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This draft proposes the use of Aggregate FEC for label summarization in large-scale MPLS networks. The goal is to scale MPLS to 30,000 PEs while keeping convergence time below 200ms by reducing prefixes and labels. It discusses the benefits of label summarization in addition to route summarization, especially in a flat IGP setup. Inter-Area LDP Draft and the LDP modifications provide detailed routes and faster convergence. Hierarchy in labels improves aggregation efficiency. The draft introduces Aggregate-IPv4 FEC for de-aggregation labels for specific IP routes. Routing summarization and label distribution are key topics for efficient MPLS routing.
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Aggregate IP FEC<draft-swallow-mpls-aggregate-FEC-00.txt>swallow@cisco.com
Purpose / Intro Goal Scale MPLS to at least 30,000 PEs Keep convergence time below 200ms Summarization works well to reduce prefixes Can easily be reduced by a factor of 10 or more What about labels? Summarization is need there too Aggregate FEC summarizes labels where the IGP summarizes routes
IGP Convergence Lab Results • 2500 ISIS prefixes • Tier 1 ISP Topology • Time measured by traffic loss Time Prefix Number • Time for ISIS LSP generation, SPF recalculation is very quick • Substantial time is required for update of structures on linecards for individual prefixes and labels
10.10.2.1 Label 21 10.10.2.2 Label 22 10.10.2.3 Label 23 10.11.5.7 Label 57 10.11.5.8 Label 58 A Flat IGP • Adjacency Pointer for each route must be updated • For MPLS Labels must be updated as well IGP LDI Adjacency IGP LDI IGP LDI IGP LDI Adjacency IGP LDI
10.10.2.1 10.10.2.2 10.10.2.3 Aggregation & the FIB • Only labels and adjacency pointers for aggregate routes are updated • But what about LSPs to individual PEs? • Label for aggregate route will only get you to the place that advertised the aggregate - the ABR • We need something else to get to individual PEs 10.10.2/24 Label 20 Adjacency IGP LDI 10.10.5/24 Label 50 10.11.5.7 Adjacency IGP LDI 10.11.5.8
Inter-Area LDP Draft Allows address aggregation at ABR Exposes detailed routes through LDP Using Ordered Mode, ABR originates labels for the /32 routes that were summarized LDP modified to set up LSPs based on best match (as opposed to exact match)
P12 P01 P02 P11 ABR1 ABR2 PE1 PE2 Area 0 Area 2 Area 1 Inter-Area LDP Draft - Operation • Within Area 2 labels are distributed for IPv4/32 routes • ABR2 advertises 10.10.2/24 • ABR2 distributes labels into Area 0 for every /32 in Area 2 • LDP propagates this throughout areas which have the route 10.10.2/24 10.10.0.1 10.10.0.2 10.10.1.1 10.10.2.2
Flat labels with hierarchical routing 10.10.2.1 Label 21 • Only adjacency pointers for prefix routes are updated • But all the labels must be updated! LDP LDI 10.10.2/24 10.10.2.2 Label 22 Adjacency LDP LDI IGP LDI 10.10.2.3 Label 23 LDP LDI 10.11.5.7 Label 57 LDP LDI 10.10.5/24 Adjacency IGP LDI 10.11.5.8 Label 58 LDP LDI
Inter-Area LDP & IGP Convergence Prefixes + Labels Prefixes Time Prefix/Label Number • IGP summarization greatly reduce load • But still many labels per IGP prefix to update • Reduces the slope slightly, but actually makes the combined number of prefixes and labels a bit larger
Label for Area/ABR Label for Area/ABR Label for PE Label for PE ABR2 ABR1 PE1 PE2 Area 0 Area 2 Area 1 Adding Hierarchy to Labels • IP aggregates routes at the ABR • For fast convergence we need to aggregate labels • But still need to specify particular PEs in the attached area • Answer: use two labels, one to get to the area and another to get to the PE • Each area • Only has detailed labels for PEs in its area • Forwards on aggregate labels for PEs in all other areas • Allows much faster convergence Label for PE
