Segment Routing (SR) is a source-based routing method that uses extensions to OSPF and IS-IS to advertise Segment IDs (SID). An SID uniquely identifies a router or a link in an SR domain. An SID can be a Prefix SID which is unique in an SR domain, or it can be an Adjacency SID which is locally significant to a router. Segment Routing is an MPLS control plane technology that can replace LDP and RSVP to signal prefix-to-label bindings.
The benefit of SR is that it can connect two PE routers within an MPLS core network without the complexity and periodic keepalive messages required by LDP and RSVP. SR does not change the data plane operation of the MPLS core network. This means, MPLS label switching, the label push, pop and swap operations, and the logic of the Label Forwarding Information Base (LFIB) are the same with SR, LDP and RSVP.
SR changes the way MPLS labels are assigned to IP prefixes, and how these labels (Segment IDs) are communicated between devices in the MPLS core network. A Prefix SID is configured on an MPLS-enabled router and advertised using the IGP. An ingress PE receives the Prefix SID value and the loopback IP of the egress PE. The Prefix SID indicates the MPLS label that should be assigned to an outgoing packet.
SR uses a specific label range for its operation. The label range is called the Segment Routing Global Block (SRGB), and it is independent from the one used by LDP or RSVP. On Cisco devices the default SRGB is between 16000 and 23999. When tracing a route from an ingress PE router to an egress PE router, the following example output can be seen.
This indicates that the egress PE advertised the Prefix SID value 6. The value 6 is an offset value. Each router in the SR domain uses the same SRGB (labels between 16000 - 23999). The offset value 6 is calculated from the starting point of 16000. Thus, when the ingress PE R1 receives the IP 6.6.6.6 with the Prefix SID value 6, it will calculate and assign the MPLS label 16000 + 6 (16006) to reach IP 6.6.6.6. Each router in the SR domain comes to the same conclusion, and for this reason the same MPLS label is visible in the above traceroute.
Segment Routing configuration with OSPF - IOS XE
In the following example scenario, SR is enabled for the OSPF area 0 on all routers. Each router has its own Node SID, which uniquely represents that device in the SR domain. The Node SID is configured as an Index value on each router. The Index value works together with the SRGB. There is a default SRGB range starting at 16000. Index 1 means the MPLS label 16001 is assigned. Index 2 is 16002. Thus, the Index value represents an offset to the start of the SRGB.
Configuration:
R1
R1#show run int Gi1 | sec int
interface GigabitEthernet1
description ** to R2 **
ip address 10.1.0.2 255.255.255.252
ip ospf network point-to-point
negotiation auto
no mop enabled
no mop sysid
R1#show run int Lo10 | sec int
interface Loopback10
ip address 1.1.1.1 255.255.255.255
R1#show run | sec ^segment
segment-routing mpls
!
connected-prefix-sid-map
address-family ipv4
1.1.1.1/32 index 1 range 1
exit-address-family
!
R1#show run | sec ^router
router ospf 10
router-id 1.1.1.1
segment-routing area 0 mpls
segment-routing mpls
network 1.1.1.1 0.0.0.0 area 0
network 10.1.0.0 0.0.0.3 area 0
R2
R2#show run int Gi1 | sec int
interface GigabitEthernet1
description ** to R1 **
ip address 10.1.0.1 255.255.255.252
ip ospf network point-to-point
negotiation auto
no mop enabled
no mop sysid
R2#show run int Gi2 | sec int
interface GigabitEthernet2
description ** to R3 **
ip address 10.2.0.1 255.255.255.252
ip ospf network point-to-point
negotiation auto
no mop enabled
no mop sysid
R2#show run int Lo10 | sec int
interface Loopback10
ip address 2.2.2.2 255.255.255.255
R2#show run | sec ^segment
segment-routing mpls
!
connected-prefix-sid-map
address-family ipv4
2.2.2.2/32 index 2 range 1
exit-address-family
!
R2#show run | sec ^router
router ospf 10
router-id 2.2.2.2
segment-routing area 0 mpls
segment-routing mpls
network 2.2.2.2 0.0.0.0 area 0
network 10.1.0.0 0.0.0.3 area 0
network 10.2.0.0 0.0.0.3 area 0
R3
R3#show run int Gi1 | sec int
interface GigabitEthernet1
description ** to R2 **
ip address 10.2.0.2 255.255.255.252
ip ospf network point-to-point
negotiation auto
no mop enabled
no mop sysid
R3#show run int Gi2 | sec int
interface GigabitEthernet2
description ** to R4 **
ip address 10.3.0.1 255.255.255.252
ip ospf network point-to-point
negotiation auto
no mop enabled
no mop sysid
R3#show run int Lo10 | sec int
interface Loopback10
ip address 3.3.3.3 255.255.255.255
R3#show run | sec ^segment
segment-routing mpls
!
connected-prefix-sid-map
address-family ipv4
3.3.3.3/32 index 3 range 1
exit-address-family
!
