The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.
 Note |
Using segment routing with OSPF is similar to using it with IS-IS. Before reading this chapter, see Using Segment Routing with IS-IS. |
Segment Routing (SR) is a new paradigm of source routing driven by the IGPs and centered on inserting a list of instructions called segments into each packet. SR forwarding can be instantiated by MPLS or IPv6.
OSPF as an IGP propagates the segment information and may either use it internally (for example, to compute Fast Rerouting repair paths) or hand over the segment information to other clients within the router (for example, TE).
To provide a base SR functionality, OSPF interacts with several components:
You can enable segment routing for IGPs under the router configuration sub mode, through commands. However, IGP segment routing are enabled only after the global SR is configured.
SR functionality configurable globally, that is outside the context of any particular IGP instance, include:
After SR is enabled globally on the router, it must be enabled in those OSPF instances that need to run SR.
segment-routing [area N ] {mpls | disable }
This command is accepted only if SR is already enabled globally. If the area keyword is specified then SR is enabled in that area only, otherwise it is enabled in all areas attached to this OSPF instance. The keyword disable can be specified only when the area keyword is also present.
Enabling this command causes OSPF to originate RI LSA, Extended Prefix and Extended Link LSAs. It enables MPLS on all interfaces in area(s) enabled for SR and programs SR MPLS labels for forwarding.
Global SR configuration may contain prefix-to-SID mapping entries for prefixes that are not local to the router. Each of these entries specify the range of prefixes. Remote mapping entries can be used to find SIDs for prefixes connected to routers that do not supporting SR and hence, are not capable of advertising SIDs themselves. This is part of SR-LDP inter-working functionality.
OSPF learns the ranges configured in the global SR configuration through the SR application and advertises them in the Extended Prefix Range TLVs.
The following command allows an OSPF instance to advertise mapping entries configured in the global SR mode:
[no] segment-routing prefix-sid-map advertise-local
This command is configurable in the router mode. Its default state is disabled , That is, no mapping ranges are advertised by OSPF even if they are configured in the global SR mode.
By default, OSPF considers prefix ranges received from mapping servers in the network when computing SIDs for prefixes. This is the core of SR-LDP inter-working label computation. However, if the SR-LDP feature needs to be disabled, use the following command:
[no] segment-routing prefix-sid-map receive
Default state of this command is enabled . That is, OSPF processes mapping ranges received from mapping servers in the network.
When this command is configured in its non-default form, or the no form, it prohibits OSPF from considering prefix-SID mappings from mapping servers.
Note |
This command does not affect processing of 'native' SIDs, that is, those SIDs that are advertised in the Extended Prefix TLVs by routers to whom the prefix is locally connected. |
Extended Prefix TLV of the Extended Prefix LSA carries flags for the prefix and one of them is N-flag (Node). The N-flag indicates that any traffic sent along to the prefix is destined to the router originating the LSA. This flag typically marks the host routes of the router's loop-back.
By default, OSPF advertises the routes of its loopback with the N-flag. If the IP address of a loopback is not unique (for example, it is part of an anycast address), then OSPF must be configured to not advertise the N-flag with the prefix.
Use the following command to cause OSPF to clear N-flag on all prefixes configured on interface in the interface-mode:
ip ospf prefix-attributes n-flag-clear
IP Fast Reroute is a set of techniques that allow rerouting IP traffic around a failed link or a failed node in the network within a very short time (< 50ms). One of the techniques to do this is Loop Free Alternates (LFA). Effective with Cisco IOS XE Release 3.18S, OSPF supports per-prefix directly connected LFA and remote LFA (RLFA).
The per-prefix directly connected LFA provides loop-free alternate path for most triangular topologies, but does not provide good coverage for rectangular or circular topologies. However, the RLFA, which uses MPLS forwarding with LDP signaling for tunneling the rerouted traffic to an intermediate node, extends the IPFRR coverage in ring or rectangular topologies. For each link, RLFA defines the P-Space (the set of nodes reachable from the calculating node without crossing the protected link) and Q-Space (the set of nodes that can reach the neighbor on the protected link without crossing the protected link itself). The nodes that belong to both P- and Q-Spaces are called PQ nodes and can be used as the intermediate nodes for the protected traffic. However, for topologies where the P- and Q-Spaces are completely disjoint, there is still no coverage by RLFA.