10.10.2/24 10.10.5/24 Label 20 Label 50 Hierarchical Routes and Labels 10.10.2.1 Label 21 • Only labels & adjacency pointers for prefix routes are updated LDP LDI 10.10.2.2 Label 22 Adjacency LDP LDI IGP LDI 10.10.2.3 Label 23 LDP LDI 10.11.5.7 Label 57 LDP LDI Adjacency IGP LDI 10.11.5.8 Label 58 LDP LDI
Aggregate-IPv4 Label • New Label (FEC) Type • Semantics are the same as the IPv4 FEC except label • Indicates that the next label is a de-aggregation label for a specific (/32) IP route • Provides context to interpret de-aggregation label • Allows consistent labels across multiple ABRs without pre-allocation • Allows ABR to handle multiple aggregates • The de-aggregation labels are determined algorithmically • Requires processing two labels at aggregating router
ABR2 ABR1 PE2 PE1 Area 0 Area 2 Area 1 Routing Summarization & Aggregate Label Distribution Aggregate- IPv4 FEC: 10.10.2/24 Label: 51 Aggregate- IPv4 FEC: 10.10.2/24 Label: 11 • Within Area 2 labels are distributed for IPv4/32 routes • ABR2 advertises 10.10.2/24 • ABR2 selects an aggregate label for 10.10.2/24 and distributes it in LDP • LDP propagates this throughout areas which have route 10.10.2/24 10.10.0.1 10.10.0.2 10.10.1.1 10.10.2.2
P-Router 2 ABR2 ABR1 PE2 PE1 Area 0 Area 2 Area 1 Label Operations:L3-VPN (1) VPN Addr: 192.169.0.22 Next Hop: 10.10.2.2 Label Stk: 47 Aggregate- IPv4 FEC: 10.10.2/24 Label: 51 Aggregate- IPv4 FEC: 10.10.2/24 Label: 11 • PE2 advertises normal VPN Route • PE1 sends to VPN Route 192.169.0.22 pushes label 47 • Selects Aggregate-IPv4 FEC 10.10.2/24 as best FEC matching NH • Algorithmically derives De-aggregation label 18 and pushes onto stack • Push LDP label for Aggregate-IPv4 FEC received from IGP next hop 10.10.2.2 10.10.0.2 10.10.0.1 10.10.1.1 11 18 47
P-Router 2 ABR2 ABR1 PE2 PE1 Area 0 Area 2 Area 1 36 47 51 11 18 18 47 47 Label Operations:L3-VPN (2) VPN Addr: 192.169.0.22 Next Hop: 10.10.2.2 Label Stk: 47 Aggregate- IPv4 FEC: 10.10.2/24 Label: 51 • Packet is label-switched to ABR2 • ABR2 receives packet label stack 51/18/47 • ABR2 pops label 51 and locates the indicated ILM • ABR2 looks up label 18 and maps it to a label (36) received for the IPv4 FEC 10.10.2.2 • Label processing from this point is exactly as current L3VPNs 10.10.2.2 10.10.0.2 10.10.0.1 10.10.1.1 47
Aggregate Labels & IGP Convergence Prefixes + Labels Time Prefix/Label Number • Summarization + Aggregate labels greatly reduce load • Just one label per IGP prefix to update • Greatly reduces number of FIB objects that need updating
Addressing, IGP summarization and Reachability • BGP Next-Hop Tracking and BGP PIC Edge are essential to support fast routing convergence upon loss of PE’s or PE-CE links. • They rely on fast and scalable detection of BGP nhop failure. IGP convergence normally provides this capability in fast and scalable way. • The proposal draft-swallow-isis-detailed-reach preserves this capability when introducing summarization across ISIS domains. • A similar draft will be done for OSPF
Example • Area 2 has 10.10.2.0/25 assigned as its address range • The following addresses appear in ABR2’s database for Area 2 • 10.10.2.1 - 10.10.2.27 • 10.10.2.46 • 10.10.2.74 - 10.10.2.87 • then the bit mask encoding would advertise a summary route to 10.10.2.0/25 with an associated 128-bit mask like this: • 0 1 2 3 • 01234567890123456789012345678901 • -------------------------------- • 01111111111111111111111111110000 • 00000000000000100000000000000000 • 00000000001111111111111100000000 • 00000000000000000000000000000000
Summary • IGP control-plane convergence is fast even with > 1000 nodes • Gating item is how fast the data-plane can be updated • Applying hierarchical routing to MPLS • Helps control-plane convergence • Does little to speed data-plane convergence • Need to apply hierarchy to both routing and labels • Aggregate FEC provides the hierarchical labels Do we want to take this on as a work item???
