*: support replace-peer-as (aka as-override)

we use the term replace-peer-as instead of as-override
since openconfig is using it.

cli

```
$ gobgp n add <neighbor-addr> as <asn> replace-peer-as
```

config

```
neighbor:
  config:
    peer-as: <asn>
    neighbor-address: <neighbor-addr>
  as-path-options:
    config:
      replace-peer-as: true
```

Signed-off-by: Wataru Ishida <ishida.wataru@lab.ntt.co.jp>

2 files changed, 40 insertions, 0 deletions

diff --git a/table/path.go b/table/path.go
index 3b0fc40d..0453139a 100644
--- a/table/path.go
+++ b/table/path.go
@@ -713,6 +713,32 @@ func (path *Path) RemovePrivateAS(localAS uint32, option config.RemovePrivateAsO
 	return
 }
 
+func (path *Path) ReplaceAS(localAS, peerAS uint32) *Path {
+	original := path.GetAsPath()
+	if original == nil {
+		return path
+	}
+	newASParams := make([]bgp.AsPathParamInterface, 0, len(original.Value))
+	changed := false
+	for _, param := range original.Value {
+		asParam := param.(*bgp.As4PathParam)
+		newASParam := make([]uint32, 0, len(asParam.AS))
+		for _, as := range asParam.AS {
+			if as == peerAS {
+				as = localAS
+				changed = true
+			}
+			newASParam = append(newASParam, as)
+		}
+		newASParams = append(newASParams, bgp.NewAs4PathParam(asParam.Type, newASParam))
+	}
+	if changed {
+		path = path.Clone(path.IsWithdraw)
+		path.setPathAttr(bgp.NewPathAttributeAsPath(newASParams))
+	}
+	return path
+}
+
 func (path *Path) GetCommunities() []uint32 {
 	communityList := []uint32{}
 	if attr := path.getPathAttr(bgp.BGP_ATTR_TYPE_COMMUNITIES); attr != nil {
diff --git a/table/path_test.go b/table/path_test.go
index 29a296ac..ea4e23d5 100644
--- a/table/path_test.go
+++ b/table/path_test.go
@@ -353,3 +353,17 @@ func TestRemovePrivateAS(t *testing.T) {
 	assert.Equal(t, list[2], uint32(1))
 	assert.Equal(t, list[3], uint32(2))
 }
+
+func TestReplaceAS(t *testing.T) {
+	aspathParam := []bgp.AsPathParamInterface{bgp.NewAs4PathParam(2, []uint32{64512, 64513, 1, 2})}
+	aspath := bgp.NewPathAttributeAsPath(aspathParam)
+	nlri := bgp.NewIPAddrPrefix(24, "30.30.30.0")
+	path := NewPath(nil, nlri, false, []bgp.PathAttributeInterface{aspath}, time.Now(), false)
+	path = path.ReplaceAS(10, 1)
+	list := path.GetAsList()
+	assert.Equal(t, len(list), 4)
+	assert.Equal(t, list[0], uint32(64512))
+	assert.Equal(t, list[1], uint32(64513))
+	assert.Equal(t, list[2], uint32(10))
+	assert.Equal(t, list[3], uint32(2))
+}