1. Scaling LDP based PW Service Across
Multiple Regions
Bhupesh Kothari

2. Agenda
Introduction
Objectives
Existing solutions
Proposed Solution
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3. Introduction
Reference Model ‘1’ (Inter-AS)
Intra-region PWs (Single Segment PWs)
Region 1 Region 2
Inter-region PWs
Inter-region LSP
Intra-region LSPs
Emulated Service
Terminating Provider Edge (T-PE)
Switching Provider Edge (S-PE)
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4. Introduction
Reference Model ‘2’ (Inter-Area)
Intra-region PWs (Single Segment PWs)
Region 1 Region 3
(Metro) Region 2
(Metro)
(WAN)
Intra-region LSPs
Emulated Service
Terminating Provider Edge (T-PE)
Switching Provider Edge (S-PE)
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5. Objectives
Provide PW service that extends from a T-PE in one region
to a T-PE in another region
 Regions can be BGP Autonomous Systems belonging to same or different
Service Providers
 Regions can be IGP areas belonging to a given Service Provider
Minimize complexity in provisioning large number of PWs
across regions
 Provisioning tens of thousands of stitched PW paths is very complex
Scale PW service in presence of large number of T-PEs
 T-PEs are typically low cost devices with limited processing power.
Number of T-PEs could be in tens of thousands
Maximize reuse of existing T-PE’s functional capabilities
 Requiring support for routing protocols such as BGP on T-PEs might not be
feasible
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6. Existing Solutions
BGP based Virtual Private Wire Service (VPWS)
 Two Inter-AS mechanism that provide dynamic placement of PWs across
regions
 Addresses scaling and PW provisioning objectives
 Requires BGP on T-PEs
LDP based Virtual Private Wire Service (VPWS)
 Static Provisioning of PWs
 Addresses neither scaling nor PW provisioning objectives
 Dynamic placement of PWs
 Two solutions:
– PW routing: Use of routing protocol (BGP) to advertise PW routes
– BGP-AD: Use of BGP to auto-discover T-PEs
 Addresses scaling and PW provisioning objectives
 Requires BGP on T-PEs
 Requires LDP FEC-129 deployment
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7. Proposed Solution
Use LDP signaling for setting up PWs within a region that
contains T-PEs
 LDP signaling can either be FEC-128 or FEC-129
Use BGP for setting PWs among the transit regions and for
stitching intra-region PWs at the regions boundaries
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8. Objectives Met
Inter-region PWs
 BGP stitching allows PW service originating from one region to be
terminated in another region
Provisioning
 Use of BGP simplifies provisioning on both T-PEs and S-PEs
Significant reuse of T-PE’s functional capabilities
 Support of FEC-128 with minor extension allows reuse of existing
deployment while extending the PW service across regions
 Requires no routing protocol such as BGP to advertise PW routes
Scaling
 LDP sessions are only confined within a region
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9. Solution Overview: Based on model ‘1’ and use of FEC-129
AS 1 Labeled MP-EBGP
AS 2
ASBR1 ASBR2
T-PE1 T-PE2
PW Stitching
T-PE3 T-PE4
Emulated Service
LDP PWs Labeled BGP PWs PSN Tunnel
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10. Provisioning: Based on model ‘1’ and use of FEC-129
What is provisioned on T-PEs:
 <SAI,TAI> (Source/Target Attachment Identifier)
 SAI: <AGI, SAII>
 AGI: Attachment Group Identifier
– Configured in Route Target format
 SAII: Source Attachment Individual Identifier
– Configured as 4 byte number
 Peer address for Targeted LDP session:
 On T-PE1, peer address is ASBR1 address
 On T-PE2, peer address is ASBR2 address
 Note that on T-PEs, the remote T-PE address is not part of
provisioning. Use of BGP on S-PEs help discover the remote T-PEs.
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11. Provisioning: Based on model ‘1’ and use of FEC-129
What is provisioned on ASBRs:
 Global configuration to enable the PW stitching
 Global configuration policy for setting Targeted LDP sessions
There is no per PW or per VPN configuration required on
ASBRs
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12. Signaling: Based on model ‘1’ and use of FEC-129
T-PE1 and ASBR1 forms a targeted LDP session and does regular
LDP-FEC129 signaling
 FEC-129 carries <SAI, TAI>
ASBR1 operations on receiving FEC-129 advertisement from T-PE1:
 Extracts the Route Target from the AGI
 Constructs the NLRI based on signaling information from T-PE
 NLRI contains:
– SAI
– TAI
– PW Label
 Sets the BGP next hop to self
 Allocates new label and creates the PW stitching state (label swap)
 ASBR1 floods this advertisement to all other ASBRs over EBGP, after
applying RT constrains if any
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13. Signaling: Based on model ‘1’ and use of FEC-129
ASBR2 operations:
 ASBR2 establishes a targeted LDP session to T-PE2
 On receiving the BGP route from ASBR1, ASBR2 translates the BGP
NLRI into FEC-129 and sends it to T-PE2
 Based on the AGI, ASBR2 can identify which PEs in AS2 are interested in the
FEC-129 advertisement. In this example, on slide 9, it is T-PE2
 Allocates new label and creates the PW stitching state in data path
T-PE2 operations:
 Establishes a targeted LDP session to ASBR2 and sends the FEC-129
advertisement carrying <SAI, TAI>
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14. Signaling: Based on model ‘1’ and use of FEC-129
Optional TLVs at the end of the NLRI
All LDP TLVs such as PW Status, Interface Parameters, etc. are
carried as TLVs at the end of the BGP NLRI
If RT constrains are in use, ASBR originates Route Target
Membership NLRI Advertisements for all AGIs in the FEC-129
advertisements received from the T-PEs
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15. Solution Overview: Use of FEC-128
Operations with FEC-128 are very similar to those explained in
previous slides
Key differences in provisioning:
 Instead of SAI and TAI, VC-ID is provisioned on the T-PEs
 Remote PE address is part of provisioning on the T-PEs
Key differences in signaling:
 BGP NLRI is agnostic of whether LDP signaling is based on FEC-128 or
FEC-129, and therefore, operations on ASBRs remain same
 ASBRs construct the NLRI based on information contained in FEC-128
advertisement
 SAI is constructed based on source PE address and TAI is constructed
based on target PE address
 Route Targets facilitate the flooding of BGP advertisements towards the
target T-PEs
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16. Model ‘2’
Area 2
Area 1 IBGP Session Area 3
ABR1 ABR2
T-PE2
T-PE1
PW Stitching
T-PE3 T-PE4
Emulated Service
LDP PWs Labeled BGP PWs PSN Tunnel
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17. Variant of Model ‘1’
Multihop EBGP
Route Route LDP
LDP Reflector Reflector Session
Session
T-PE2
Labeled BGP ASBR2
ASBR1
AS 1 AS 2
T-PE3 T-PE4
LDP PWs End-to-end Tunnel PSN Tunnel
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18. Summary
T-PEs do not need new routing protocol support such as BGP for
dynamic placement of PWs
LDP signaling on the PEs can be either FEC-128 or FEC-129
 Minor extensions are needed for FEC-128 and FEC-129
LDP signaling for setting up PWs among regions is not needed
 BGP is used for stitching inter-region PW
 Standard BGP procedures and policy mechanisms can be used for inter-region
communication
Use of BGP on S-PEs to stitch LDP PWs avoids the need to have full
mesh of LDP adjacencies
 Addresses scalability of T-PEs
QoS
 Proposed solution is only a signaling framework and thus is agnostic to data path
features
OAM
 Existing OAM procedures are sufficient
 No new procedures defined as part of this solution
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19. Questions?
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