add docs

Author: deng.zhou

Diff for: docs/bgp.md

@@ -158,3 +158,15 @@ kubectl annotate node ip-172-20-46-87.us-west-2.compute.internal "kube-router.io
 
 By default kube-router populates GoBGP RIB with node IP as next hop for the advertised pod CIDR's and service VIP. While this works for most cases, overriding the next hop for the advertised rotues is necessary when node has multiple interfaces over which external peers are reached. Next hop need to be as per the interface local IP over which external peer can be reached. `--override-nexthop` let you override the next hop for the advertised route. Setting `--override-nexthop` to true leverages BGP next-hop-self functionality implemented in GoBGP. Next hop will automatically selected appropriately when advertising routes irrespective of the next hop in the RIB. 
 
+## Overriding the next hop and enable IPIP/tuennel
+
+ In a scenario there are multiple groups of nodes in different subnets and user wants to peer mulitple upstream routers for each of their node (.e.g., a cluster has seperate public and private networking, and the nodes in this cluster are located into two different racks, which has their own routers. So there would be two upstream routers for each node, and there are two different subnets in this case), The override-nexthop and tunnel cross subnet features need to be used together to achive the goal.
+
+ to support the above case, user need to set `--enable-overlay` and `--override-nexthop` to true together. This configuration would have the following effect.
+* Peering Outside the Cluster (https://github.com/cloudnativelabs/kube-router/blob/master/docs/bgp.md#peering-outside-the-cluster) via one of the many means that kube-router makes that option available
+* Overriding Next Hop
+* Enabling overlays in either full mode or with nodes in different subnets
+
+The warning here is that when using `--override-nexthop` in the above scenario, it may cause kube-router to advertise an IP address other than the node IP which is what kube-router connects the tunnel to when the `--enable-overlay` option is given. If this happens it may cause some network flows to become un-routable.
+
+Specifically, people need to take care when combining `--override-nexthop` and `--enable-overlay` and make sure that they understand their network, the flows they desire, how the kube-router logic works, and the possible side-effects that are created from their configuration. Please refer to this PR for the risk and impact discussion https://github.com/cloudnativelabs/kube-router/pull/1025.