8.5.14. HA-SEA with Load Sharing
The great advantage of the SEA failover is the higher availability due to the redundancy (2 SEAs). However, the SEA failover configuration also has a disadvantage: only one of the two SEAs is active (Primary) and forwards the network traffic. This means that only one of the two physical adapters of the two SEAs is active, receiving and transmitting network packets. Consequently, only 50% of the theoretically available bandwidth can be used.
With the introduction of SEA load sharing, IBM changed this. The two SEAs split the VLANs among themselves and each of the two SEAs forwards the network traffic for its VLANs. This means that both physical network adapters of the SEAs can be used at the same time, and thus a higher network throughput can be achieved (theoretically the sum of the bandwidths of the physical network adapters).
Figure 8.12 shows a load sharing configuration with a separate control channel. Both SEAs have 2 trunking adapters, whereby the left SEA (primary_sh) forwards network traffic for the first trunking adapter with VLANs 11, 12 and 13 and the right SEA (backup_sh) forwards network traffic for the second trunking adapter with the VLANs 14 and 15. The figure also shows Ethernet frames from the upper left LPAR for VLAN 12, which are forwarded to the SEA at the bottom left via the active trunking adapter for VLAN 12 and then finally passed on to the external network by the physical network adapter ent3. The second LPAR, with VLAN 14, in turn sends an Ethernet frame for VLAN 14, which is forwarded via the active trunking adapter for VLAN 14 to the right SEA, which then transfers the Ethernet frame by the physical network adapter ent5 to the same external network.
Within both SEAs, half of the trunking adapters are active and the other half inactive. In the event of an error, the remaining SEA takes over all VLANs by activating all trunking adapters.
The procedure for distributing the VLANs between the two SEAs is as follows:
- The first half of the trunking adapters are assigned to the SEA with the higher trunking priority (lower value for trunk_priority), the second half of the trunking adapters are assigned to the SEA with the lower trunking priority. The sequence of the trunking adapters of the SEA with the higher trunking priority (primary SEA) counts.
- If the number of trunking adapters is odd, one more trunking adapter is assigned to the SEA with the higher trunking priority (primary SEA).
Note: As in the case of failover, the use of a control channel is optional for load sharing.
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