Configure Segment Routing

By default, Cisco Crosswork Planning creates and routes RSVP LSPs. However, you can create Segment Routing (SR) LSPs by changing the LSP Type property to SR. These SR LSPs use autoroute, forwarding adjacencies, and class-based forwarding, just like RSVP LSPs. Once the packet enters the tunnel, the different routing mechanisms determine the path. An RSVP LSP is first established along a path, with each node on the path accepting the reservation request and maintaining the reservation state. In contrast, SR LSPs rely on a segment list that is created at the LSP’s source node (head-end). This segment list, which is stored in the packet (demand), directs the traffic over a series of configured node segments and interface segments. SR LSPs use IP routing (and thus, potentially ECMP) between segments.


Note
Cisco Crosswork Planning uses the terms SR LSPs that use segment lists, which contain segment list hops. In Cisco routing terminology, these terms are SR tunnels that use segment ID (SID) lists, which contain SIDs.

This section contains the following topics:

SR LSP Segment Types

The segment list of an SR LSP contains one or more segment list hops, which can be nodes, interfaces, SR LSPs, or anycast groups.

To view a segment list, select the SR LSP path and choose Cross Table Filter icon > Filter to segment lists.

For details on creating segment lists and their hops, see Create Segment Lists.


Note
The process of creating and optimizing segment lists and their segment list hops can be automated using the SR-TE optimization tool. For information, see Optimize SR-TE. You can also create SR LSPs and their hops by optimizing bandwidth. For information, see Optimize and Analyze SR-TE Bandwidth.

Node Segment List Hops

When using node segment list hops, the SR LSP takes the shortest path to the specified node. Example Node Segment List Hop shows an example SR LSP from cr2.kcy node to cr2.mia node using one node segment list hop, which is cr2.okc (as indicated by orange circle around the node). Because the IGP metrics are equal, the traffic is routed using ECMP to reach okc.

Figure 1. Example Node Segment List Hop

Interface Segment List Hops

On routers, interfaces can only be used in segment lists if the previous segment list hop is the node containing the interface, or if the interface is the egress of the source node. In Cisco Crosswork Planning, there is no restriction that an interface segment list hop be local. There is no requirement that the segment list contain a preceding segment to the node containing the interface.

Example Interface Segment List Hop shows an example of an interface segment list hop from cr2.chi to cr2.wdc. To ensure the path reaches cr2.chi (the node the packet must reach to use this interface segment list hop), a node segment list hop is configured first to reach chi. This figure also shows how a demand through an SR LSP reroutes when a node fails. The demand uses IGP to route around the failure and back to the segment, if possible.

Figure 2. Example Interface Segment List Hop

LSP Segment List Hops

SR LSPs can use other SR LSPs as segment list hops, and in turn, these SR LSPs segment list hops can contain other SR LSPs as segment list hops. Two rules apply:

  • The LSP segment list hop must be an SR LSP, not RSVP LSP.

  • An LSP segment list hop cannot reference another SR LSP that directly or indirectly references it. For example, if the segment for SR LSP A contains SR LSP B as a segment list hop, SR LSP B cannot contain SR LSP A as a segment list hop.

Example LSP Segment List Hop shows that LSP A (from er1.lax to er1.nyc) contains an LSP B segment list hop, whose source node is cr1.lax. By deselecting LSP A and then selecting LSP B, you see that LSP B also contains an LSP segment list hop, which is LSP C whose source node is cr1.atl.

To view the LSP segment list hops:

  1. Select the LSP and choose Cross Table Filter icon > Filter to LSP paths. The LSP Paths page appears.

  2. Select the LSP paths and choose Cross Table Filter icon > Filter to segment lists. The Segment Lists page appears.

  3. Select the Segment list and click Edit icon to see the relevant segment list hops.

Figure 3. Example LSP Segment List Hop

Anycast Group Segment List Hops

An SR LSP routes through the node in an anycast group segment list hop that has the shortest path to it. In case of a tie between multiple nodes in an anycast group, ECMP is applied. This mechanism lets you impose routing restrictions in terms of potential intermediate segment destinations. It also lets the SR LSP choose among the possible next hops (next segment list hops), potentially reducing latency and improving load balancing.

