AS1                           :    AS 2
                                                    :
                       :                            :
         Area 1        :      Area 0                :
                       :                            :
+-----+             +-----+-IP3--1-IP4--+-----+     :
| R1  +-IP1--1-IP2--+ R2  |             | R3  |     :
+--+--+             +--+--+-IP5--3-IP6--+--+--+-IP15-IP16- 
   |                   |                   |        :     \
  IP3                 IP7                IP13       :      \
   |                   |                   |        :    +--+--+
   1                   1                   1        :    |  R7 |
   |                   |                   |        :    +--+--+
  IP4                 IP8                IP14       :      /
   |                   |                   |        :     /
+--+--+             +--+--+             +--+--+-IP17-IP18- 
|  R4 +-IP19-2-IP20-+ R5  |-IP11-2-IP12-| R6  |     :
+-----+             +-----+             +-----+     :
                       :                            :
                       :                            :
                       :                            :
	  

Figure 14: Sample Topology

5.1.1. Transport IP addresses and router-IDs

• R1: 192.168.1.1
• R2: 192.168.1.2
• R3: 192.168.1.3
• R4: 192.168.1.4
• R5: 192.168.1.5
• R6: 192.168.1.6
• R7: 192.168.1.7

5.1.2. Link IP addresses

• R1 to R2 link: 10.0.0.1, 10.0.0.2
• R1 to R4 link: 10.0.0.3, 10.0.0.4
• R2 to R3 link #1: 10.0.0.3, 10.0.0.4
• R2 to R3 link #2: 10.0.0.5, 10.0.0.6
• R2 to R5 link: 10.0.0.7, 10.0.0.8
• R3 to R6 link: 10.0.0.13, 10.0.0.14
• R3 to R7 link: 10.0.0.15, 10.0.0.16
• R4 to R5 link: 10.0.0.19, 10.0.0.20
• R5 to R6 link: 10.0.0.11, 10.0.0.12
• R6 to R7 link: 10.0.0.17, 10.0.0.18

The IGP link metrics are displayed in the middle of the link. All of them are assumed to be bi-directional.

5.2. One-hop LSP to an adjacent Router

If R1 would advertise a label <N> bound to a one-hop LSP from R1 to R2 it would encode as follows:

• LSA 149, LSA-ID <N>:
  • IPv4 Prefix ERO TLV: 192.168.1.2/32, Strict

5.3. One-hop LSP to an adjacent Router using a specific link

If R2 would advertise a label <N>bound to a one-hop LSP from R2 to R3, using the link #2 it would encode as follows

• LSA 149, LSA-ID <N>:
  • IPv4 Prefix ERO TLV: 10.0.0.6/32, Strict

5.4. One-hop LSP to an adjacent external Router

If R3 would advertise a label <N> bound to a one-hop LSP from R3 to R7 (which is outside of the IGP domain), it would encode as follows:

• LSA 149, LSA-ID <N>:
  • IPv4 Prefix ERO TLV: 192.168.1.7/32, Strict

As you can see the representation of an MPLS label crossing an external link is identical as an internal link Section 5.2.

5.5. Advertisement of an RSVP LSP

Consider a RSVP LSP name "R2-to-R6" traversing (R2 to R3 using link #1, R6):

If R2 would advertise a label <N> bound to the RSVP LSP named 'R2-to-R6', it would encode as follows

• LSA 149, LSA-ID <N>:
  • IPv4 Prefix ERO TLV: 10.0.0.4/32, Strict
  • IPv4 Prefix ERO TLV: 192.168.1.6/32, Strict

5.6. Advertisement of an LDP LSP

Consider R2 that creates a LDP label binding for FEC 172.16.0.0./12 using label <N>.

If R2 would re-advertise this label binding in OSPF it would encode as follows

• LSA 149, LSA-ID <N>:
  • IPv4 Prefix ERO TLV: 172.16.0.0/12, Loose

5.7. Interarea advertisement of diverse paths

Consider two R2->R6 paths: {R2, R3, R6} and {R2, R5, R6}

Consider two R5->R3 paths: {R5, R2, R3} and {R5, R6, R3}

R2 encodes its two paths to R6 as follows:

• LSA 149, LSA-ID <N1>:
  • IPv4 Prefix ERO TLV: 192.168.1.3, Loose
  • IPv4 Prefix ERO TLV: 192.168.1.6, Loose
  • Flags TLV: Down
• LSA 149, LSA-ID <N2>:
  • IPv4 Prefix ERO subTLV: 192.168.1.5, Loose
  • IPv4 Prefix ERO subTLV: 192.168.1.6, Loose
  • Flags TLV: Down

R5 encodes its two paths to R3 as follows:

• LSA 149, LSA-ID <N1>:
  • IPv4 Prefix ERO subTLV: 192.168.1.2, Loose
  • IPv4 Prefix ERO subTLV: 192.168.1.3, Loose
  • Flags TLV: Down
• LSA 149, LSA-ID <N2>:
  • IPv4 Prefix ERO subTLV: 192.168.1.6, Loose
  • IPv4 Prefix ERO subTLV: 192.168.1.3, Loose
  • Flags TLV: Down

A receiving non-backbone router does see now all 4 paths and may decide to load-balance across all or a subset of them.

