The purpose of SR is to control the label allocation that the PE and P routers will use for their loopbacks and the transit links. The key difference between SR and LDP is SR allocates the label to the loopback interface. LDP does not do this, static labeling is supported but configuration intensive. SR uses a dedicated block of labels, the SRGB with a range of 16000-23999.
LDP is deployed along side of IGP but as a different process, IGP needs to be converged before LDP converges or micro loops can occur.
SR is configured under the IGP process for both OSPF and IS-IS. The SR labels are propagated inside of the IS-IS TLVs and OSPF Opaque LSAs.
There are 2 different label allocations, the loopback of the P or PE router and the connected links between the P and PE routers.
The loopback label is called the "Prefix SID" or Prefix Segment Identifier.
The transit label is called the "Adjacency SID" or Adjacency Segment Identifier.
The Prefix SID comes from the 16000-23999 label range, the SRGB.
The Adjacency SID comes from the dynamic label range 24000-1048575.
The only thing that changes in the MPLS L3VPN deployment here is SR is the labeling technique, VRFs, MP-BGP, VRF Aware BGP PE-CE and IGP routing are still needed. The above IOS routers, R1-R4 R1 is ASN 101, R2 is ASN 102 and so forth. The ASN in the core is ASN1. XR6 is a RR to the PE routers.
The configuration and verification outputs are below.
XR1
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid absolute 16021
!
interface GigabitEthernet0/0/0/0.112
!
interface GigabitEthernet0/0/0/0.115
XR2
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid absolute 16022
!
interface GigabitEthernet0/0/0/0.112
!
interface GigabitEthernet0/0/0/0.123
!
interface GigabitEthernet0/0/0/0.126
XR3
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid absolute 16023
!
interface GigabitEthernet0/0/0/0.123
!
interface GigabitEthernet0/0/0/0.134
!
interface GigabitEthernet0/0/0/0.137
XR4
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid absolute 16024
!
interface GigabitEthernet0/0/0/0.134
!
interface GigabitEthernet0/0/0/0.148
XR5
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid absolute 16025
!
interface GigabitEthernet0/0/0/0.115
!
interface GigabitEthernet0/0/0/0.156
XR6
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid absolute 16026
!
interface GigabitEthernet0/0/0/0.126
!
interface GigabitEthernet0/0/0/0.156
!
interface GigabitEthernet0/0/0/0.167
XR7
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid absolute 16027
!
interface GigabitEthernet0/0/0/0.137
!
interface GigabitEthernet0/0/0/0.167
!
interface GigabitEthernet0/0/0/0.178
XR8
router ospf 1
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback0
prefix-sid index 28
!
interface GigabitEthernet0/0/0/0.148
!
interface GigabitEthernet0/0/0/0.178
XR8 is running 5.3 XR code, so the "absolute" option isn't supported, Index and absolute do the same thing, index just calls the label value that will get added to 16000 where absolute defines it completely.
RP/0/0/CPU0:XR1#sh mpls interfaces detail
Wed May 9 19:17:00.751 UTC
Interface GigabitEthernet0/0/0/0.112:
LDP labelling not enabled
LSP labelling not enabled
MPLS enabled
Interface GigabitEthernet0/0/0/0.115:
LDP labelling not enabled
LSP labelling not enabled
MPLS enabled
RP/0/0/CPU0:XR2#show mpls interfaces detail
Wed May 9 19:18:20.711 UTC
Interface GigabitEthernet0/0/0/0.112:
LDP labelling not enabled
LSP labelling not enabled
MPLS enabled
Interface GigabitEthernet0/0/0/0.123:
LDP labelling not enabled
LSP labelling not enabled
MPLS enabled
Interface GigabitEthernet0/0/0/0.126:
LDP labelling not enabled
LSP labelling not enabled
MPLS enabled
As you can see, LDP is not being used here, Segment Routing is.
R1 and R2 have now peered with the SP and advertised their loopbacks into BGP.
R1#sh ip route bgp | b Gateway
Gateway of last resort is not set
192.0.2.0/32 is subnetted, 2 subnets
B 192.0.2.2 [20/0] via 100.64.101.11, 10:18:02
R2#sh ip route bgp | b Gateway
Gateway of last resort is not set
192.0.2.0/32 is subnetted, 2 subnets
B 192.0.2.1 [20/0] via 100.64.103.14, 10:18:48
Now we'll do some trace routes to see how Segment Routing will look different than what LDP will look. NOTE - BGP VPNv4 is still used to allocate labels for customer learned routes, these labels are pulled from the global dynamic label pool.
R2#traceroute 192.0.2.1 source loopback 0 numeric
Type escape sequence to abort.
Tracing the route to 192.0.2.1
VRF info: (vrf in name/id, vrf out name/id)
1 100.64.103.14 23 msec 14 msec 8 msec
2 100.64.134.13 [MPLS: Labels 16021/24004 Exp 0] 107 msec 92 msec 91 msec
3 100.64.123.12 [MPLS: Labels 16021/24004 Exp 0] 99 msec 97 msec 100 msec
4 100.64.112.11 [MPLS: Label 24004 Exp 0] 99 msec 85 msec 88 msec
5 100.64.101.1 89 msec * 110 msec
The 16021/24004 is the 2 label stack we would normally see with LDP, the top label, the transport label, 16021 wouldn't be in the range of 16000-23999.
In this case, the label 16021 isn't LDP allocating labels arbitrarily, this label value is configured on XR1 on the loopback interface and propagated to the other P/PE routers inside of OSPF Opaque LSAs. All of the routers in the core know that to reach XR1 via a labeled path, they must use label 16021 to get there.
