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Static Adjacency Segment Identifier for ISIS

Adjacency segment is a strict forwarded single-hop tunnel that carries packets over a specific link between two nodes, irrespective of the link cost. You can configure static adjacency segment identifier (SID) labels for an interface or an interface group.

Configuring a static adjacency SID on an interface causes the existing dynamically allocated adjacency SID to be removed along with the transit route for the same.

For static adjacency SIDs, the labels are picked from either a static reserved label pool or from an ISIS segment routing global block (SRGB).

You can reserve a label range to be used for static allocation of labels using the following configuration:

user@host# set protocols mpls label-range static-label-range start-value end-value

The static pool can be used by any protocol to allocate a label in this range. You need to ensure that no two protocols use the same static label. ISIS adjacency SIDs can be allocated from this label block through the configuration using keyword label. The label value for the specific adjacency SIDs need to be explicitly configured. The specific label is advertised as the adjacency SIDs for that interface for the specific level and address family. The following is a sample configuration:

SRGB is a global label space that is allocated for the protocol based on configuration. The labels in the entire SRGB is available for ISIS to use and are not allocated to other applications/protocols. Prefix SIDs (and Node SIDs) are indexed from this SRGB.

ISIS Adj-SIDs can be allocated from ISIS SRGB using keyword ‘index’ in the configuration. In such cases, it should be ensured that the Adj-SID index does not conflict with any other prefix SID in the domain. Like Prefix-SIDs, Adj-SIDs will also be configured by mentioning the index with respect to the SRGB. However, the Adj-SID subtlv will still have the SID as a value and the L and V flags are set. The following is a sample configuration:

Static adjacency SIDs can be configured per address family and also based on whether the protection is required or not. Adjacency SIDs should be configured per level per interface at the [edit protocols isis interface interface-name level level-num] hierarchy level.

  • Protected—Ensures adjacency SID is eligible to have a backup path and a B-flag is set in an adjacency SID advertisement.

  • Unprotected—Ensures no backup path is calculated for a specific adjacency SID and a B-flag is not set in an adjacency SID advertisement.

The following is a sample configuration:

You can use the same adjacent SID for multiple interfaces by grouping a set of interfaces under an interface group and configuring the adjacency SID for that interface group and traffic can be load balanced among the interfaces under the interface group using weight. This can be configured under the [edit protocols isis interface-group interface_group_name] hierarchy level.

When segment routing is used in LAN subnetworks, each router in the LAN may advertise the adjacency SID of each of its neighbors. To configure adjacency SID for a LAN interface to a specific neighbor, you should configure the adjacency SIDs under the lan-neighbor configuration at the [edit protocols isis interface interface_name level level_num lan-neighbor neighbor-sysid] hierarchy level. The following is a sample configuration:

An adjacency set can be configured by declaring a set of interfaces under an interface group and configuring the adjacency segment for that interface group. The adjacency SID can be picked from the reserved static label pool or ISIS SRGB. Unlike normal interfaces, dynamic adjacency SID is not allocated by default under interface group, in which case the dynamic CLI statement is configured. Interfaces configured under an interface group can also be configured separately as independent interfaces as long as the link-group-protection is not configured. The following is a sample configuration:

Use the following CLI hierarchy for configuring adjacency SID:

Use the following operational CLI commands to verify the configuration:

show isis adjacency detail

The following sample output displays the details of configured and dynamic adjacency SID.

show isis database extensive

The following sample output displays the details of LAN/PTP adjacency SID.

show isis interface-group

The following sample output displays the status information about the specified interface group.