Nested Cluster Routing

2.33 Nested Cluster Routing

When a node switches traffic to remote (non-adjacent) nodes, it is possible for STP to use at least one route that differs from the other members of a cluster. This typically occurs when the node is directly connected to the member of a cluster. The nested cluster routing feature provides a mechanism that enables both cluster and member routes to be provisioned within the same cluster.

Nested Clusters and Cluster Members

The cluster routing feature requires that routes to a cluster and members of that cluster be in the same routeset. However, with the nested cluster routing feature, users can have certain members of the provisioned cluster with different full point code routesets. These routesets may be entirely different, partially different, or identical.

With this feature, routes to these members can be modified, removed, or added. Removing a full point code route entry within a cluster will result in the member using the cluster entry for routing. Deletion of a cluster route entry will not delete the full point code route entry. even if they share the same route.

The vSTP sends cluster network management messages (TCA, TCR, and TCP) based on the least restrictive status of the cluster's routeset, and any full point code entries within the cluster.

The nested cluster routing feature provides a new routing model. vSTP supports multiple routing models. The below table describes coupling between a cluster and its members. Coupling defines the relationship between the cluster and member routes.

Table 2-50 Routing Models

vSTP Routing Model	Characteristics	Issues and Resolution
Full Point Code Routing (FPR) No coupling	vSTP will behave as an FPC router when only FPC destinations are provisioned. vSTP will never generate TCX messages concerning clusters of provisioned members. Received TCX messages are applied to all members of the concerned cluster.	No issues. There is no coupling between cluster status and member status due to the lack of clusters.
Cluster Routing and Management Diversity (CRMD) Full coupling NCAI=No	In this mode, vSTP allows provisioning of clusters as well as members of same clusters. Here cluster and member have the same route set and they are fully coupled. All TCX messages are applied to members and TCX messages generated by vSTP reflect member status. In this mode, member status cannot be less restrictive than cluster.	No issues regarding network management message generation and processing. Cluster and members cannot have different route set.
Nested Cluster Routing No coupling NCAI=Yes	In this mode, the NCAI parameter is specified as "Yes" for cluster. The user can enter a cluster route set, then enter a different route set for a member of that cluster. In this case, member route set status can be less restricted than cluster route set status. There is an issue concerning the broadcast of (TCA, TCR, TCP) and the preventive TCP generation.	There is an issue regarding broadcasting network management messages. As members can be less restricted than the cluster, broadcast of cluster messages (TCA, TCR, TCP) is based on the least restrictive of the following: The cluster’s route set status. The route set status of any Full Point Code entries within the cluster. Also, when if NCAI is Yes, vSTP will not generate preventive TCPs.

vSTP Routing Model

Characteristics

Issues and Resolution

Full Point Code Routing (FPR) No coupling

vSTP will behave as an FPC router when only FPC destinations are provisioned.

vSTP will never generate TCX messages concerning clusters of provisioned members.

Received TCX messages are applied to all members of the concerned cluster.

No issues. There is no coupling between cluster status and member status due to the lack of clusters.

Cluster Routing and Management Diversity (CRMD) Full coupling NCAI=No

In this mode, vSTP allows provisioning of clusters as well as members of same clusters. Here cluster and member have the same route set and they are fully coupled. All TCX messages are applied to members and TCX messages generated by vSTP reflect member status. In this mode, member status cannot be less restrictive than cluster.

No issues regarding network management message generation and processing. Cluster and members cannot have different route set.

Nested Cluster Routing No coupling NCAI=Yes

In this mode, the NCAI parameter is specified as "Yes" for cluster. The user can enter a cluster route set, then enter a different route set for a member of that cluster. In this case, member route set status can be less restricted than cluster route set status. There is an issue concerning the broadcast of (TCA, TCR, TCP) and the preventive TCP generation.

There is an issue regarding broadcasting network management messages. As members can be less restricted than the cluster, broadcast of cluster messages (TCA, TCR, TCP) is based on the least restrictive of the following:

The cluster’s route set status.
The route set status of any Full Point Code entries within the cluster.

Also, when if NCAI is Yes, vSTP will not generate preventive TCPs.

Administration

The Nested Cluster Routing feature is provisioned using the NCAI parameter in Remote Signaling Point Managed Object. The NCAI parameter can only be specified for cluster point codes.

If the NCAI parameter is yes, the vSTP allows a certain members of the provisioned cluster to have a different full point code routeset.