We'll take the next several outputs and examine them to breakdown how we the label values above were allocated and understand where they fit in.
Let's see what routes we received in from XR1 via the RR of XR6.
RP/0/0/CPU0:XR4#sh bgp vpnv4 unicast neighbors 192.0.2.26 routes | b Network
Wed May 9 19:31:25.218 UTC
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 1:1 (default for vrf A)
*>i192.0.2.1/32 192.0.2.21 0 100 0 101 i
Processed 1 prefixes, 1 paths
We can see that we learned 192.0.2.1 from 192.0.2.21, let's expand that to see what label value VPNv4 applied.
RP/0/0/CPU0:XR4#sh bgp vrf A 192.0.2.1/32
Wed May 9 19:25:24.652 UTC
BGP routing table entry for 192.0.2.1/32, Route Distinguisher: 1:1
Versions:
Process bRIB/RIB SendTblVer
Speaker 4 4
Last Modified: May 9 09:00:02.407 for 10:25:22
Paths: (1 available, best #1)
Advertised to CE peers (in unique update groups):
100.64.103.2
Path #1: Received by speaker 0
Advertised to CE peers (in unique update groups):
100.64.103.2
101
192.0.2.21 (metric 4) from 192.0.2.26 (192.0.2.21)
Received Label 24004
Origin IGP, metric 0, localpref 100, valid, internal, best, group-best, import-candidate, imported
Received Path ID 0, Local Path ID 1, version 4
Extended community: RT:1:1
Originator: 192.0.2.21, Cluster list: 192.0.2.26
Source AFI: VPNv4 Unicast, Source VRF: A, Source Route Distinguisher: 1:1
We see that label 24004 was allocated by VPNv4 for the 192.0.2.1/32 route advertised by XR1. We have the VPN label, now we need to know what to configure as the transport label.
RP/0/0/CPU0:XR4#sh route 192.0.2.21
Wed May 9 19:34:21.986 UTC
Routing entry for 192.0.2.21/32
Known via "ospf 1", distance 110, metric 4, labeled SR, type intra area
Installed May 8 22:23:31.979 for 21:10:50
Routing Descriptor Blocks
100.64.134.13, from 192.0.2.21, via GigabitEthernet0/0/0/0.134
Route metric is 4
No advertising protos.
We see that the route was learned via OSPF intra area propagation, more importantly, labeled SR is propagated as well.
RP/0/0/CPU0:XR4#show cef 192.0.2.21
Wed May 9 19:36:06.659 UTC
192.0.2.21/32, version 16, internal 0x1000001 0x81 (ptr 0xa12b3a74) [1], 0x0 (0xa12994f4), 0xa28 (0xa150d140)
Updated May 8 22:23:32.049
local adjacency 100.64.134.13
Prefix Len 32, traffic index 0, precedence n/a, priority 1
via 100.64.134.13/32, GigabitEthernet0/0/0/0.134, 9 dependencies, weight 0, class 0 [flags 0x0]
path-idx 0 NHID 0x0 [0xa0f592a4 0x0]
next hop 100.64.134.13/32
local adjacency
local label 16021 labels imposed {16021}
Checking the CEF table we can see that both the imposed label and the local label are both 16021. Imposed means that 16021 will be used to forward these packets through the core.
RP/0/0/CPU0:XR4#show mpls forwarding labels 16021
Wed May 9 19:38:12.840 UTC
Local Outgoing Prefix Outgoing Next Hop Bytes
Label Label or ID Interface Switched
------ ----------- ------------------ ------------ --------------- ------------
16021 16021 SR Pfx (idx 21) Gi0/0/0/0.134 100.64.134.13 5628
This is a prefix SID that is applied to XR1's loopback. It is both the local label and the outgoing label.
RP/0/0/CPU0:XR4#sh ospf database opaque-area adv-router 192.0.2.21
Wed May 9 19:39:44.484 UTC
OSPF Router with ID (192.0.2.24) (Process ID 1)
Type-10 Opaque Link Area Link States (Area 0)
LS age: 926
Options: (No TOS-capability, DC)
LS Type: Opaque Area Link
Link State ID: 4.0.0.0
Opaque Type: 4
Opaque ID: 0
Advertising Router: 192.0.2.21
LS Seq Number: 80000027
Checksum: 0x8ceb
Length: 52
Router Information TLV: Length: 4
Capabilities:
Graceful Restart Helper Capable
Stub Router Capable
All capability bits: 0x60000000
Segment Routing Algorithm TLV: Length: 1
Algorithm: 0
Segment Routing Range TLV: Length: 12
Range Size: 8000
SID sub-TLV: Length 3
Label: 16000
LS age: 664
Options: (No TOS-capability, DC)
LS Type: Opaque Area Link
Link State ID: 7.0.0.1
Opaque Type: 7
Opaque ID: 1
Advertising Router: 192.0.2.21
LS Seq Number: 80000027
Checksum: 0xc5ee
Length: 44
Extended Prefix TLV: Length: 20
Route-type: 1
AF : 0
Flags : 0x40
Prefix : 192.0.2.21/32
SID sub-TLV: Length: 8
Flags : 0x0
MTID : 0
Algo : 0
SID Index : 21
Looking at the bolded parts of the OSPF Opaque LSA, we see that the Prefix SID begins at 16000 and carries for 8000 which ranges from 16000 - 23999. Below that we see the Prefix of 192.0.2.21 with an index of 21. 16000 plus 21 gets us 16021. This boils down that 16021 will be the transport label for every SP core router, XR2 through XR8 to reach XR1.
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