os-frr: No possibility to select specific IP address as BGP source if interface contains multiple IP

[n3wtype]

Describe the bug
No possibility to select specific ip address as a source of bgp sessions. If interface has multiple ip addresses address selection is undeterministic. This is causing bgp neighbor mismatches on both side of bgp session depending on which.

There is possibility to select source interface, but it does not guarantee any specific IP from interface to be used.
The "Local initiater IP field is not used for update-source statement in frr.

# grep neighbor.localip * -r
net/frr/src/opnsense/mvc/app/controllers/OPNsense/Quagga/forms/dialogEditBGPNeighbor.xml:    <id>neighbor.localip</id>
net/frr/src/opnsense/service/templates/OPNsense/Quagga/sa_policies.conf:dst={{ neighbor.localip }}
net/frr/src/opnsense/service/templates/OPNsense/Quagga/sa_policies.conf:src={{ neighbor.localip }}


# grep update-source * -r
net/frr/src/opnsense/service/templates/OPNsense/Quagga/bgpd.conf: neighbor {{ peergroup.name }} update-source {{ physical_interface(peergroup.updatesource) }}
net/frr/src/opnsense/service/templates/OPNsense/Quagga/bgpd.conf: neighbor {{ neighbor.address }} update-source {{ physical_interface(neighbor.updatesource) }}

To Reproduce
Steps to reproduce the behavior:

1. Go to "Routing/BGP/Neighbor"
2. set "local initiater IP"
3. specified address is not used in frr config as value of update-source for given neighbor

Expected behavior
When "Local initiater IP" field is set, update-source within frr confguration should contain this IP, not an interface name. "Local initiater IP" should take precedence over source interface

Environment
OPNsense 26.1.5-amd64
FreeBSD 14.3-RELEASE-p9
OpenSSL 3.0.19

[AdSchellevis]

In most cases I would advice to match the traffic with NAT and use the carp address, which usually works like a charm and is highly flexible (eases administration too). If someone wants to add a specific address as well, I don't mind that much, as long as he/she also adds the documentation required.

[OPNsense-bot]

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[n3wtype]

I have created PR for this change.

In our scenario BGP source address and CARP VIP are not always the same. It was confirmed that on BGP session with also bfd enabled we get most stable and predictable result by setting update-source to specific address.