uplink between Cisco and Dell switches.

When attempting to link a Cisco switch to a Dell server, specifically for network traffic, users may encounter situations where the interface status shows “line protocol down,” despite the physical link appearing to be up. This scenario is illustrated by a user trying to connect a Dell S5248F-ON switch to a Cisco network. Both devices are equipped with 25GB ports and corresponding SFP28 modules from their respective vendors (Dell and Cisco). However, upon connection, no uplink is established.

Let’s examine a typical interface configuration on the Cisco switch in such a scenario:

interface ethernet 1/1/47
 description uplink
 no shutdown
 switchport mode trunk
 switchport access vlan 2
 switchport trunk allowed vlan 40-41
 flowcontrol receive on
 flowcontrol transmit off
 spanning-tree port type edge

And the corresponding interface status:

Ethernet 1/1/47 is up, line protocol is down
 Description: uplink
 Hardware is Eth, address is 74:86:02:11:22:33
 Current address is 74:86:2:11:22:33
 Pluggable media present, SFP28 type is SFP28 25GBASE-LR
 Wavelength is 1310
 Interface index is 60
 Internet address is not set
 Mode of IPy4 Address Assignment: not set
 Interface IPv6 oper status: Disabled
 MTU 1532 bytes, IP MTU 1500 bytes
 LineSpeed 0, Auto-Negotiation off
 Configured FEC is off, Negotiated FeC is off
 Flowcontrol ry off tx off
 ARP type: ARPA, ARP Timeout: 60
 Last clearing of "show interface" counters: 00:19:33
 Queuing strategy: fifo
 Input statistics:
     0 packets, 0 octets
     0 64-byte pts, 0 over 64-byte pts, 0 over 127-byte pts
     0 over 255-byte pts, 0 over 511-byte pts, 0 over 1023-byte pts
     0 multicasts, 0 broadcasts, 0 unicasts
     0 runts, 0 giants, 0 throttles
     0 CRC, 0 overrun, 0 discarded
 Output statistics:
     0 packets, 0 octets
     0 64-byte pts, 0 over 64-byte pkts, 0 over 127-byte pts
     0 over 255-byte prts, 0 over 511-byte prts, 0 over 1023-byte pts
     0 multicasts, 0 broadcasts, 0 unicasts
     0 throttles, 0 discarded, 0 collisions, wred drops
 Rate Info(interval 30 seconds):
   Input 0 Mbits/sec, 0 packets/sec, o% of line rate
   Output 0 Mbits/sec, 0 packets/sec, 0% of line rate
 Time since last interface status change: 00:19:34

The key indicator here is “line protocol is down” despite “Ethernet 1/1/47 is up”. This typically points to a Layer 2 issue preventing proper communication between the Cisco and Dell switches.

Potential Reasons and Troubleshooting Steps:

1. SFP Module Compatibility: While both SFP28 modules are designed for 25GBASE-LR, vendor-specific firmware or compatibility requirements can sometimes cause issues.

   • Action: Verify if both the Cisco and Dell switches officially support the SFP modules being used. Check the vendor documentation for compatibility matrices. Try using identical SFP modules on both ends if possible, or test with generic, compatible SFPs.

2. Auto-Negotiation and Speed/Duplex Mismatches: Although auto-negotiation is turned off in the provided Cisco configuration (Auto-Negotiation off), ensure that the speed and duplex settings are correctly configured and compatible on both the Cisco and Dell switch ports. In this case, it’s a 25G connection, so speed should be fixed at 25G on both sides. Duplex is typically full-duplex for fiber connections.

   • Action: Double-check the Dell switch port configuration. While not shown, ensure it’s configured for 25G and full duplex, and that auto-negotiation is either disabled or properly negotiated if enabled. Try explicitly setting speed and duplex on both sides if auto-negotiation is problematic.

3. VLAN Configuration: VLAN mismatches or incorrect trunking configurations can prevent Layer 2 connectivity.

   • Action: Confirm that the VLANs configured on the Cisco side (switchport trunk allowed vlan 40-41) are correctly configured and exist on the Dell switch. If VLAN 40 and 41 are intended to be trunked, the Dell port must also be configured as a trunk port allowing these VLANs. If VLAN 2 is the access VLAN, ensure this is also correctly configured and intended for communication between these devices. Verify native VLAN settings if applicable.

4. Cable Issues (Though Less Likely): While the user mentioned the cable works with a Dell server, it’s still worth re-verifying the cable, especially for longer runs or if it has been handled roughly.

   • Action: If possible, try a different, known-good fiber cable, especially if it’s a long run. Inspect the cable connectors for any damage or debris.

5. Port Shutdown or Errors on Dell Switch: The problem might not be on the Cisco side. The Dell switch port could be administratively down, have errors, or be misconfigured.

   • Action: Check the configuration and status of the corresponding port on the Dell switch. Ensure it is administratively up (no shutdown equivalent command on Dell), and check for any error counters on the interface.

6. Flow Control Mismatches: While flow control mismatch can sometimes cause issues, it’s less likely to be the primary cause of a “line protocol down” state. However, inconsistencies are worth noting.

   • Action: While configured differently (receive on, transmit off on Cisco), consider testing with flow control disabled on both sides to rule out any flow control related problems, although this is less likely the root cause here.

Conclusion:

Successfully integrating Cisco TOR switches with Dell BCF servers is generally achievable, but requires careful configuration and attention to detail, especially at the physical and data link layers. The “line protocol down” issue often points to a mismatch or misconfiguration in SFP compatibility, speed/duplex settings, or VLAN configurations. By systematically checking these areas and following the troubleshooting steps outlined above, network administrators can effectively diagnose and resolve connectivity problems between Cisco and Dell network devices. If problems persist, consulting the hardware compatibility lists and support documentation for both Cisco and Dell is highly recommended.