AS1                           :    AS 2
                                                    :
                       :                            :
           Level 1     :     Level 2                :
                       :                            :
                       :                            :
+-----+             +-----+-IP3--1-IP4--+-----+     :
| R1  +-IP1--1-IP2--+ R2  |             | R3  |     :
+--+--+             +--+--+-IP5--3-IP6--+--+--+-IP15-IP16- 
   |                   |                   |        :     \
  IP9                 IP7                IP13       :      \
   |                   |                   |        :    +--+--+
   1                   1                   1        :    |  R7 |
   |                   |                   |        :    +--+--+
 IP10                 IP8                IP14       :      /
   |                   |                   |        :     /
+--+--+             +--+--+             +--+--+-IP17-IP18- 
|  R4 +-IP19-2-IP20-+ R5  |-IP11-2-IP12-| R6  |     :
+-----+             +-----+             +-----+     :
                       :                            :
                       :                            :
                       :                            :
	  

Figure 11: Sample Topology

4.1.1. 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

4.1.2. Link IP addresses

• R1 to R2 link: 10.0.0.1, 10.0.0.2
• R1 to R4 link: 10.0.0.9, 10.0.0.10
• 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.

4.2. 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:

• TLV 149: MPLS label <N>, Flags {}:
  • IPv4 Prefix ERO subTLV: 192.168.1.2/32, Strict

4.3. 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

• TLV 149: MPLS label <N>, Flags {}:
  • IPv4 Prefix ERO subTLV: 10.0.0.6/32, Strict

4.4. Advertisement of Fast Re-Route LSP for One-Hop LSP

R2 may advertise a one-hop LSP from R2 to R3, along with a Link Protection Bypass for the directly adjacent links between those two nodes. The Link Protection Bypass would use the path: {R2, R5, R6, R3}. R2 would encode both the primary LSP and Link Protection Bypass LSP as follows:

• TLV 149: MPLS label <N>, Flags {}:
  • IPv4 Prefix ERO subTLV: 192.168.1.3/32, Strict
  • IPv4 Prefix Bypass ERO subTLV: 192.168.1.5/32, Strict
  • IPv4 Prefix Bypass ERO subTLV: 192.168.1.6/32, Strict
  • IPv4 Prefix Bypass ERO subTLV: 192.168.1.3/32, Strict

4.5. 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

• TLV 149: MPLS label <N>, Flags {}:
  • IPv4 Prefix ERO subTLV: 10.0.0.4/32, Strict
  • IPv4 Prefix ERO subTLV: 192.168.1.6/32, Strict

4.6. 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 binding in IS-IS it would encode as follows

• TLV 149: MPLS label <N>, Flags {}:
  • IPv4 Prefix ERO subTLV: 172.16.0.0/12, Loose

4.7. 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:

• TLV 149: MPLS label <N1>, Flags {}:
  • IPv4 Prefix ERO subTLV: 192.168.1.3, Strict
  • IPv4 Prefix ERO subTLV: 192.168.1.6, Strict
• TLV 149: MPLS label <N2>, Flags {}:
  • IPv4 Prefix ERO subTLV: 192.168.1.5, Strict
  • IPv4 Prefix ERO subTLV: 192.168.1.6, Strict

R5 encodes its two paths to R3 as follows:

• TLV 149: MPLS label <N1>, Flags {}:
  • IPv4 Prefix ERO subTLV: 192.168.1.2, Strict
  • IPv4 Prefix ERO subTLV: 192.168.1.3, Strict
• TLV 149: MPLS label <N2>, Flags {}:
  • IPv4 Prefix ERO subTLV: 192.168.1.6, Strict
  • IPv4 Prefix ERO subTLV: 192.168.1.3, Strict

A receiving L1 router does see now all 4 paths and may decide to load-balance across all or a subset of them.

4.8. 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:

• TLV 149: MPLS Label <N1>, Flags {}:
  • All Router Block subTLV: Block Size 10, Algo 0, Topology 0
  • All Router ID IPv4 Map subTLV: 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:

• TLV 149, MPLS Label <N2>, Flags {}:
  • All Router Block subTLV: Block Size 10, Algo 0, Topology 0
  • All Router ID IPv4 Map subTLV: 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:

• TLV 149, MPLS Label <N3>, Flags {}:
  • All Router Block subTLV: Block Size 10, Algo 0, Topology 0
  • All Router ID IPv4 Map subTLV: 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:

• TLV 149, MPLS Label <N4>, Flags {}:
  • All Router Block subTLV: Block Size 10, Algo 0, Topology 0
  • All Router ID IPv4 Map subTLV: 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 10.0.0.4 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.

4.9. Expansion of an 'All Router Block' subTLV

All routers within a given area MUST advertise their Label Blocks along with an 'ID'. Now assume that the initial label block size assignment is too small to support all routers which generate an ordinal within an IGP domain. Consider the seven routers in Figure 11, and assume that R7 advertises a new ID '15' using an 'All Router ID Map' subTLV. ID '15' is outside of the range of '10' as per the previous example in Section 4.8. Now all the routers in an IGP domain need to advertise one more label block in order to map the ID '15' to an actual label value.

All routers would advertise in addition to their label block <N> with a size of 10, a second label block <N2> with a size sufficient enough that the new ordinal can get covered. In this example the same block size 10 is used also for the second label block. For example router R2 would advertise the following label bindings.

• TLV 149: MPLS Label <N1>, Flags {}:
  • All Router Block subTLV: Block Size 10, Algo 0, Topology 0
  • All Router ID IPv4 Map subTLV: ID 2, 192.168.1.2
• TLV 149: MPLS Label <N2>, Flags {}:
  • All Router Block subTLV: Block Size 10, Algo 0, Topology 0

Now the upstream router can map the new ID of R7 to an actual label value, as ID '15' corresponds to the 5th label inside the second Label block.

5. Inter Area Protocol Procedures

5.1. Applicability

Propagation of a MPLS LSP across a level boundary is a local policy decision.

5.2. Data plane operations

If local policy dictates that a given L1L2 router needs to re-advertise a MPLS LSPs from one Level 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 level. Depending on how to reach the re-advertised LSP, this is typically done using a MPLS 'SWAP' or 'SWAP/PUSH' data plane operation.

5.3. Control plane operations

5.3.1. MPLS Label operations

If local policy dictates that a given L1L2 router re-advertises a MPLS LSPs into another Level then it MUST prepend its "Traffic-Engineering-ID" as a loose hop in the Prefix ERO subTLV list. If the LSP is propagated from a higher Level to a lower Level then the 'Down' bit MUST be set.

5.3.2. MPLS Label Block operations

If local policy dictates that a given L1L2 router advertises its 'All Router Block' into another Level, then it also MUST re-advertise all known 'ID' ordinals (again gated by policy) to the respective other Level. Without knowledge of all 'ID's in the network no router is able to construct SPT label switched paths. If a Label Block and its ID mappings are propagated from a higher Level to a lower Level then the 'Down' bit MUST be set.

6. Acknowledgements

Many thanks to Yakov Rekhter and John Drake for their useful comments.

7. IANA Considerations

This documents request allocation for the following TLVs and subTLVs.