Currently, there is something very odd going on when using EtherChannel (Network Interface Backup mode) if the backup adapter is a virtual adapter. PMR 68839.300.624 clarified that it is currently designed that the backup virtual adapter is receiving traffic, even though it is in backup mode. However, this introduces an additional problem: even though the backup virtual adapter is receiving traffic, it is not replying to it. It is the primary channel that responds, which creates an unbalanced situation on the physical network, resulting in flooding.
- All AIX versions up till now
- POWER5(+) firmware <= SF240_320
Consider that the ARP tables on client LPAR A and server LPAR B are empty aswell as the MAC table on the Ethernet Switch. Client LPAR A wishes to send data to LPAR B.
ent1 (Virtual Ethernet) - MAC address 22:f1:30:00:70:06
en1 - IP address 10.226.32.145
ent3 (EtherChannel in NIB mode (Active/Passive)) - MAC address 00:14:5e:c6:46:80
en3 - IP address 10.226.32.139
Primary Channel: ent2 (Physical Ethernet)
Backup Channel: ent1 (Virtual Ethernet)
ent3 (Shared Ethernet Adapter) - MAC address 00:14:5e:48:2c:7a
Physical Ethernet: ent2 - MAC address 00:14:5e:48:2c:7a
Virtual Ethernet: ent1 - MAC address 22:f1:30:00:30:06
Source IP address: 10.226.32.145
Destination IP address: 10.226.32.139
Source MAC address: 22:f1:30:00:70:06
Destination MAC address: unknown
Since client LPAR A does not know the destination MAC address of server LPAR B, client LPAR A is broadcasting an ARP request (Who has 10.226.32.139, tell 10.226.32.145) on the internal Layer 2 PHYP switch. Even though the EtherChannel on server LPAR B is in Primary Channel Mode, the PHYP delivers this packet to the backup Virtual Ethernet adapter of the EtherChannel and also delivers the broadcast to the SEA for bridging. As a result, the MAC table on the physical switch is updated with MAC address of client LPAR A, located on physical port X. Server LPAR B will form a unicast reply but sends this unicast reply via the Primary Channel to the Ethernet Switch. The Ethernet Switch receives the unicast reply on port Y, links the source MAC address of server LPAR B to port Y in the MAC table. Since the frame contains a destination MAC address which has a valid MAC table entry on the physical switch, it is delivered to port X and it ultimately received by client LPAR A through the SEA. Client LPAR A updates the ARP table with the MAC address of server LPAR B.
Now client LPAR A can start communicating with server LPAR B since it now knows the destination MAC address. The PHYP is delivering the packets via the backup Virtual Ethernet adapter of the EtherChannel. After the TTL of the MAC table entry for client LPAR A expires, flooding is observed on the physical switch, meaning that the switch will act as a simple repeater for all communication from server LPAR B to client LPAR A and hereby sending it to all trunk ports and access ports defined in the same VLAN. Ofcourse, the frames are also forwarded to port X (it's in the same VLAN) and are ultimately received by client LPAR A though the SEA.
When client LPAR A is sending jumbo frames (data) to server LPAR B, approximately 2 Mbit/s of TCP ACK flooding was observed. It gets really bad when the process is reversed, in which server LPAR B is sending data to client LPAR A. As a result, all data will be flooded on the switch and only the TCP acks are delivered via the backup Virtual Ethernet Adapter.
According to IBM, this is working as designed and a DCR was created to address this issue.
- Reduce ARP table TTL on the LPARs (arpt_killc network tunable) OR
- Increase MAC table TTL on the physical switch OR
- Replace Virtual Ethernet adapter by a Physical Ethernet adapter for the EtherChannel backup channel.