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This chapter describes how Segment Routing (SR) works with IS-IS.
Segment Routing relies on a small number of extensions to Cisco Intermediate System-to-Intermediate System (IS-IS) and Open Shortest Path First (OSPF) protocols. There are two levels of configuration required to enable segment routing for a routing protocol instance. The top level segment routing configuration which is managed by segment routing infrastructure component enables segment routing, whereas, segment routing configuration at the router level enables segment routing for a specific address-family of a routing protocol instance. There are three segment routing states:
Segment routing configuration under the IGPs is allowed only if the SR state is either SR_DISABLED or SR_ENABLED. The SR_ENABLED state indicates that there is at least a valid SRGB range reserved through the MFI successfully. 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.
 Note |
IS-IS protocol SR command is based on per topology (IPv4 address family). |
The SR_ENABLED is a necessary state for any protocol to enable SR, however, it is not a sufficient for enabling SR for a protocol instance. The reason being that the IS-IS still does not have any information about segment routing global block (SRGB) information. When the request to receive information about the SRGB is processed successfully, the IS-IS SR operational state is enabled.
Segment Routing requires each router to advertise its segment routing data-plane capability and the range of MPLS label values that are used for segment routing in the case where global SIDs are allocated. Data-plane capabilities and label ranges are advertised using the SR-capabilities sub-TLV inserted into the IS-IS Router Capability TLV-242 that is defined in RFC4971.
ISIS SR-capabilities sub TLV includes all reserved SRGB ranges. However, the Cisco implementation supports only one SRGB range. The supported IPv4 prefix-SID sub TLV are TLV-135 and TLV-235.
Prefix SIDs received in a label switched path (LSP) with a reachability TLV (TLV 135 and 235) are downloaded to the routing information base (RIB) in the same way as BGP downloads per prefix VPN labels, only if the following conditions are met:
Note |
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Effective with Cisco IOS-XE Release 3.17, IS-IS supports the advertisement of segment routing adjacency SID. An Adjacency Segment Identifier (Adj-SID) represents a router adjacency in Segment Routing.
A segment routing-capable router may allocate an Adj-SID for each of its adjacencies and an Adj-SID sub-TLV is defined to carry this SID in the Adjacency TLVs. IS-IS adjacencies are advertised using one of the IS-Neighbor TLVs below:
IS-IS allocates the adjacency SID for each IS-IS neighbor only if the IS-IS adjacency state is up and IS-IS segment routing internal operational state is enabled. If an adjacency SID allocation failure is due to out-of-label resource, IS-IS retries to allocate the Adj-SID periodically in a default interval (30 seconds).
Effective with Cisco IOS-XE Release 3.18, multiple adjacency-SIDs are supported. For each protected P2P/LAN adjacency, IS-IS allocates two Adj-SIDs. The backup Adj-SID is only allocated and advertised when FRR (local LFA) is enabled on the interface. If FRR is disabled, then the backup adjacency-SID is released. The persistence of protected adj-SID in forwarding plane is supported. When the primary link is down, IS-IS delays the release of its backup Adj-SID until the delay timer expires. This allows the forwarding plane to continue to forward the traffic through the backup path until the router is converged.
Cisco IOS-XE Release 3.18, IS-IS Adj-SID is changed to be per level based since the forwarding plane is unaware of protocol-specific levels. The allocated and advertised backup Adj-SIDs can be displayed in the output of show isis neighbor detail and show isis data verbose commands.
Segment Routing Mapping Server (SRMS) allows configuration and maintenance of the Prefix-SID mapping policy entries. Effective with Cisco IOS-XE Release 3.17, the IGPs use the active policy of the SRMS to determine the SID values when programming the forwarding plane.
The SRMS provides prefixes to SID/Label mapping policy for the network. IGPs, on the other hand, are responsible for advertising prefixes to SID/Label mapping policy through the Prefix-SID/Label Binding TLV. Active policy information and changes are notified to the IGPs, which use active policy information to update forwarding information.
Sometimes, a router may install a prefix with a SID that is different than what it advertises to the LSP. For example, if more than one protocol or more than one IGP instance is announcing the same prefix with different SIDs to the SRMS, the SRMS resolves the conflict and announces the winning prefix and SID that may not be the same as the local instance. In that case, the IGP always advertises what it learns from its source LSP although it still tries to install the SID which may be different than what it learns in its LSP. This is done to prevent the IGP from redistributing the SIDs from another protocol or another protocol instance.
When IS-IS segment routing is configured, IS-IS must request an interaction with the SRGB before IS-IS SR operational state can be enabled. If no SRGB range is created, IS-IS will not be enabled.