Topology Independent Fast st Reroute (TI-FRR) is a technique that uses Segment Routing to provide link protection in any topology, assuming the metric on the links in the topology is symmetrical. Even TI-LFA does not guarantee a backup in cases where the bandwidth on a single link is asymmetrical. TI-LFA only considers loop-free paths that are on the post-convergence path that helps you to better plan the capacity of the network.
Segment Routing allows creating a full explicit path through the network, but using such a fully specified path is not scalable in larger topologies due to the number of segments along the path. Specifying the whole path is, however, not necessary, and only a subset of the path is needed to carry the traffic to an intermediate node (release node) which does not loop the traffic back to the protecting node. When the release node is one of the neighbors of S (source or sender node), then we have a directly connected TI-LFA.
Effective with Cisco IOS XE Release 3.18S, an SR Tunnel is constructed to the release node (if remote) and used as the TI- LFA. The tunnel is constructed by explicitly forwarding through a set of one or more repair nodes. The tunnel is created using a SID stack consisting of one SID (Node SID or Adjacency SID) for every repair node and the corresponding label stack is pushed to the protected traffic.
TI-LFA can be enabled on interface, area, and process scopes. By default, TI-LFA is disabled globally.
TI-LFA can be enabled using following commands.
fast-reroute per-prefix ti-lfa
However, when TI-LFA is enabled globally, you have the option to disable it per area level, using following command:.
[no] fast-reroute per-prefix ti-lfa [area area disable]
This command is accepted even if Segment-Routing is not enabled on the OSF instance or area. In this case, a TI-LFA backup path is created if the post-convergence path has a PQ node with an LDP label to reach it. That is, RLFA through a post-convergence path.
At the process level:
At the interface level:
The following show command displays the details about TI-LFA configuration:
show ip ospf fast-reroute ti-lfa
OSPF Router with ID (13.13.13.13) (Process ID 100)
Microloop avoidance is enabled for protected prefixes, delay 5000 msec
Loop-free Fast Reroute protected prefixes:
Area Topology name Priority Remote LFA Enabled TI-LFA Enabled
0 Base High Yes Yes
Repair path selection policy tiebreaks (built-in default policy):
0 post-convergence
10 primary-path
20 interface-disjoint
30 lowest-metric
40 linecard-disjoint
50 broadcast-interface-disjoint
256 load-sharing
OSPF/RIB notifications:
Topology Base: Notification Enabled, Callback Registered
Last SPF calculation started 00:02:42 ago and was running for 8 ms.show ip ospf database opaque-area type ext-link
OSPF Router with ID (13.13.13.13) (Process ID 100)
Type-10 Opaque Area Link States (Area 0)
LS age: 1378
Options: (No TOS-capability, DC)
LS Type: Opaque Area Link
Link State ID: 8.0.0.7
Opaque Type: 8 (Extended Link)
Opaque ID: 7
Advertising Router: 4.4.4.4
LS Seq Number: 80000056
Checksum: 0xFEF8
Length: 76
TLV Type: Extended Link
Length: 52
Link connected to : another Router (point-to-point)
(Link ID) Neighboring Router ID: 12.12.12.12
(Link Data) Interface IP address: 0.0.0.7
Sub-TLV Type: Adj SID
Length : 7
Flags : L-Bit, V-bit
MTID : 0
Weight : 0
Label : 27
Sub-TLV Type: Adj SID
Length : 7
Flags : L-Bit, V-bit, B-bit
MTID : 0
Weight : 0
Label : 34
Sub-TLV Type: Local / Remote Intf ID
Local Interface ID : 7
Remote Interface ID : 0
show ip ospf database opaque-area type ext-prefix
OSPF Router with ID (13.13.13.13) (Process ID 100)
Type-10 Opaque Area Link States (Area 0)
LS age: 1392
Options: (No TOS-capability, DC)
LS Type: Opaque Area Link
Link State ID: 7.0.0.0
Opaque Type: 7 (Extended Prefix)
Opaque ID: 0
Advertising Router: 4.4.4.4
LS Seq Number: 80000055
Checksum: 0xD156
Length: 44
TLV Type: Extended Prefix
Length: 20
Prefix : 4.4.4.4/32
AF : 0
Route-type: Intra
Flags : N-bit
Sub-TLV Type: Prefix SID
Length: 8
Flags : None
MTID : 0
Algo : IGP metric based SPT
