Hello Experts,

I have an issue, pending from long, I would really apricate if someone could help me out.

I have Debian 10, and KVM installed on it. hardware has two interface i.e. eth1 and eth0, I have created a bridge interfaces. Br1 mapped to eth1 and br0 mapped with eth0. everything is working fine, my virtual machine is able to send traffic through bridge interface, I am able to assign IP through DCHP or Static. no issue at all.

the only issue I have is: when i unplugged the cable from physical interface i.e. eth1 or eth0 and check the status "ip a" eth1 and eth0 shows down but bridge interface shows up and inside the VM ports shows up as well.

now, after unplugging the cable if i restart the network services manually through /etc/init.d/networking restart or services start networking.services then bridge interface status shows down. after that it continue works normal till I reboot the host. after rebooting the host the br0/1 status changes from down to up and eth1/0 status remain down.

note- virtual machine port station always remain up it never goes down is it a normal behavior?

when i checked the network service status it shows active/exited
below are some output of my interfaces script and ip a output.
~# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br1 state UP group default qlen 1000
link/ether 94:b8:c5:9a:6b:6c brd ff:ff:ff:ff:ff:ff
3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br0 state UP group default qlen 1000
link/ether 94:b8:c5:9a:6b:6d brd ff:ff:ff:ff:ff:ff
4: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 94:b8:c5:9a:6b:6d brd ff:ff:ff:ff:ff:ff
inet 192.168.0.3/24 brd 192.168.0.255 scope global br0
valid_lft forever preferred_lft forever
inet6 fe80::96b8:c5ff:fe9a:6b6d/64 scope link
valid_lft forever preferred_lft forever
5: br1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 94:b8:c5:9a:6b:6c brd ff:ff:ff:ff:ff:ff
inet 192.168.1.3/24 brd 192.168.1.255 scope global br1
valid_lft forever preferred_lft forever
inet6 fe80::96b8:c5ff:fe9a:6b6c/64 scope link
valid_lft forever preferred_lft forever
8: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br1 state UNKNOWN group default qlen 1000
link/ether fe:54:00:03:a5:17 brd ff:ff:ff:ff:ff:ff
inet6 fe80::fc54:ff:fe03:a517/64 scope link
valid_lft forever preferred_lft forever
9: vnet1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN group default qlen 1000
link/ether fe:54:00:2a:3f:ba brd ff:ff:ff:ff:ff:ff
inet6 fe80::fc54:ff:fe2a:3fba/64 scope link
valid_lft forever preferred_lft forever

cat /etc/network/interfaces
# The loopback network interface
auto lo
iface lo inet loopback

# ("internal") network interface
auto eth0
iface eth0 inet manual

auto br0
iface br0 inet static
address 192.168.0.4
netmask 255.255.255.0
bridge_ports eth0
bridge_stp off
bridge_fd 0
bridge_maxwait 0

# (RJ45) network interface
auto eth1
iface eth1 inet manual

auto br1
iface br1 inet dhcp
#address 192.168.1.4
#netmask 255.255.255.0
#gateway 192.168.1.1
bridge_ports eth1
bridge_stp off
bridge_fd 0
bridge_maxwait 0

Do you have the output that eth0 or eth1 is down but br0 or br1 is up?