R3#show run | sec ^router
router ospf 10
router-id 3.3.3.3
segment-routing area 0 mpls
segment-routing mpls
network 3.3.3.3 0.0.0.0 area 0
network 10.2.0.0 0.0.0.3 area 0
network 10.3.0.0 0.0.0.3 area 0
R1#show mpls forwarding-table
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
16 Pop Label 10.1.0.1-A 0 Gi1 10.1.0.1
16002 Pop Label 2.2.2.2/32 0 Gi1 10.1.0.1
16003 16003 3.3.3.3/32 0 Gi1 10.1.0.1
16004 16004 4.4.4.4/32 0 Gi1 10.1.0.1
16005 16005 5.5.5.5/32 0 Gi1 10.1.0.1
16006 16006 6.6.6.6/32 0 Gi1 10.1.0.1 « Labels are assigned based on received Prefix SID index value
A - Adjacency SID
R1#show segment-routing mpls connected-prefix-sid-map ipv4
PREFIX_SID_CONN_MAP ALGO_0
Prefix/masklen SID Type Range Flags SRGB
1.1.1.1/32 1 Indx 1 Y
PREFIX_SID_PROTOCOL_ADV_MAP ALGO_0
Prefix/masklen SID Type Range Flags SRGB Source
1.1.1.1/32 1 Indx 1 Y OSPF Area 0 1.1.1.1
2.2.2.2/32 2 Indx 1 Y OSPF Area 0 2.2.2.2
3.3.3.3/32 3 Indx 1 Y OSPF Area 0 3.3.3.3
4.4.4.4/32 4 Indx 1 Y OSPF Area 0 4.4.4.4
5.5.5.5/32 5 Indx 1 Y OSPF Area 0 5.5.5.5
6.6.6.6/32 6 Indx 1 Y OSPF Area 0 6.6.6.6 « Index 6 means that the label is 16006R1#show ip ospf database opaque-area prefix 6.6.6.6/32
OSPF Router with ID (1.1.1.1) (Process ID 10)
Type-10 Opaque Area Link States (Area 0)
LS age: 1382
Options: (No TOS-capability, DC)
LS Type: Opaque Area Link
Link State ID: 7.0.0.0
Opaque Type: 7 (Extended Prefix)« Opaque LSA type-7 carrying Prefix SID
Opaque ID: 0
Advertising Router: 6.6.6.6« R6 is the advertising router, this is the OSPF router ID
LS Seq Number: 80000002
Checksum: 0xD494
Length: 44
TLV Type: Extended Prefix
Length: 20
Prefix : 6.6.6.6/32« The prefix is 6.6.6.6/32
AF : 0
Route-type: Intra
Flags : N-bit
Sub-TLV Type: Prefix SID
Length: 8
Flags : None
MTID : 0
Algo : SPF
SID : 6« The prefix SID is index 6, which is MPLS label 16006R1#show ip route ospf | beg Ga
Gateway of last resort is not set
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 10.1.0.1, 00:55:56, GigabitEthernet1
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/3] via 10.1.0.1, 00:55:56, GigabitEthernet1
4.0.0.0/32 is subnetted, 1 subnets
O 4.4.4.4 [110/4] via 10.1.0.1, 00:55:56, GigabitEthernet1
5.0.0.0/32 is subnetted, 1 subnets
O 5.5.5.5 [110/5] via 10.1.0.1, 00:55:01, GigabitEthernet1
6.0.0.0/32 is subnetted, 1 subnets
O 6.6.6.6 [110/6] via 10.1.0.1, 00:55:56, GigabitEthernet1
10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
O 10.2.0.0/30 [110/2] via 10.1.0.1, 00:55:56, GigabitEthernet1
O 10.3.0.0/30 [110/3] via 10.1.0.1, 00:55:56, GigabitEthernet1
O 10.4.0.0/30 [110/4] via 10.1.0.1, 00:55:56, GigabitEthernet1
O 10.5.0.0/30 [110/5] via 10.1.0.1, 00:55:56, GigabitEthernet1
R1#show ip route 6.6.6.6
Routing entry for 6.6.6.6/32
Known via "ospf 10", distance 110, metric 6, type intra area
Last update from 10.1.0.1 on GigabitEthernet1, 00:56:15 ago
SR Incoming Label: 16006« Segment Routing label also shows in RIB
Routing Descriptor Blocks:
* 10.1.0.1, from 6.6.6.6, 00:56:15 ago, via GigabitEthernet1, prefer-non-rib-labels, merge-labels
Route metric is 6, traffic share count is 1
MPLS label: 16006
MPLS Flags: NSF
R1#trace 6.6.6.6 pr 1
Type escape sequence to abort.