When an SRGB change event occurs, IS-IS makes the corresponding changes in its sub-block entries. IS-IS also advertises the newly created or extended SRGB range in SR-capabilities sub-TLV and updates the prefix-sid sub TLV advertisement.
Note |
In Cisco IOS-XE Release 3.16 only one SRGB range and SRGB extension for the modification are supported. |
When IS-IS receives an SRGB deletion event, it looks for an SRGB entry in the IS-IS SRGB queue list. If an SRGB entry does not exist, IS-IS makes sure that there is no pending SRGB created event. If a pending SRGB creation event is found, then IS-IS removes the SRGB creation event, and completes the SRGB delete processing,
If an SRGB entry is found in the IS-IS SRGB queue, IS-IS locks the SRGB, redistributes the RIBs and un-advertises all prefixed-SIDs that have SID value within the pending delete SRGB range, and un-advertises the SRGB range from SR-capabilities sub TLV. Once IS-IS has completed the SRGB deletion processing, it unlocks the SRGB and deletes the SRGB from its SR sub-block entry.
If there is no valid SRGB after the deletion of the SRGB, IS-IS SR operational state becomes disabled.
MPLS forwarding must be enabled before segment routing can use an interface. IS-IS is responsible for enabling MPLS forwarding on an interface.
When segment routing is enabled for a IS-IS topology, or IS-IS segment routing operational state is enabled, IS-IS enables MPLS for any interface on which the IS-IS topology is active. Similarly, when segment routing is disabled for a IS-IS topology, IS-IS disables the MPLS forwarding on all interfaces for that topology.
The command sr-label-preferred allows the forwarding interface to prefer the segment routing labels over LDP labels for all prefixes in a topology.
IS-IS announces the SR information to TE when it detects that both, IS-IS SR and TE are enabled for at least one level. IS-IS announce only the information that is obtained from the level for which TE is configured.
Similarly, IS-IS instructs TE to delete all announcements when it detects that SR is not enabled or TE is no longer configured on any level.
How to Configure Segment Routing —IS-IS v4 Node SID
Before configuring IS-IS to support segment routing you must first configure the segment routing feature in global configuration mode.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
| Step 2 |
configure terminal Example: |
Enters global configuration mode. |
| Step 3 |
segment-routing mpls Example: |
Enables the segment feature using the mpls data plane. |
| Step 4 |
connected-prefix-sid-map Example: |
Enters a sub-mode where you can configure address-family specific mappings for local prefixes and SIDs. |
| Step 5 |
address-family ipv4 Example: |
Specifies IPv4 address prefixes. |
| Step 6 |
1.1.1.1/32 index 100 range 1 Example: |
Associates SID 100 with the address 1.1.1.1/32. |
| Step 7 |
exit-address-family Example: |
Exits the address family. |
Before you configure segment routing on IS-IS network, IS-IS must be enabled on your network.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
router isis Example: |
Enables the IS-IS routing protocol and enters router configuration mode. |
| Step 2 |
net network-entity-title Example: |
Configures network entity titles (NETs) for the routing instance. |
| Step 3 |
metric-style wide Example: |
Configures the device to generate and accept only wide link metrics. |
| Step 4 |
segment-routing mpls Example: |
Configures segment routing operation state. |
| Step 5 |
exit Example: |
Exits segment routing mode and returns to the configuration terminal mode. |
| Step 6 |
show isis segment-routing Example: |
Displays the current state of the IS-IS segment routing. |
The following example displays output from the show isis segment-routing state command for the segment routing under IS-IS:
Device# show isis segment-routing
ISIS protocol is registered with MFI
ISIS MFI Client ID:0x63
Tag 1 - Segment-Routing:
SR State:SR_ENABLED
Number of SRGB:1
SRGB Start:16000, Range:8000, srgb_handle:0x4500AED0, srgb_state: created
Address-family IPv4 unicast SR is configured
Operational state:EnabledThis task explains how to configure prefix segment identifier (SID) index under each interface.
Segment routing must be enabled on the corresponding address family.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
enable Example: |
Enables privileged EXEC mode. |
| Step 2 |
configure terminal Example: |
Enters global configuration mode. |
| Step 3 |
segment-routing mpls Example: |
Configures segment routing mpls mode. |
| Step 4 |
connected-prefix-sid-map Example: |
Enters a sub-mode where you can configure address-family specific mappings for local prefixes and SIDs. |
| Step 5 |
address-family ipv4 Example: |
Specifies the IPv4 address family and enters router address family configuration mode. |
| Step 6 |
1.1.1.1/32 index 100 range 1 Example: |
Associates SID 100 with the address 1.1.1.1/32. |
| Step 7 |
exit Example: |
Exits segment routing mode and returns to the configuration terminal mode. |
By default, a flag called N-flag is set by IS-IS when advertising a SID which is associated with a loopback address. If you wish to clear this flag add explicit configuration.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
| Step 2 |
configure terminal Example: |
Enters global configuration mode. |
| Step 3 |
interface loopback3 Example: |
Specifies the interface loopback. |
| Step 4 |
isis prefix n-flag-clear Example: |
Clears the prefix N-flag. |
To disable penultimate-hop-popping (PHP) and add explicit-Null label, explicit-null option needs to be specified. Once the option is given, IS-IS sets the E flag in the prefix-SID sub TLV.