Example Anycast Group Segment List Hop shows an example of anycast group nodes cr2.chi and cr1.chi. In one instance, the IGP metrics to those nodes have equal IGP metrics, so the SR LSP uses ECMP to route through both of them. When the IGP metric increases from cr2.lax to cr1.chi, the SR LSP routes only through cr2.chi because it has the shortest path. However, as Example Anycast Group Rerouting Around Failure shows, if a failure prevents the shortest path from being used, the SR LSP can still route through the anycast group using the next-highest cost path.

Figure 4. Example Anycast Group Segment List Hop
Figure 5. Example Anycast Group Rerouting Around Failure

SR LSP Paths

SR LSPs can have multiple LSP paths, where each LSP path has its own segment list. There is no distinction between standby and non-standby LSP paths for SR LSPs.


Note
If both the SR LSP and its SR LSP paths have segments, the segments on the LSP paths override those on the SR LSP.

Example SR LSP shows an SR LSP from kcy to mia that has a primary and a secondary path. The primary path is configured with a chi node segment list hop and chi-to-wdc interface segment list hop, and the secondary path is configured with a node segment list hop in okc. This can be seen by selecting the LSP paths individually from the LSP Paths table.

Figure 6. Example SR LSP

SR LSP Routing

Assuming the common LSP mechanisms such as autoroute and forwarding adjacencies have determined that a demand will enter an SR LSP, the LSP traffic is routed as follows. If the destination node or segment list hops are not reachable, the SR LSP is not established.


Note
The process of creating final segment list hops can be automated using the Simulate Traffic Flow from Source to Destination Using Demands tool.

  • If the SR LSP does not have a segment list, the demand is routed via IGP to the destination node.

    If the SR LSP has a segment list, the demand is routed using the segment list hops within the segment list in the sequential order in which they appear.

  • If it is using a node segment list hop, the demand takes the shortest path to that node.

    If it is using an interface segment list hop, the demand takes the shortest path of the source node of that interface, and then routes to that interface’s destination node.

    If it is using an anycast group segment list hop, the demand routes through the node with the shortest path. In case of a tie between multiple nodes in an anycast group, the SR LSP uses ECMP to route.

  • If you select both a node and an anycast group containing that node when creating the segment list hop, the node segment has precedence.

  • If the SR LSP contains LSP paths, the demand is routed on the LSP path with the lowest path option for which the demand can be routed from source to destination. Demands for SR LSP paths route in the same manner as described above for SR LSPs.

  • If the SR LSP contains multiple SR candidate paths, the demand is routed on the LSP path with the highest preference option for which the demand can be routed from source to destination. Demands for SR LSP paths route in the same manner as described above for SR LSPs.

  • If an SR LSP path is configured as 'Dynamic', the LSP path is simulated based on the specified 'Metric type'. This is applicable for IGP, TE, and Delay metric types. As ECMP is not allowed along the path, interfaces with the lowest IP address are preferred. If there is a segment list configured, the LSP path is not dynamically allocated.

  • If the demand cannot be routed on the segments defined in the SR LSP or its LSP paths (for example, because of a failure in a node segment list hop), the demand is routed to the destination using the IGP shortest path.

  • If a hop (nodes, interfaces, LSPs, or anycast groups) contains a segment ID (SID), the segment list hop inherits that SID.

Inter-AS SR LSP Routing

SR LSPs with source and destination nodes in different ASes can route provided the following hop configurations exist:

  • There is a node segment list hop on the border of the AS that the LSP is exiting. This node is the exit node.

  • One of these hops exists:

    • A node segment list hop exists on the first node traversed in another AS. This node is the entry node. If there are multiple peering interfaces, their metrics determine which one the LSP uses.

    • An interface segment list hop connects the exit node of one AS to an entry node in another AS.

Using demands with inter-AS SR LSPs requires that two conditions be met:

  • The demand must be private to the SR LSP. For information on creating private LSPs, see Configure MPLS Routing.

