Hi, all
In bird, BFD sessions are distinguished only by their peers' address, so if
multiple routing protocols request BFD session for the same neighbor, they can
share and be controlled by one session. Normally it won't be a problem.
However, it is not the case when it come to the IPv6 link-local addresses.
Since fe80:: addresses are link-local, the same address on different interfaces
refers to different nodes. So when requesting BFD sessions for neighbors with
link-local address, the destination interface should also be considered.
This issue can be reproduced by the following configuration with the latest
commit in legacy branch, and a similar configuration with the latest commit in
master branch:
```
router id 192.168.0.1;
protocol bfd bfd1{
interface "eth*";
};
protocol device {
};
protocol static {
route 2001:db8::/32 multipath
via fe80::1%eth0 bfd
via fe80::1%eth1 bfd
;
};
```
As shown in `show bfd session`, only one BFD session is created:
```
# ./birdc sh bfd sess
BIRD 1.6.8 ready.
bfd1:
IP address Interface State Since Interval Timeout
fe80::1 eth0 Down 01:33:24 1.000 0.000
```
Expected behavior: two BFD sessions are created with both routers on both
interfaces.
Cheers,
Miao Wang
