bgpd: EVPN route type-5 to type-2 recursive resolution using gateway IP

bgpd: EVPN route type-5 to type-2 recursive resolution using gateway IP

When EVPN prefix route with a gateway IP overlay index is imported into the IP
vrf at the ingress PE, BGP nexthop of this route is set to the gateway IP.
For this vrf route to be valid, following conditions must be met.
- Gateway IP nexthop of this route should be L3 reachable, i.e., this route
  should be resolved in RIB.
- A remote MAC/IP route should be present for the gateway IP address in the
  EVI(L2VPN table).

To check for the first condition, gateway IP is registered with nht (nexthop
tracking) to receive the reachability notifications for this IP from zebra RIB.
If the gateway IP is reachable, zebra sends the reachability information (i.e.,
nexthop interface) for the gateway IP.
This nexthop interface should be the SVI interface.

Now, to find out type-2 route corresponding to the gateway IP, we need to fetch
the VNI for the above SVI.

To do this VNI lookup effitiently, define a hashtable of struct bgpevpn with
svi_ifindex as key.

struct hash *vni_svi_hash;

An EVI instance is added to vni_svi_hash if its svi_ifindex is nonzero.

Using this hash, we obtain struct bgpevpn corresponding to the gateway IP.

For gateway IP overlay index recursive lookup, once we find the correct EVI, we
have to lookup its route table for a MAC/IP prefix. As we have to iterate the
entire route table for every lookup, this lookup is expensive. We can optimize
this lookup by adding all the remote IP addresses in a hash table.

Following hash table is defined for this purpose in struct bgpevpn
Struct hash *remote_ip_hash;

When a MAC/IP route is installed in the EVI table, it is also added to
remote_ip_hash.

It is possible to have multiple MAC/IP routes with the same IP address because
of host move scenarios. Thus, for every address addr in remote_ip_hash, we
maintain list of all the MAC/IP routes having addr as their IP address.

Following structure defines an address in remote_ip_hash.
struct evpn_remote_ip {
        struct ipaddr addr;
        struct list *macip_path_list;
};

A Boolean field is added to struct bgp_nexthop_cache to indicate that the
nexthop is EVPN gateway IP overlay index.

bool is_evpn_gwip_nexthop;

A flag BGP_NEXTHOP_EVPN_INCOMPLETE is added to struct bgp_nexthop_cache.

This flag is set when the gateway IP is L3 reachable but not yet resolved by a
MAC/IP route.

Following table explains the combination of L3 and L2 reachability w.r.t.
BGP_NEXTHOP_VALID and BGP_NEXTHOP_EVPN_INCOMPLETE flags

*                | MACIP resolved | MACIP unresolved
*----------------|----------------|------------------
* L3 reachable   | VALID      = 1 | VALID      = 0
*                | INCOMPLETE = 0 | INCOMPLETE = 1
* ---------------|----------------|--------------------
* L3 unreachable | VALID      = 0 | VALID      = 0
*                | INCOMPLETE = 0 | INCOMPLETE = 0

Procedure that we use to check if the gateway IP is resolvable by a MAC/IP
route:
- Find the EVI/L2VRF that belongs to the nexthop SVI using vni_svi_hash.
- Check if the gateway IP is present in remote_ip_hash in this EVI.

When the gateway IP is L3 reachable and it is also resolved by a MAC/IP route,
unset BGP_NEXTHOP_EVPN_INCOMPLETE flag and set BGP_NEXTHOP_VALID flag.

Signed-off-by: Ameya Dharkar <adharkar@vmware.com>