Tracing the route to 6.6.6.6
VRF info: (vrf in name/id, vrf out name/id)
1 10.1.0.1 [MPLS: Label 16006 Exp 0] 4 msec « The label is assigned at each hop
2 10.2.0.2 [MPLS: Label 16006 Exp 0] 4 msec
3 10.3.0.2 [MPLS: Label 16006 Exp 0] 3 msec
4 10.4.0.2 [MPLS: Label 16006 Exp 0] 2 msec
5 10.5.0.2 4 msec
As visible in the above outputs, R6 has the Prefix SID Index 6 configured. This translates to MPLS label 16006 on all routers, because the SRGB range starts with 16000. All other routers in the OSPF area install the label that is advertised by R6. As a result, the same label is visible throughout the entire traceroute.
In this example scenario, each router has the same SRGB by default, which is the range 16000 - 23999. So when a router receives an index of 6, they locally do the translation to 16000 + 6, and install the MPLS label 16006. The default SRGB range values can change between routers, for example different vendors may have a different default SRGB range.
Segment Routing configuration with OSPF - IOS XR
In this example topology, SR is configured with OSPF using IOS-XR. Each router is assigned a Node SID based on its Loopback IP address. The Node SID is advertised through the OSPF SR extension with the LSA type-7. Thus, each router can assign an MPLS label to all other routers in the network.
Configuration:
R1
RP/0/0/CPU0:R1#show run | beg hostname
Tue Jan 9 15:44:36.384 UTC
Building configuration...
hostname R1
domain lookup disable
line console
escape-character 0x51
!
interface Loopback10
ipv4 address 1.1.1.1 255.255.255.255
!
interface MgmtEth0/0/CPU0/0
shutdown
!
interface GigabitEthernet0/0/0/0
description ** to R2 **
ipv4 address 10.1.0.2 255.255.255.252
!
interface GigabitEthernet0/0/0/1
shutdown
!
interface GigabitEthernet0/0/0/2
shutdown
!
router ospf 10
router-id 1.1.1.1
address-family ipv4 unicast
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback10
prefix-sid index 1
loopback stub-network enable
!
interface GigabitEthernet0/0/0/0
network point-to-point
!
!
!
end
R2
RP/0/0/CPU0:R2#show run | beg hostname
Tue Jan 9 15:46:51.433 UTC
Building configuration...
hostname R2
domain lookup disable
line console
escape-character 0x51
!
interface Loopback10
ipv4 address 2.2.2.2 255.255.255.255
!
interface MgmtEth0/0/CPU0/0
shutdown
!
interface GigabitEthernet0/0/0/0
description ** to R3 **
ipv4 address 10.2.0.1 255.255.255.252
!
interface GigabitEthernet0/0/0/1
description ** to R1 **
ipv4 address 10.1.0.1 255.255.255.252
!
interface GigabitEthernet0/0/0/2
shutdown
!
router ospf 10
router-id 2.2.2.2
address-family ipv4 unicast
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback10
prefix-sid index 2
loopback stub-network enable
!
interface GigabitEthernet0/0/0/0
network point-to-point
!
interface GigabitEthernet0/0/0/1
network point-to-point
!
!
!
end
R3
RP/0/0/CPU0:R3#show run | beg hostname
Tue Jan 9 15:47:15.601 UTC
Building configuration...
hostname R3
domain lookup disable
line console
escape-character 0x51
!
interface Loopback10
ipv4 address 3.3.3.3 255.255.255.255
!
interface MgmtEth0/0/CPU0/0
shutdown
!
interface GigabitEthernet0/0/0/0
description ** to R2 **
ipv4 address 10.2.0.2 255.255.255.252
!
interface GigabitEthernet0/0/0/1
description ** to R4 **
ipv4 address 10.3.0.1 255.255.255.252
!
interface GigabitEthernet0/0/0/2
shutdown
!
router ospf 10
router-id 3.3.3.3
address-family ipv4 unicast
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback10
prefix-sid index 3
loopback stub-network enable
!
interface GigabitEthernet0/0/0/0
network point-to-point
!
interface GigabitEthernet0/0/0/1
network point-to-point
!
