Once upon a time, Rick Stevens <ricks(a)alldigital.com> said:
Now, if we want one of the uplinks to remain unavailable, it is
"administratively down" (in Cisco IOS-speak). For Linux, this would
essentially be "ip link dev <devname> down". This takes the link down,
but doesn't change the IP address for it.
The difference is that Cisco can leave an interface configured but
inactive, while Linux doesn't really support that. On a Cisco, the IP
will still show up configured on the interface, but things like "show ip
route <IP>" will not show it (it is not actually active when the
interface is down).
If you configure a Cisco interface for 10.0.0.1/24 and the link drops,
the route for 10.0.0.0/24 is removed from the routing table. The
interface shows down, but it is still watching for link to come back.
When that happens, 10.0.0.0/24 is re-added to the routing table.
On Linux, with static config, the route would still point out the down
interface. If you shut the link down, the route will be removed, but
then the interface will not come back automatically when link returns.
You have to have some type of user-space active network management to
get basic router-like behavior (functionally, that's how Cisco actually
works, it's just all under the hood).
Chris Adams <linux(a)cmadams.net>