By default, a flag called E-flag (Explicit-Null flag) is set to 0 by ISIS when advertising a Prefix SID which is associated with a loopback address. If you wish to set this flag add explicit configuration.
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
| Step 2 |
configure terminal Example: |
Enters global configuration mode. |
| Step 3 |
segment-routing mpls Example: |
Configures segment routing mpls mode. |
| Step 4 |
set-attributes Example: |
Sets the attribute. |
| Step 5 |
address-family ipv4 Example: |
Specifies the IPv4 address family and enters router address family configuration mode. |
| Step 6 |
explicit-null Example: |
Specifies the explicit-null. |
| Step 7 |
exit-address-family Example: |
Exits the address family. |
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
| Step 2 |
configure terminal Example: |
Enters global configuration mode. |
| Step 3 |
segment-routing mpls Example: |
Configures segment routing mpls mode. |
| Step 4 |
set-attributes Example: |
Sets the attribute. |
| Step 5 |
address-family ipv4 Example: |
Specifies the IPv4 address family and enters router address family configuration mode. |
| Step 6 |
sr-label-preferred Example: |
Specifies SR label to be preferred over the LDP. |
| Step 7 |
exit-address-family Example: |
Exits the address family. |
The following command enables the IS-IS SRMS and allows IS-IS to advertise local mapping entries. IS-IS does not send remote entries to the SRMS library. However, IS-IS uses the SRMS active policy, which is computed based only on the locally configured mapping entries.
[no] segment-routing prefix-sid-map advertise-local
By default, the IS-IS SRMS client mode is enabled. IS-IS always sends remote prefix-sid-mapping entries received through LSP to SRMS. The SRMS active policy is calculated based on local and remote mapping entries.
The following command disables the prefix-sid-mapping client functionality and it is configured on the receiver side.
segment-routing prefix-sid-map receive [disable]
By default, the IS-IS SRMS server does not flood SID binding entries within the routing domain. From Cisco IOS-XE Release 3.18, the optional keyword domain-wide is added in the IS-IS SRMS server mode command to enable the SID and Label binding TLV flooding functionality.
segment-routing prefix-sid-map advertise-local [domain-wide]
The domain-wide keyword enables the IS-IS SRMS server to advertise SID binding TLV across the entire routing domain.
Note |
The option is valid only if IS-IS SRMS performs in the SRMS server mode. |
Configuration Examples for Segment Routing —IS-IS v4 Node SID
The following example shows how to configure prefix segment identifier (SID) index under each interface:
Device(config)#segment-routing mpls
Device(config-srmpls)#connected-prefix-sid-map
Device(config-srmpls-conn)#address-family ipv4
Device(config-srmpls-conn-af)#10.1.2.2/32 index 2 range 1
Device(config-srmpls-conn-af)#exit-address-family
Device(config-srmpls-conn-af)#end
The following is an example for configuring explicit null attribute:
Device(config)# segment-routing mpls
Device(config-srmpls)# set-attributes
Device(config-srmpls-attr)# address-family ipv4
Device(config-srmpls-attr-af)# explicit-null
Device (config-srmpls-attr-af)# exit-address-family|
Related Topic |
Document Title |
|---|---|
|
Cisco IOS commands |
|
|
IP Routing ISIS commands |
|
RFC |
Title |
|---|---|
|
RFC4971 |
Intermediate System to Intermediate System (IS-IS) Extensions for Advertising Router Information |
|
RFC5305 |
IS-IS Extensions for Traffic Engineering. Defines the advertisement of router IDs for IPv4. |
|
RFC6119 |
IPv6 Traffic Engineering in IS-IS. Defines the advertisement of router IDs for IPv6. |
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.|
Feature Name |
Releases |
Feature Information |
|---|---|---|
|
Segment Routing—IS-IS v4 Node SID |
Cisco IOS XE Release 3.16S Cisco IOS XE Fuji 16.7.1 |
The Segment Routing—ISIS v4 node SID feature provides support for segment routing on IS-IS networks. The following commands were introduced or modified: connected-prefix-sid-map , show isis segment-routing , isis prefix n-flag-clear , explicit-null In Cisco IOS XE Fuji 16.7.1, this feature is supported on Cisco 4000 Series Integrated Service Routers. |
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