!
!
end
RP/0/0/CPU0:R1#show mpls forwarding
Tue Jan 9 15:52:59.060 UTC
Local Outgoing Prefix Outgoing Next Hop Bytes
Label Label or ID Interface Switched
------ ----------- ------------------ ------------ --------------- ------------
16002 Pop SR Pfx (idx 2) Gi0/0/0/0 10.1.0.1 0
16003 16003 SR Pfx (idx 3) Gi0/0/0/0 10.1.0.1 544
16004 16004 SR Pfx (idx 4) Gi0/0/0/0 10.1.0.1 0
16005 16005 SR Pfx (idx 5) Gi0/0/0/0 10.1.0.1 0
16006 16006 SR Pfx (idx 6) Gi0/0/0/0 10.1.0.1 31040 « MPLS forwarding table contains Segment Routing labels
24000 Pop SR Adj (idx 0) Gi0/0/0/0 10.1.0.1 0
RP/0/0/CPU0:R1#show ospF database opaque-area 6.6.6.6/32
Tue Jan 9 15:55:00.972 UTC
OSPF Router with ID (1.1.1.1) (Process ID 10)
Type-10 Opaque Link Area Link States (Area 0)
LS age: 1778
Options: (No TOS-capability, DC)
LS Type: Opaque Area Link
Link State ID: 7.0.0.1
Opaque Type: 7« OSPF Opaque LSA type-7 advertises Prefix SID
Opaque ID: 1
Advertising Router: 6.6.6.6
LS Seq Number: 80000001
Checksum: 0xcc9c
Length: 44
Extended Prefix TLV: Length: 20
Route-type: 1
AF : 0
Flags : 0x40
Prefix : 6.6.6.6/32« /32 Node Prefix SID is advertised by R6
SID sub-TLV: Length: 8
Flags : 0x0
MTID : 0
Algo : 0
SID Index : 6« An index value 6 is added to the SRGBRP/0/0/CPU0:R1#show ip route 6.6.6.6
Wed Jan 10 13:16:27.937 UTC
Routing entry for 6.6.6.6/32
Known via "ospf 10", distance 110, metric 6, labeled SR, type intra area « Information about SR included in RIB
Installed Jan 10 13:16:08.709 for 00:00:19
Routing Descriptor Blocks
10.1.0.1, from 6.6.6.6, via GigabitEthernet0/0/0/0
Route metric is 6
No advertising protos.
RP/0/0/CPU0:R1#trace 6.6.6.6 pr 1
Tue Jan 9 15:55:20.590 UTC
Type escape sequence to abort.
Tracing the route to 6.6.6.6
1 10.1.0.1 [MPLS: Label 16006 Exp 0] 9 msec « Traffic is sent label switched with SR label
2 10.2.0.2 [MPLS: Label 16006 Exp 0] 0 msec
3 10.3.0.2 [MPLS: Label 16006 Exp 0] 0 msec
4 10.4.0.2 [MPLS: Label 16006 Exp 0] 0 msec
5 10.5.0.2 19 msec
Alternative configuration in IOS XR
In IOS XR the following configurations achieve the same result. This is due to configuration inheritance. Namely, if the configuration is applied at the OSPF instance level, then the OSPF area 0 inherits that configuration. However, the same configuration can also be applied to the OSPF area only, then it takes effect only in that single area.
Alternative configurations
At the OSPF instance level
At the OSPF area level
RP/0/0/CPU0:R1#show run router ospf
Tue Jan 9 16:08:22.107 UTC
router ospf 10
router-id 1.1.1.1
segment-routing mplssegment-routing forwarding mpls
address-family ipv4 unicast
area 0
interface Loopback10
prefix-sid index 1
loopback stub-network enable
!
interface GigabitEthernet0/0/0/0
network point-to-point
!
!
!
RP/0/0/CPU0:R1#show run router ospf
Tue Jan 9 16:06:14.056 UTC
router ospf 10
router-id 1.1.1.1
address-family ipv4 unicast
area 0
segment-routing forwarding mplssegment-routing mpls
interface Loopback10
prefix-sid index 1
loopback stub-network enable
!
interface GigabitEthernet0/0/0/0
network point-to-point
!
!
!
Change Segment Routing SRGB
The Segment Routing Global Block (SRGB) can be changed on each router, however it is best to have an identical SRGB throughout a network. In the following example the SRGB is changed on IOS XR, and this has an effect on the assigned MPLS label values.