  • The demand must have the same source and destination as the SR LSP.

Create SR LSPs and Their LSP Paths

SR LSPs and mesh SR LSPs are created the same way RSVP-TE LSPs are created. The difference is that you must set the Type property to SR. For more information, see Create and Visualize LSPs.

The SR LSP has a Color property which is a 32-bit numerical value.

Once created, the next step is to either create the segment list or create LSP paths and their segment lists.

Likewise, SR LSP paths are created in the same way as RSVP-TE LSP paths (see Create LSP Paths). After creating the SR LSP paths, you can set the Protocol origin (Edit LSP Path window) to identify the component or protocol that originates the SR LSP path. Choose from Local, PCE initiated, and BGP initiated.

For more information on LSPs and LSP paths, see Configure MPLS Routing.

Create Segment Lists

To create segment lists, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose Actions > Insert > LSPs > Segment list.

OR

In the Network Summary panel on the right side, click Add icon in the Segment lists page.

The Segment lists tab is available under the More tab. If it is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon) and check the Segment lists check box.

Step 3

In the Name field, enter the name of the segment list.

Step 4

From the Site and Node drop-down lists, choose the relevant site and node.

Step 5

In the Hops section, add the segment list hops and rearrange them in the order they should be followed. Click and drag the entries to reorder.

  • To add a new segment list hop, click Add icon and enter the details.

  • To edit an existing segment list hop, select it from the list and then click Edit icon.

  • To delete an existing segment list hop, select it from the list, click Delete icon.

Step 6

To continue creating or editing a segment, use the following options and then click Add. As needed, choose the site, node, interface, or anycast group.

Note

 
Interface segment list hops must be the egress interface of a node segment list hop.

  • If the node or source node (for interfaces and LSPs) is within a site, first choose the site.

  • For node segment list hops, do not choose an interface.

  • For interfaces and LSPs segment list hops, first choose the source node.

  • For anycast group segment list hops, do not choose a site, node, or interface.

After the segment list is created, another way to manage segments is to double-click one from the Segment List Hops table. From there you can change the site, node, and interface, but not the sequence.

Create Anycast Groups

To create Anycast groups, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose Actions > Insert > Others > Anycast group.

OR

In the Network Summary panel on the right side, click Add icon in the Anycast groups tab.

The Anycast groups tab is available under the More tab. If it is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon) and check the Anycast groups check box.

Step 3

In the Group name field, enter a unique name to identify the anycast group.

Step 4

For each node you want to include in the anycast group, check the check boxes in the Included column.

Step 5

Click Add.

Create SIDs

Use the following procedures to create new SIDs.

Create Node SID

To create a node SID, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose Actions > Insert > SIDs > Node SID.

OR

In the Network Summary panel on the right side, click Add icon in the Node SIDs table.

The Node SIDs tab is available under the More tab. If it is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon) and check the Node SIDs check box.

Step 3

Select the values for Site, Node, and Type fields.

Step 4

Enter values for SID and SR algorithm.

Step 5

Use the check box to select Protected or IPv6, as necessary.

Step 6

Click Save.

Create SRv6 Node SID

To create SRv6 node SIDs, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose Actions > Insert > SIDs > SRv6 node SID.

OR

In the Network Summary panel on the right side, click Add icon in the SRv6 node SIDs table.

The SRv6 node SIDs tab is available under the More tab. If it is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon) and check the SRv6 node SIDs check box.

Step 3

Select values for Site and Node from the drop-down.

Step 4

Enter values for SID and SR algorithm.

Step 5

Use the check box to select Protected as necessary.

Step 6

Click Save.

Create Interface SID

To create interface SIDs, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose Actions > Insert > SIDs > Interface SID.

OR

In the Network Summary panel on the right side, click Add icon in the Interface SIDs table.

The Interface SIDs tab is available under the More tab. If it is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon) and check the Interface SIDs check box.

Step 3

Select values for Site, Node, Interface, and Type fields.

Step 4

Enter the value for SID.

Step 5

Use the check box to select Protected or IPv6, as necessary.

Step 6

Click Save.

Create SRv6 Interface SID

To create an SRv6 interface SID, do the following:

Procedure

Step 1

From the toolbar, choose Actions > Insert > SIDs > SRv6 interface SID.

OR

In the Network Summary panel on the right side, click Add icon in the SRv6 interface SIDs table.

The SRv6 interface SIDs tab is available under the More tab. If it is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon) and check the SRv6 interface SIDs check box.

Step 2

Select values for Site, Node, and Interface fields.

Step 3

Enter values for SID and SR algorithm.

Step 4

Use the check box to select Protected, as necessary.

Step 5

Click Save.

Create Flex Algorithm

To create Flex Algorithm, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose Actions > Insert > SIDs > Flex algorithm.

OR

In the Network Summary panel on the right side, click Add icon in the Flex algorithms table.

The Flex algorithms tab is available under the More tab. If it is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon) and check the Flex algorithms check box.

Figure 7. Add Flex Algorithm Window

Step 3

In the Basic tab, enter the value for SR algorithm.

Step 4

Select values for IGP process ID, OSPF area, ISIS level, and Metric type.

Step 5

Click the Flex Algo Affinities tab and choose the inclusion or exclusion rule for each affinity.

Step 6

Click Submit.

SR-TE Protection

If the network contains a configurable topology-independent loop-free alternate (TI-LFA), you can simulate SR-TE tunnels before and after convergence.


Note
To see the simulation before convergence, you must be in Fast reroute mode (click Network options or Actions > Edit > Network options; in the Simulation convergence mode area, choose Fast reroute). In the IGP and LSP Reconvergence mode, you can only see the after-convergence routes.

Simulate SR-TE Tunnels Before Convergence

For each SR tunnel in a network model, and for each SID in the path, Cisco Crosswork Planning determines the no-failure route.

In a normal path state, Cisco Crosswork Planning determines the no-failure route from segment list hop to segment list hop. Between segment list hops, Cisco Crosswork Planning follows the IGP shortest path to the next segment list hop. At each interface along the path, if the connected circuit is down, Cisco Crosswork Planning determines whether the next segment list hop is an interface or a node, and whether the SR tunnel is routable.

  • If SR FRR is enabled for the interface in the Interfaces table, Cisco Crosswork Planning configures the protect path state:

    • Cisco Crosswork Planning derives the post-convergence path from the local node to the next SID hop with the subject circuit removed from the topology.

    • The next SID hop depends on the SID hop type. For an interface SID, the next hop is the remote node of that interface. For a node SID, the next hop is that node.

    • Cisco Crosswork Planning follows the derived after-convergence path hop by hop until it reaches the next SID hop. (If a failure occurs along the way, the SR FRR and the SR LSP do not route.)

    • When Cisco Crosswork Planning reaches the next SID hop, it returns to the normal path state and resumes following the no-fail path to the SID hop.

  • If SR FRR is not enabled for the interface in the Interfaces table, the SR tunnel does not route.

Simulate SR-TE Tunnels After Convergence

Cisco Crosswork Planning routes to each segment list hop through the IGP shortest path under all failure conditions.


Note
An SR LSP might be routable before convergence through a TI-LFA, but unroutable after convergence.

Constraints

  • In the Interfaces table, if the SR FRR enabled column displays "true" for a given interface, all SR LSPs with a node or interface SID as the next hop are protected over that interface. If the SRR FRR enabled column displays "false", all SR LSP SIDs are unprotected over that interface.

  • Protected versus unprotected adjacency SIDs are not supported. All adjacency SIDs are considered protected.

  • Cisco Crosswork Planning does not impose a limit on the TI-LFA path’s label stack depth.

  • Cisco Crosswork Planning does not explicitly derive P and Q nodes, but instead derives a protect path that aligns with the after-convergence shortest path from the PLR to the next segment list hop.

  • If any part of the TI-LFA path encounters a failure before reaching the next segment list hop, the TI-LFA path fails and the SR LSP does not route.