5.8. Advertisement of SPT labels using 'All Router Block' TLV

All routers within a given area MUST advertise their Label Blocks along with an 'ID'.

If R2 would advertise a label block <N1> with a size of 10, declaring SPT label forwarding support to all routers within a given domain, it would encode as follows:

• LSA 149, LSA-ID <N1>:
  • All Router Block TLV: Block Size 10
  • All Router ID IPv4 Map TLV: ID 2, 192.168.1.2

If R3 would advertise a label block <N2> with a size of 10, declaring SPT label forwarding support to all routers within a given domain, it would encode as follows:

• LSA 149, LSA-ID <N2>:
  • All Router Block TLV: Block Size 10
  • All Router ID IPv4 Map TLV: ID 3, 192.168.1.3

If R5 would advertise a label block <N3> with a size of 10, declaring SPT label forwarding support to all routers within a given domain, it would encode as follows:

• LSA 149, LSA-ID <N3>:
  • All Router Block TLV: Block Size 10
  • All Router ID IPv4 Map TLV: ID 5, 192.168.1.5

If R6 would advertise a label block <N4> with a size of 10, declaring SPT label forwarding support to all routers within a given domain, it would encode as follows:

• LSA 149, LSA-ID <N4>:
  • All Router Block TLV: Block Size 10
  • All Router ID IPv4 Map TLV: ID 6, 192.168.1.6

Consider now R2 constructing a SPT label for R6. R2s SPT to R6 is {R2, IP4, R3, R6}. R2 first determines if its downstream router (R3) has advertised a label-block. Since R3 has advertised a label block 'N2' and it has received R6 'ID' of 6 it will be picking the 6th label value inside the advertised range of its downstream neighbor. Specifically R2 MUST be program a MPLS SWAP for its own label range Label(N1+6) to Label(N2+6), NH 10.0.0.4 into its MPLS transit RIB. Furthermore R2 MAY program a MPLS PUSH operation for IP 192.168.1.6 to Label (N2+6), NH 10.0.0.04 into its IPv4 tunnel RIB.

Next walk down to R3, which is the next router on the SPT tree towards R6. R3s SPT to R6 is {R3, R6}. R3 determines if its downstream router (R6) has advertised a label-block. Since R6 has advertised a label block 'N4' and it has received R6 'ID' of 6 it will be picking the 6th label value inside the advertised range of its downstream neighbor. Since R3 is the penultimate router to R6 it MUST program a MPLS POP for its own label range Label(N2+6) NH 10.0.0.14 into its MPLS transit RIB. Furthermore R3 MAY program a MPLS NOP for IP 192.168.1.6, NH 10.0.0.14 into its IPv4 tunnel RIB.

6. Inter Area Protocol Procedures

6.1. Applicability

Propagation of a MPLS LSPs and MPLS Block LSPs across an area boundary is a local policy decision.

6.2. Data plane operations

If local policy dictates that a given ABR router needs to re-advertise a MPLS LSPs from one area to another then it MUST allocate a new label and program its label forwarding table to connect the new label to the path in the respective other area. Depending on how to reach the re-advertised LSP, this is typically done using a MPLS 'SWAP' or 'SWAP/PUSH' data plane operation.

6.3. Control plane operations

6.3.1. MPLS Label operations

If local policy dictates that a given ABR router needs to re-advertise MPLS LSPs from one area to another then it must prepend its "Traffic-Engineering-ID" as a loose hop in the Prefix ERO TLV list. Furthermore it MUST append the Flags TLV and set the 'Down' Bit.

6.3.2. MPLS Label Block operations

If local policy dictates that a given ABR router advertises its 'All Router Block' into another area, then it also MUST re-advertise all known 'ID' ordinals (again gated by policy) to the respective other area. Without knowledge of all 'ID's in the network no router is able to construct SPT label switched paths. Furthermore an ABR MUST append the Flags TLV and set the 'Down' Bit for all re-advertised 'CE' IDs.

7. Acknowledgements

Many thanks to Yakov Rekhter and Shraddha Hedge for their useful comments.

8. IANA Considerations

This documents request allocation for one common OSPFv2 and OSPFv3 LSA Type and TLVs contained within.