RP/0/0/CPU0:R1#show run | begin ospf
Tue Jan 9 16:17:58.347 UTC
Building configuration...
router ospf 10
router-id 1.1.1.1
address-family ipv4 unicast
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback10
prefix-sid index 1
loopback stub-network enable
!
interface GigabitEthernet0/0/0/0
network point-to-point
!
!
!
segment-routing
global-block 20000 20010
!
end
Now that the SRGB block is changed on every router, the index value offset is counted from 20000 upwards, as seen in the following output. The MPLS labels start from 20000. Index 0 would be 20000.
RP/0/0/CPU0:R1#show mpls forwarding
Tue Jan 9 16:20:21.468 UTC
Local Outgoing Prefix Outgoing Next Hop Bytes
Label Label or ID Interface Switched
------ ----------- ------------------ ------------ --------------- ------------
20002 Pop SR Pfx (idx 2) Gi0/0/0/0 10.1.0.1 0
20003 20003 SR Pfx (idx 3) Gi0/0/0/0 10.1.0.1 0
20004 20004 SR Pfx (idx 4) Gi0/0/0/0 10.1.0.1 0
20005 20005 SR Pfx (idx 5) Gi0/0/0/0 10.1.0.1 0
20006 20006 SR Pfx (idx 6) Gi0/0/0/0 10.1.0.1 0
24000 Pop SR Adj (idx 0) Gi0/0/0/0 10.1.0.1 0
Prefix SID absolute value
Instead of using an index value to specify the offset from the SRGB, an absolute value can also be used to provide an exact number for the Prefix SID. The following table shows how to configure absolute SIDs in IOS XE and XR.
Alternative configurations
IOS XE
IOS XR
R1#show run | section ^segment
segment-routing mpls
!
connected-prefix-sid-map
address-family ipv4
1.1.1.1/32 absolute 16010 range 1
exit-address-family
!
RP/0/0/CPU0:R1#show run router ospf
Tue Jan 9 16:32:37.337 UTC
router ospf 10
router-id 1.1.1.1
address-family ipv4 unicast
area 0
segment-routing forwarding mpls
segment-routing mpls
interface Loopback10
prefix-sid absolute 16010
loopback stub-network enable
!
interface GigabitEthernet0/0/0/0
network point-to-point
!
!
!
Adjacency SID
An Adjacency SID is a locally significant value which is automatically assigned by a router. The Adjacency SID has a role in Fast Reroute (TI-LFA), traffic engineering and load balancing. The following command can be used to view the Adjacency SID on a router.
R2#show ip ospf segment-routing adjacency-sid
OSPF Router with ID (2.2.2.2) (Process ID 10)
Flags: S - Static, D - Dynamic, P - Protected, U - Unprotected, G - Group, L - Adjacency Lost
Adj-Sid Neighbor ID Interface Neighbor Addr Flags Backup Nexthop Backup Interface
-------- --------------- ------------------ --------------- ------- --------------- ------------------
16 3.3.3.3 Gi1 10.2.0.2 D U
17 1.1.1.1 Gi2 10.1.0.2 D U
Packet capture: OSPF Opaque LSA type-7 and type-8
OSPF uses extensions to the routing protocol to encapsulate Segment Routing information. There are special types of Link State Update (LSU) messages advertised between OSPF neighbors to communicate details related to SR, such as the Prefix SID or Adjacency SID. The Prefix SID is encoded in the OSPF LSA type-7 as shown in the following packet capture.
The Adjacency SID is encoded in the OSPF LSA type-8, which is displayed below. The Adjacency SID is locally significant only, and is used in Segment Routing Topology Independent Loop Free Alternate (TI-LFA), which is a SR Fast Reroute mechanism.
Download section
Disclaimer: You download and use files from networkstudysite.com at your own risk.
ospf-opaque-lsa-type-7-and-8-segment-routing.pcap — The opaque LSA type can be found under each "LSA-type 10 (Opaque LSA, Area-local scope)". For Type-7 it is defined as "Link State ID Opaque Type: OSPFv2 Extended Prefix Opaque LSA (7)", and for Type-8 it is defined as "Link State ID Opaque Type: OSPFv2 Extended Link Opaque LSA (8)", these two LSA types are relevant for Segment Routing, Type-7 is Extended Prefix information (SR SID Label), and Type-8 is Extended Link information (SR Adjacency Label)
Use at your own risk: networkstudysite.com makes no representations as to accuracy, completeness, currentness, suitability, or validity of any information found on this website. Full disclaimer on the About page.
Privacy Policy:
networkstudysite.com does not install browser cookies to collect or store your data.
Thank you for your interest in this blog post!
Looking for something else? View infographics, explore the archives or read the recommended posts below:
