feat(bgp_policies.go): don't override-nexthop for internal peers

Previously when a user selected to override the next-hop via GoBGP's
NextHopActions: Self functionality, we did it for all exported routes.
However, in a dual-stack use-case this causes problems for internal pod
IP routes that are spread via BGP advertisements.

Currently, kube-router only peers with an internal peer once over
whatever it's primary IP is according to it's Kubernetes node
information. This means that when overriding next-hop the IP is either
an IPv4 or IPv6 address depending on how the node has configured itself.
Therefore when it attempts to add a route for an IPv6 address and
override next-hop is configured, if the node's primary IP was an IPv4
address this will not succeed as a next-hop for an IPv6 address cannot
be an IPv4 gateway.

Rather than making the code base overly complicated with both an IPv4
and IPv6 peering for internal nodes, this change presents a bit of a
middle ground. By choosing not to override the next-hop for pod subnet
advertisements to internal (Kubernetes node) peers, we eliminate this
problem.

This does change the functionality of kube-router a bit, but one of the
foundational aspects to Kubernetes networking is that all nodes should
be able to contact each other. So I cannot currently think of a good
use-case where overriding the next-hop for pod subnets of internal peers
would be necessary, so I think that this is an ok concession to make.

pkg/controllers/routing/bgp_policies.go

@@ -605,9 +605,6 @@ func (nrc *NetworkRoutingController) addExportPolicies() error {
 		actions := gobgpapi.Actions{
 			RouteAction: gobgpapi.RouteAction_ACCEPT,
 		}
-		if nrc.overrideNextHop {
-			actions.Nexthop = &gobgpapi.NexthopAction{Self: true}
-		}
 
 		// statement to represent the export policy to permit advertising node's IPv4 & IPv6 pod CIDRs
 		for _, podSet := range []string{podCIDRSet, podCIDRSetV6} {