SID : 4
show ip ospf database opaque-area type router-information
OSPF Router with ID (13.13.13.13) (Process ID 100)
Type-10 Opaque Area Link States (Area 0)
LS age: 1402
Options: (No TOS-capability, DC)
LS Type: Opaque Area Link
Link State ID: 4.0.0.0
Opaque Type: 4 (Router Information)
Opaque ID: 0
Advertising Router: 4.4.4.4
LS Seq Number: 80000055
Checksum: 0x7B86
Length: 52
TLV Type: Router Information
Length: 4
Capabilities:
Graceful Restart Helper
Stub Router Support
Traffic Engineering Support
TLV Type: Segment Routing Algorithm
Length: 1
Algorithm: IGP metric based SPT
TLV Type: Segment Routing Range
Length: 12
Range Size: 8000
Sub-TLV Type: SID/Label
Length: 3
Label: 16000
show ip ospf interface
Loopback0 is up, line protocol is up
Internet Address 13.13.13.13/32, Interface ID 29, Area 0
Attached via Interface Enable
Process ID 100, Router ID 13.13.13.13, Network Type LOOPBACK, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Enabled by interface config, including secondary ip addresses
Loopback interface is treated as a stub Host
BDI2104 is up, line protocol is up
Interface is unnumbered, Interface ID 40, Area 0
Using address of Loopback0 (13.13.13.13)
Attached via Interface Enable
Process ID 100, Router ID 13.13.13.13, Network Type POINT_TO_POINT, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State POINT_TO_POINT, BFD enabled
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:05
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Can be protected by per-prefix Loop-Free FastReroute
Can be used for per-prefix Loop-Free FastReroute repair paths
Not Protected by per-prefix TI-LFA
Segment Routing enabled for MPLS forwarding
Index 1/3/3, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 2
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 8.8.8.8
Suppress hello for 0 neighbor(s)show ip ospf [ AS ] segment-routing
OSPF Router with ID (13.13.13.13) (Process ID 100)
Global segment-routing state: Enabled
Segment Routing enabled:
Area Topology name Forwarding
0 Base MPLS
SR Attributes
Prefer SR Labels
Do not advertise Explicit Null
Local MPLS label block (SRGB):
Range: 16000 - 23999
State: Created
Registered with SR App, client handle: 10
Connected map notifications active (handle 0x18), bitmask 0x1
Active policy map notifications active (handle 0x19), bitmask 0xC
Registered with MPLS, client-id: 100
Bind Retry timer not running
Adj Label Bind Retry timer not running
Adj Protected Label Bind Retry timer not running
Note |
In the absence of a mis-configuration, there is only one prefix reported for each SID value. |
show ip ospf [ AS ] segment-routing sid-database [ SID ]
OSPF Router with ID (13.13.13.13) (Process ID 100)
OSPF Segment Routing SIDs
Flags: L - local, N - label not programmed,
M - mapping-server
SID Prefix/Mask Adv-Rtr-Id Area-Id
-------- ------------------ ------------------ -------------
8 8.8.8.8/32 8.8.8.8 0
10 10.10.10.10/32 10.10.10.10 0
13 (L) 13.13.13.13/32 13.13.13.13 0Note |
This show command provides information only about locally-configured prefixes. Information about prefixes received from other routers is available either through LSDB (show ip ospf database opaque-area ) or LRIB (show ip ospf rib local ) show commands. |
show ip ospf [AS ] segment-routing local-prefix
OSPF Router with ID (13.13.13.13) (Process ID 100)
Area 0:
Prefix: Sid: Index: Interface:
13.13.13.13/32 13 0.0.0.0 Loopback0
Use the following command to debug issues
| Related Topic | Document Title |
|---|---|
|
Cisco IOS commands |
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mcl/allreleasemcl/all-book.html |
| Standard/RFC | Title |
|---|---|
|
No specific Standards and RFCs are supported by the features in this document. |
— |
| MIB | MIBs Link |
|---|---|
| — |
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: |
| Description | Link |
|---|---|
|
The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies. To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds. Access to most tools on the Cisco Support website requires a Cisco.com user ID and password. |
Feedback