mLDP Signalling: In Band and Out of Band

In my previous post, I discussed about the overall architecture of mLDP. This post is more focused on mLDP Signalling. mLDP signalling provides two functions
1. FEC Discovery for a MP LSP
2. Assigning multicast flow to a MP LSP

mLDP can use two signalling methods; In-band Signalling and Out-of-Band Signalling. FEC uniquely defines the MP LSP within the network by using combination of VPN-ID and it’s opaque value. The signalling maps the streams that will run over that MP LSP.
• In-Band Signalling
Opaque Value is used to map an MP LSP
Opaque value is derived from the multicast flow

• Out-Of-Band Signalling
 Uses an overlay protocol to build the tree
 Opaque value are assigned by statically configured
 MP LSP creation is on-demand

In-Band Signalling Operation
It is called in-band signalling because the Egress PE uses the multicast stream information to create the Opaque Value and the Ingress PE uses this Opaque value to learn what multicast flow to send on the MP LSP.
1. The Egress PE receives a (S, G) IGMP join from a receiver. It creates a label mapping message containing the FEC TLV with the Opaque Value and a label TLV based on this information. The root address in the FEC Element will be derived from the BGP next-hop or (S). All ingress PE routers will create exactly the same FEC Element.


Debugs of inband signalling:-
mpls_ldp[1042]: DBG-mLDP[4142372544], Peer(9.9.0.7:0)/Root(9.9.0.3): ldp_mldp_sig_peer_msg_rx_nfn: 'Label-Mapping' msg (type 0x400), msg_id 167, lbl 23, MP2MP-Dn FEC { root 9.9.0.3, opaque-len 14 }, no MP-Status, status_code 0x0
2. The Egress PE then builds an MP LSP towards the Ingress LSP (root of the tree) using the label mapping message with the downstream label( lbl 23, MP2MP-Dn FEC { root 9.9.0.3, opaque-len 14 }). At each hop along the way, the P routers will use the same FEC Element, but the downstream label will change. When the ingress PE receives the label mapping message, it parses the Opaque Value to extract the multicast stream information and creates the appropriate (S, G) state and mapping information.
3. When the Ingress PE receives the (S, G} stream it will then forward it onto the correct MP LSP using it’s label information.


Out-of-Band (Overlay) Signalling Operation
Overlay protocol signals the mapping of the IP multicast flow to a MP LSP.
1. The Egress PE creates the FEC through a static procedure and builds the MP LSP hop-by-hop to the root based on FEC information.
2. Using overlay signalling, the Egress PE signals the Ingress PE to forward the IP multicast stream over the MP LSP with the unique FEC value.
3. The Ingress PE then forwards the (S, G) traffic onto the tree using the mapping information.

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