1. The document discusses scaling networks using segment routing. It describes using a PCE server to collect topology and SID information via BGP-LS to compute paths on demand. 2. An NSO controller is used to configure services, and nodes can request path computations from the PCE server if they do not already have an LSP to reach another node. 3. Examples are given for intra-domain and inter-domain routing using SIDs, including assigning SRGB ranges to different domains. The PCE server can compute multi-domain paths with TE optimization across the different routing domains.

1. Scaling Networks with Segment Routing
VS Kandaswamy
Senior Technical Marketing Engineer
Cisco SPI Team

2. Why ACE ?
Unified MPLS/
Seamless
MPLS
AC
E
Simplicity
Scalability
Stability
Flexibility
Programmabilit
y
overlay
underlay
SR
BGP
SDN
overlay
underlay

3. DC WAN Access
Unicast-SID 17001
Anycast-SID 18001 Unicast-SID 16001
Tail-f NSO controller
Unicast-SID 17002
Anycast-SID 18001
Unicast-SID 17003
Anycast-SID 18002
Unicast-SID 17004
Anycast-SID 18002
ToR1
ToR2
ABR1
ABR2
ABR3
ABR4
AC1
Unicast-SID 16002
Unicast-SID 16001
Unicast-SID 16002
PCE Server
AC2
BGP Link State
Hint:
1. PCE collect topology and SID via BGP
LS
On demand SR Next Hop

4. XML YANG:
- PW-123 from ToR1 to AC1
Hint:
1. PCE collect topology and SID via BGP LS
2. NSO to configure service
DC WAN Access
Unicast SID 17001
Anycast SID 18001 Unicast-SID 16001
Tail-f NSO controller
Unicast SID 17002
Anycast SID 18001
Unicast SID 17003
Anycast SID 18002
Unicast SID 17004
Anycast SID 18002
ToR1
ToR2
ABR1
ABR2
ABR3
ABR4
AC1
Unicast-SID 16002
Unicast-SID 16001
Unicast-SID 16002
PCE Server
AC2
XML YANG:
- PW-123 from AC1 to ToR1
On demand SR Next Hop

5. DC WAN Access
Unicast SID 17001
Anycast SID 18001 Unicast-SID 16001
Tail-f NSO controller
Unicast SID 17002
Anycast SID 18001
Unicast SID 17003
Anycast SID 18002
Unicast SID 17004
Anycast SID 18002
ToR1
ToR2
ABR1
ABR2
ABR3
ABR4
AC1
Unicast-SID 16002
Unicast-SID 16001
Unicast-SID 16002
PCE Server
AC2
Hint:
1. PCE collect topology and SID via BGP LS
2. NSO to configure service
3. ToR1 check if he has LSP to AC1
Yes -> use it
No -> next slide
Do I have LSP
to AC1 ?
On demand SR Next Hop

6. DC WAN Access
Unicast SID 17001
Anycast SID 18001 Unicast-SID 16001
Tail-f NSO controller
Unicast SID 17002
Anycast SID 18001
Unicast SID 17003
Anycast SID 18002
Unicast SID 17004
Anycast SID 18002
ToR1
ToR2
ABR1
ABR2
ABR3
ABR4
AC1
Unicast-SID 16002
Unicast-SID 16001
Unicast-SID 16002
PCE Server
AC2
Hint:
1. PCE collect topology and SID via BGP LS
2. NSO to configure service
3. ToR1 check if he has LSP to AC1
4. ToR1 request LSP to PCE
PCEP reply
- ERO is: 18001,18002,16001
PCEP request
- Could you provide me the
ERO to reach AC1 ?
1 2
3
On demand SR Next Hop

