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The Segment Routing—ISIS v4 node SID feature provides support for segment routing on Cisco Intermediate System-to-Intermediate System (IS-IS) networks.
Information About Segment Routing IS-IS v4 Node SID
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.
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.
1.
enable
2.
configure
terminal
3. segment-routing mpls
4. connected-prefix-sid-map
5. address-family ipv4
6. 1.1.1.1/32 index 100 range 1
7. exit-address-family
Before you configure segment routing on IS-IS network, IS-IS must be enabled on your network.
1. router isis
2. net network-entity-title
3. metric-style wide
4. segment-routing mpls
5. exit
6. show isis 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:Enabled
This task explains how to configure prefix segment identifier (SID) index under each interface.
Segment routing must be enabled on the corresponding address family.
1. enable
2. configure terminal
3.
segment-routing
mpls
4.
connected-prefix-sid-map
5.
address-family ipv4
6. 1.1.1.1/32 index 100 range 1
7. exit
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.
1.
enable
2.
configure
terminal
3. interface loopback3
4. isis prefix n-flag-clear
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.
1.
enable
2.
configure
terminal
3. segment-routing mpls
4. set-attributes
5.
address-family ipv4
6.
explicit-null
7.
exit-address-family
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 |
Cisco IOS Master Command List, All Releases http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mcl/allreleasemcl/all-book.html |
IP Routing ISIS commands |
Cisco IOS IP Routing ISIS commands http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mcl/allreleasemcl/all-book.html |
Description |
Link |
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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.
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Feature Name |
Releases |
Feature Information |
---|---|---|
Segment Routing—IS-IS v4 Node SID |
Cisco IOS XE Release 3.16S |
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 |