Configuring segment routing in shortest path

The user enables segment routing in an IGP routing instance using the following sequence of commands.

First, the user configures the global label block, known as Segment Routing Global Block (SRGB), which is reserved for assigning labels to segment routing prefix SIDs originated by this router. This range is derived from the system dynamic label range and is not instantiated by default:

config>router>mpls-labels>sr-labels start start-value end end-value

Next, the user enables the context to configure segment routing parameters within an IGP instance:

config>router>isis>segment-routing
config>router>ospf>segment-routing

The key parameter is the configuration of the prefix SID index range and the offset label value that this IGP instance uses. Because each prefix SID represents a network global IP address, the SID index for a prefix must be unique network-wide. Thus, all routers in the network are expected to configure and advertise the same prefix SID index range for an IGP instance. However, the label value used by each router to represent this prefix, that is, the label programmed in the ILM, can be local to that router by the use of an offset label, referred to as a start label:

Local Label (Prefix SID) = start-label + {SID index}

The label operation in the network is similar to LDP when operating in the independent label distribution mode (RFC 5036) with the difference that the label value used to forward a packet to each downstream router is computed by the upstream router based on advertised prefix SID index using the above formula.

Figure: Packet label encapsulation using segment routing tunnel shows an example of a router advertising its loopback address and the resulting packet label encapsulation throughout the network.

Figure: Packet label encapsulation using segment routing tunnel

Router N-6 advertises loopback 10.10.10.1/32 with a prefix index of 5.Routers N-1 to N-6 are configured with the same SID index range of [1,100] and an offset label of 100 to 600 respectively. The following are the actual label values programmed by each router for the prefix of PE2.

N-6 has a start label value of 600 and programs an ILM with label 605.
N-3 has a start label of 300 and swaps incoming label 305 to label 605.
N-2 has a start label of 200 and swaps incoming label 205 to label 305.

Similar operations are performed by N-4 and N-5 for the alternate path.

N-1 has an SR tunnel to N-6 with two ECMP paths. It pushes label 205 when forwarding an IP or service packet to N-6 via downstream next-hop N-2 and pushes label 405 when forwarding via downstream next-hop N-4.

The CLI commands for configuring the prefix SID index range and offset label value for an IGP instance are as follows:

config>router>isis>segment-routing>prefix-sid-range {global | start-label label-value max-index index-value}
config>router>ospf>segment-routing>prefix-sid-range {global | start-label label-value max-index index-value}

There are two mutually-exclusive modes of operation for the prefix SID range on the router. In the global mode of operation, the user configures the global value and this IGP instance takes the start label value as the lowest label value in the SRGB and the prefix SID index range size equal to the range size of the SRGB. After one IGP instance selected the global option for the prefix SID range, all IGP instances on the system are restricted to do the same.

The user must shutdown the segment routing context and delete the prefix-sid-range command in all IGP instances to change the SRGB. After the SRGB is changed, the user must re-enter the prefix-sid-range command. The SRGB range change fails if an already allocated SID index or label goes out of range.

In the per-instance mode of operation, the user partitions the SRGB into non-overlapping subranges among the IGP instances. The user configures a subset of the SRGB by specifying the start label value and the prefix SID index range size. All resulting net label values (start-label + index) must be within the SRGB or the configuration fails. Furthermore, the code checks for overlaps of the resulting net label value range across IGP instances and strictly enforces that these ranges do not overlap.

The user must shutdown the segment routing context of an IGP instance to change the SID index or label range of that IGP instance using the prefix-sid-range command. In addition, any range change fails if an already allocated SID index or label goes out of range.

The user can, however, change the SRGB at any time as long as it does not reduce the current per-IGP instance SID index or label range defined with the prefix-sid-range. Otherwise, the user must shutdown the segment routing context of the IGP instance and delete and reconfigure the prefix-sid-range command.

Finally, the user brings up segment routing on that IGP instances by un-shutting the context:

config>router>isis>segment-routing>no shutdown
config>router>ospf>segment-routing>no shutdown

This command fails if the user has not previously enabled the router-capability option in the IGP instance. Segment routing is a new capability and must be advertised to all routers in a domain so that routers which support the capability only program the node SID in the data path toward neighbors which support it.

config>router>isis>advertise-router-capability {area | as}
config>router>ospf>advertise-router-capability {link | area | as}

The IGP segment routing extensions are area-scoped. As a consequence, the user must configure the flooding scope to area in OSPF and to area or as in IS-IS, otherwise performing no shutdown of the segment-routing node fail.

Next, the user assigns a node SID index or label to the prefix representing the primary address of a network interface of type system or loopback using one of the following commands. A separate SID value can be configured for each IPv4 and IPv6 primary address of the interface.

config>router>isis>interface>ipv4-node-sid index value 
config>router>ospf>area>interface>node-sid index value
config>router>ospf3>area>interface>node-sid index value
config>router>isis>interface>ipv4-node-sid label value 
config>router>ospf>area>interface>node-sid label value 
config>router>ospf3>area>interface>node-sid label value 
config>router>isis>interface>ipv6-node-sid index value
config>router>isis>interface>ipv6-node-sid label value

The secondary address of an IPv4 interface cannot be assigned a node SID index and does not inherit the SID of the primary IPv4 address. The same applies to the non-primary IPv6 addresses of an interface.

In IS-IS, an interface inherits the configured IPv4 or IPv6 node SID value in any level the interface participates in: Level 1, Level 2, or both.

In OSPFv2 and OSPFv3, the node SID is configured in the primary area but is inherited in any other area in which the interface is added as secondary.

The preceding commands fail if the network interface is not of type system or loopback, or if the interface is defined in an IES or a VPRN context. Assigning the same SID index or label value to the same interface in two different IGP instances is not allowed within the same node.

For OSPF, the protocol version number and the instance number dictates if the node SID index or label is for an IPv4 or IPv6 address of the interface. Specifically, the support of address families in OSPF is as follows:

for ospfv2, always IPv4 only
for ospfv3, instance 0..31, ipv6 only
for ospfv3, instance 64..95, ipv4 only

The value of the label or index SID is taken from the range configured for this IGP instance. When using the global mode of operation, a new segment routing module checks that the same index or label value is not assigned to more than one loopback interface address. When using the per-instance mode of operation, this check is not required because the index and the label ranges of the various IGP instances cannot overlap.

For an individual adjacency, values for the label may be provisioned for an IS-IS or OSPF interface. If they are not provisioned, they are dynamically allocated by the system from the dynamic label range. The following CLI commands are used:

config>router>isis>interface
   [no] ipv4-adjacency-sid label value 
   [no] ipv6-adjacency-sid label value

config>router>ospf>area>interface
   [no] adjacency-sid label value

The value must correspond to a label in a reserved label block in provisioned mode referred to by the srlb command (see Segment routing local block for more details of SRLBs).

A static label value for an adjacency SID is persistent. Therefore, the P-bit of the Flags field in the Adjacency-SID TLV advertised in the IGP is set to 1.

By default, a dynamic adjacency SID is advertised for an interface. However, if a static adjacency SID value is configured, then the dynamic adjacency SID is deleted and only the static adjacency SID used. Changing an adjacency SID from dynamic (for example, no adjacency-sid) to static, or the other way around, may result in traffic being dropped as the ILM is reprogrammed.

For a provisioned adjacency SID of an interface, a backup is calculated similar to a regular adjacency SID when sid-protection is enabled for that interface.

Provisioned adjacency SIDs are only supported on point-to-point interfaces.