7. Some Suggestions to Mange the Label Depth

8. SRGB and SID allocation
• Many options, this is just an illustration
• All nodes use SRGB [16000- 23999]
– While the SRGB can always be extended later, try and size it to allow expected scale and growth
– e.g. SRGB could initially be set to [16000-79999]
• SID’s in range 16000-16999 are allocated to WAN nodes and are globally unique
• SID’s in range 17000-19999 are allocated to Metro nodes and are globally unique
– METRO A get [ 17000-17999]
– METRO B get [ 18000-18999]
• SID’s in range 20000-23999 are allocated to DC nodes and are only unique within their local DC
– Spine, ToR and vPE all receive a SID
• If a node is at the border of several domains, it has one single SID from the most central domain
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2

9. Intra-Domain Routing – DC A1 and B2
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2
vPE1/32
NH: vPE1
BGP-LU LABEL: POP
PREFIX-SID: 20001
(relative 4001)
vPE1/32
NH: TOR
BGP-LU LABEL: 20001
PREFIX-SID: 20001
(relative 4001)
vPE1/32
NH: SPINE
BGP-LU LABEL: 20001
PREFIX-SID: 20001
(relative 4001)
DCI2/32
NH: DCI2
BGP-LU LABEL: POP
PREFIX-SID: 18001
(relative 2001)
DCI2/32
NH: SPINE
BGP-LU LABEL: 18001
PREFIX-SID: 18001
(relative 2001)
DCI2/32
NH: TOR
BGP-LU LABEL: 18001
PREFIX-SID: 18001
(relative 2001)

10. Intra-Domain Routing – Metro A and B
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2
DCI1/32
NH: DCI1
BGP-LU LABEL: POP
PREFIX-SID: 17001
(relative 1001)
DCI1/32
NH: LSR
BGP-LU LABEL: 17001
PREFIX-SID: 17001
(relative 1001)
ISIS LSP of AGG2
Leaf: Agg2
PREFIX-SID: 16003
(relative 3)

11. Intra-Domain Routing – WAN
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2
ISIS LSP of AGG1
Leaf: Agg1
PREFIX-SID: 16001
(relative 1)

12. Inter-Domain Routing
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2
ISIS LSP of AGG2
Leaf: AGG1
PREFIX-SID: 16001
(relative 1)
AGG1
NH: DCI2
BGP-LU LABEL: 16001
PREFIX-SID: 16001
(relative 1)
AGG1
NH: SPINE
BGP-LU LABEL: 16001
PREFIX-SID: 16001
(relative 1)
AGG1
NH: ToR
BGP-LU LABEL: 16001
PREFIX-SID: 16001
(relative 1)

13. Inter-Domain Routing (Cont’d)
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2
ISIS LSP of AGG1
Leaf: AGG2
PREFIX-SID: 16003
(relative 3)
AGG2
NH: DCI1
BGP-LU LABEL: 16003
PREFIX-SID: 16003
(relative 3)
AGG2
NH: SPINE
BGP-LU LABEL: 16003
PREFIX-SID: 16003
(relative 3)
AGG2
NH: TOR
BGP-LU LABEL: 16003
PREFIX-SID: 16003
(relative 3)

14. SR PCE
• Multi-domain topology
– Realtime reactive feed via BGP-LS/ISIS/OSPF from multiple domains
– Including ip address and SID
• Multi-domain path compute with TE optimization and constraint
– SRTE algorithms (see later)
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2
BGP-LS - Multi-Domain Topology
SR PCE
STATEFUL PATH COMPUTE with TE
optimization and constraint

15. vPE2 to vPE1
Simple Connectivity
• vPE2 sends a PCE request for a stateful computation of a path to vPE1
• PCE stores the stateful request
– Upon any multi-domain topology change that would impact the connectivity, PCE would send an
updated path
• vPE2 gets a path to vPE1: {16001, 17001, 20001}
vPE1
20001
ToR
20002
Spine
20003
DCI1
17001
LSR
17002
AGG1
16001
LSR
16002
AGG2
16003
vPE2
20001
ToR
20002
Spine
20003
DCI2
18001
LSR
18002
DC A1 METRO A METRO BWAN DCB2
SR
PCE
2: {16001, 17001, 20001}
1: vPE1?

16. Thank you