Hello,
This may have been discussed, but all search engines couldn't give me a good answer...
I notice that when an interface is up/running, a local route is in the routing table:
$ ifconfig eth1
eth1 Link encap:Ethernet HWaddr 00:13:20:0e:2f:ed
inet addr:192.168.1.125 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::213:20ff:fe0e:2fed/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:35984995 errors:0 dropped:0 overruns:0 frame:0
TX packets:7409151 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3252413825 (3.2 GB) TX bytes:1340077250 (1.3 GB)
$ ip route
192.168.20.0/24 dev eth0 proto kernel scope link src 192.168.20.120
192.168.1.0/24 dev eth1 proto kernel scope link src 192.168.1.125
default via 192.168.20.254 dev eth1 metric 100
After I unplug the cable from eth1, the RUNNING flag disappears, but the route is still there:
$ ifconfig eth1
eth1 Link encap:Ethernet HWaddr 00:13:20:0e:2f:ed
inet addr:192.168.1.125 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::213:20ff:fe0e:2fed/64 Scope:Link
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:35985023 errors:0 dropped:0 overruns:0 frame:0
TX packets:7409151 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3252415633 (3.2 GB) TX bytes:1340077250 (1.3 GB)
$ ip route
192.168.20.0/24 dev eth0 proto kernel scope link src 192.168.20.120
192.168.1.0/24 dev eth1 proto kernel scope link src 192.168.1.125
default via 192.168.20.254 dev eth1 metric 100
And that *prevents* from using the default route to reach 192.168.1/24 subnet after eth1 is out.