DNM prepare for iBGP

[mtomaska]

• Adds ability to set same ASN for spines and leafs in preparation for mixed iBGP and eBGP deployment.
• conditional add route-reflector-client for spines when when remote_as is set to internal as it is incase of iBGP (same asn)
  Continuation on top of Adjusts to test eBGP (different ASN values) #3900

Related: OSPRH-32310
Assisted-By: Claude Code

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[eduolivares]

Hi Miro,

I tested the all-iBGP topology (all nodes in AS 65000, remote-as internal everywhere) on titan145 and found that router-0 receives no BGP routes from the spines. The root cause is the iBGP split-horizon rule: spine nodes learn routes from leaves via iBGP but cannot re-advertise them to router-0 (also iBGP) unless the spine is configured as a route reflector.

The fix is to add neighbor downlink route-reflector-client in the IPv4 and IPv6 address families on the spines when the downlink peering is iBGP. I verified this on titan145 — after adding the directive and restarting FRR, router-0 immediately started receiving all expected BGP routes.

I think playbooks/bgp/templates/spine-frr.conf.j2 needs a conditional block like:

  address-family ipv4 unicast
    redistribute connected
    neighbor downlink default-originate
    neighbor downlink prefix-list only-host-prefixes in
{% if cifmw_bgp_spine_downlink_remote_as | default('external') == 'internal' %}
    neighbor downlink route-reflector-client
{% endif %}

And the same for address-family ipv6 unicast. Without this, anyone who overrides to the all-iBGP topology will hit the split-horizon issue — spines silently drop leaf-learned routes instead of forwarding them to router-0.

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