If the NCAI parameter is No, standard rules apply (any full point code routeset within a cluster must have the same routeset as the cluster). If NCAI parameter is yes, new rules apply (full point code routeset can differ from the cluster routeset). The following illustration provides an example of provisioning a nested cluster and its associated members.

Figure 2-51 Nested Cluster

Table 2-51 Nested Cluster K Route Table (5-5-*)

Point Code	Linkset	Route Cost
5-5-*	LSB	10
5-5-*	LSC	20

Table 2-52 FPC Member X Route Table (5-5-1)

Point Code	Linkset	Route Cost
5-5-1	LSE	5
5-5-1	LSB	10
5-5-1	LSC	20

Table 2-53 Examples of Nested Cluster Routing Failure and Recovery Actions

Event	Action
All link sets are up and all routes are available	vSTP will not send preventive TCP (K) to vSTP-B as K is a nested cluster, start routing messages to X using LSE and K using LSB. vSTP will broadcast TCA (K) to SSP1, SSP-X, vSTP-A, and vSTP-B.
Link set between vSTP-B and SSP-Y (5-5-2) fails, vSTP-B sends a TFP (Y)	vSTP will create a (5-5-2) X-list entry and mark it to Prohibited on LSB. vSTP will broadcast TFP to SSP1, SSP-X and vSTP and sends response method TFP concerning 5-5-2 (Rule #3). vSTP will start RSP for Y on LSB.
Link set between vSTP-B and SSP-X (5-5-1) fails, vSTP-B sends a TFP (X) to vSTP	vSTP will mark FPC 5-5-1 to PROHIBITED on LSB, vSTP routes the traffic to X through LSE, vSTP will start RSP for X on LSB.
Link set between vSTP-B and SSP-Y (5-5-2) recovers and vSTP-B sends a TFA (Y) to vSTP.	vSTP will remove (5-5-2) X-list entry prohibited status on LSB, performs rerouting and start routing traffic to SSP-Y through LSB. vSTP will broadcast TFA (Y) to SSP1, SSP-X and vSTP-A. vSTP sends a preventive TFP (Y) to vSTP-B
Link set between vSTP-B and SSP-X (5-5-1) recovers and vSTP-B sends a TFA (X) to vSTP	vSTP will mark FPC 5-5-1 to allowed status on LSB.
LSB fails	vSTP will stop using LSB to send traffic to cluster K, mark Prohibited on LSB, perform forced rerouting, start T11 (K) and start using LSC to switch messages to K.
SSP1 sends an MSU with DPC=Y	MSU will be routed on LSC.
SSP1 sends an MSU with DPC=X	vSTP will route MSU to SSP-X using LSE.
LSB recovers	vSTP will stop using LSC to send traffic to cluster K, performs controlling rerouting on K, and mark cluster K as Allowed on LSB, starts routing traffic to cluster K through LSB.
SSP sends a route set test (RSR) concerning Y to vSTP	vSTP responds with a TFA (Y).
LSC fails.	vSTP will stop using LSC to send traffic to cluster K or FPC X and mark K and FPC X Prohibited on LSC.
LSC recovers.	vSTP will mark cluster K and FPC X Allowed on LSC.
LSE fails.	vSTP will stop using LSE to send traffic to SSP-X, marks Prohibited on LSE, perform forced rerouting, send preventive TFP (X) to vSTP-B and start using LSB to switch messages to FPC X.
SSP1 sends an MSU with DPC=SSP-Y	vSTP will route MSU to SSP-Y using LSB.
SSP1 sends an MSU with DPC=SSP-X	MSU will be routed to SSP-X using LSB.
LSE recovers.	vSTP will stop using LSB to send traffic to SSP-X, perform controlling rerouting on FPC X and mark FPC X as Allowed on LSE, start routing traffic to FPC X through LSE. vSTP broadcast TFA (X) to SSP1, vSTP-A, and vSTP-B.

Limitations

vSTP supports a maximum of 500 nested cluster destinations.

Note:
This limit does not apply to non-nested clusters (clusters with NCAI=NO).
If a cluster is more restrictive than one member, vSTP will broadcast status of the least restricted member and rely on response method for members of the cluster that do not have a full point code entry.
vSTP does not broadcast preventive TCPs for nested cluster destinations. As vSTP will not send preventive TCPs when it begins routing towards a nested cluster, circular routing can occur. When routing begins toward a nested cluster, circular routing may occur. However, vSTP sends response method TFPs if it receives an MSU when there is a risk of circular routing.