Dual-WAN addressing

Problem

I have two WAN interfaces from different ISPs. One is on ethernet0/0 and is static; one is on ethernet0/1 and is PPPoE. Both have default routes configured.

When both interfaces are up, the firewall prefers to use the PPPoE link. When the PPPoE link drops, the firewall automatically starts using ethernet0/0 instead.

However, I can only ping the IP on ethernet0/0 from the internet when the PPPoE link is down.

(This is because the reply-packet to the ping request is routed out the preferred default route, but the PPPoE ISP will not route packets with a source IP address of the ethernet0/0 ISP.)

Solution

Use this setting:

set flow mac-cache mgt
save

The management interfaces on ethernet0/0 are available now whenever the link on ethernet0/0 is up. The state of the PPPoE link does not affect availability.

Discussion

I frankly have no idea why this works, I just know it does.

(Source: JTAC, and this KB article)