MAD mechanisms

IRF provides MAD mechanisms by extending LACP, BFD, ARP, and IPv6 ND.

	IMPORTANT: You can configure BFD MAD, ARP MAD, and ND MAD together in an IRF fabric for prompt IRF split detection. However, do not configure any of these mechanisms together with LACP MAD, because they handle collisions differently.

Table 1 compares the MAD mechanisms and their application scenarios.

As shown in Figure 5, LACP MAD has the following requirements:

• Every IRF member must have a link with an intermediate device.

• All the links form a dynamic link aggregation group.

• The intermediate device must be a device that supports extended LACP for MAD.

The IRF member devices send extended LACPDUs that convey a domain ID and an active ID. The intermediate device transparently forwards the extended LACPDUs received from one member device to all the other member devices.

• If the domain IDs and active IDs sent by all the member devices are the same, the IRF fabric is integrated.

• If the extended LACPDUs convey the same domain ID but different active IDs, a split has occurred. LACP MAD handles this situation as described in "Collision handling."

You can use common or management Ethernet ports for BFD MAD links.

• If management Ethernet ports are used, BFD MAD has the following requirements:

  • Each IRF member device has a BFD MAD link to an intermediate device.

  • Each member device is assigned a MAD IP address on the master's management Ethernet port.

• If common Ethernet ports are used, BFD MAD has the following requirements:

  • If an intermediate device is used, each member device must have a BFD MAD link to the intermediate device. If no intermediate device is used, all member devices must have a BFD MAD link to each other. As a best practice, use an intermediate device to connect IRF member devices if the IRF fabric has more than two member devices. A full mesh of IRF members might cause broadcast loops.

  • Each member device is assigned a MAD IP address on a VLAN interface.

The BFD MAD links must be dedicated. Do not use BFD MAD links for any other purposes.

	NOTE: The MAD addresses identify the member devices and must belong to the same subnet. Of all management Ethernet ports on an IRF fabric, only the master's management Ethernet port is accessible.

Figure 6 shows a typical BFD MAD scenario that uses an intermediate device. Figure 7 shows a typical BFD MAD scenario that does not use an intermediate device.

With BFD MAD, the master attempts to establish BFD sessions with other member devices by using its MAD IP address as the source IP address.

• If the IRF fabric is integrated, only the MAD IP address of the master takes effect. The master cannot establish a BFD session with any other member. If you execute the display bfd session command, the state of the BFD sessions is Down.

• When the IRF fabric splits, the IP addresses of the masters in the split IRF fabrics take effect. The masters can establish a BFD session. If you execute the display bfd session command, the state of the BFD session between the two devices is Up.

ARP MAD detects multi-active collisions by using extended ARP packets that convey the IRF domain ID and the active ID.

You can use common or management Ethernet ports for ARP MAD.

• If management Ethernet ports are used, ARP MAD must work with an intermediate device. Make sure the following requirements are met:

  • Connect a management Ethernet port on each member device to the intermediate device.

  • On the intermediate device, you must assign the ports used for ARP MAD to the same VLAN.

• If common Ethernet ports are used, ARP MAD can work with or without an intermediate device. Make sure the following requirements are met:

  • If an intermediate device is used, connect each IRF member device to the intermediate device. Run the spanning tree feature between the IRF fabric and the intermediate device. In this situation, data links can be used.

  • If no intermediate device is used, connect each IRF member device to all other member devices. In this situation, IRF links cannot be used for ARP MAD.

Figure 8 shows a typical ARP MAD scenario that uses an intermediate device.

Each IRF member compares the domain ID and the active ID in incoming extended ARP packets with its domain ID and active ID.

• If the domain IDs are different, the extended ARP packet is from a different IRF fabric. The device does not continue to process the packet with the MAD mechanism.

• If the domain IDs are the same, the device compares the active IDs.

  • If the active IDs are different, the IRF fabric has split.

  • If the active IDs are the same, the IRF fabric is integrated.

ND MAD detects multi-active collisions by using NS packets to transmit the IRF domain ID and the active ID.

You can set up ND MAD links between neighbor IRF member devices, or between each IRF member device and an intermediate device (see Figure 9). If an intermediate device is used, you must also run the spanning tree protocol between the IRF fabric and the intermediate device.

Each IRF member device compares the domain ID and the active ID in incoming NS packets with its domain ID and active ID.

• If the domain IDs are different, the NS packet is from a different IRF fabric. The device does not continue to process the packet with the MAD mechanism.

• If the domain IDs are the same, the device compares the active IDs.

  • If the active IDs are different, the IRF fabric has split.

  • If the active IDs are the same, the IRF fabric is integrated.