Is bonding supported with direct connection using crossover cables?

Environment

• Red Hat Enterprise Linux
• Bonding link aggregation
• Two systems connected via crossover cable, without a network switch in between

Issue

• Is bonding supported with direct connection using crossover cables?
• How to bond servers in active/backup mode without switch?
• Can I directly connect servers to have high availability and failover?

Resolution

Red Hat does not support bonded network configurations where components of the bond are peer-to-peer connected with cross-over or straight cabling without using network switch infrastructure.

Red Hat's advice is to use appropriate switches in such configurations to avoid problems related to this unsupported configuration.

Root Cause

• The Linux bonding driver is not developed or tested with crossover in mind, either by Red Hat or the upstream Linux community.
• Such a configuration is at odds with accepted industry recommendations for redundancy and reliability.
• The Bonding module in MII monitoring mode concerns itself with the state of the "link" as reported by the PHY ¹ in the NIC, so with the state the hardware and the electrical signalling is reported.
• If an interface is administratively disabled in such a configuration, the PHY may still maintain electrical connectivity to the peer and failover will not work as expected, leading to a lack of bond communication.
• This is a hardware/PHY/firmware design point, and not something to be fixed in software. As this is very hardware dependant, it is best to simulate all failure scenarios prior deployment.
• Whilst the ARP monitoring mode could be used, there are few configurations where it would theoretically work, and it is easily possible to get such systems into a state where link appears up but communication does not work.

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