Cluster Routing Support

2.32 Cluster Routing Support

The Cluster Routing feature eliminates the need for a full point code (FPC) entry in the routing table to route to every signaling point in every network. The Cluster Routing feature allows the virtual Signaling Transfer Point (vSTP) to configure one routeset to a entire cluster of destinations. This feature also allows the vSTP to manage and switch traffic to more end nodes.

A cluster is defined as a group of signaling points in which point codes have identical values for the network and cluster fields. A cluster entry in the routing table is shown with an asterisk (*) in the member field of the point code, for example: 111-011-* with this feature, an ANSI destination point code (DPC) can be specified as either an FPC, example: 123-043-045, or as a cluster of signaling point codes, example: 111-011-*.

By default, Cluster Routing is automatically enabled in vSTP and users cannot disable this feature.

Note:

Cluster entries can only be provisioned as ANSI DPCs. Cluster entries cannot be provisioned for ITU international or ITU national DPCs. The ANSI alias point code for an ITU international or ITU national DPC must be an FPC.

The Cluster Routing feature supports provisioning of clusters and FPCs belonging to the same cluster as DPCs as illustrated below. The point codes 111-011-*, 111-011-005, and 111-011-045 entries can be provisioned. In the illustration, the cluster DPC 111-011-* represents all the point codes within the cluster. Cluster entries in the DPC table can also be used as DPCs for routes. A group of such routes with varying relative costs form a routeset to a cluster, similar to a routeset to an FPC.

Figure 2-50 Cluster Routing

Exception Lists (X-lists)

An exception list for a cluster is a list of point codes in a cluster whose routes are more restricted than other routes to that cluster. The term "more restricted" is used when comparing the route status of a cluster member to the route status of the cluster. A prohibited status is more restrictive than a restricted status, and a restricted status is more restrictive than an allowed status.

This list contains point codes that are not assigned to any individual routeset, and the only routeset to that node is through a cluster routeset. The exception list is a dynamic list that changes when the status of the cluster routeset changes.

The vSTP allows users to specify whether exception list entries need to be created on a per cluster basis. For each cluster, the user can specify an ELEI (Exception List Exclusion Indicator) when configuring the cluster point code. When the ELEI is Yes, the vSTP does not create Exception List entries or remove any existing exception list entries for the given cluster. When the ELEI is No, the vSTP creates and removes exception list entries. When the ELEI is No, it is not guaranteed that there will be space available to create each and every possible exception list entry for provisioned cluster entries. All such exception list entries must compete for available exception list space.

Exception list entries are stored separately from the Remote Signaling Point Code table. Maximum 10,000 exception list entries can be stored.

Exception list entries have an expiration timer. There is a single vSTP wide expiration timer value for exception list entries. The exception list expiration timer and the percentage of occupancy that generates a minor alarm can be configured in M3RL Options MO (Managed Object).

XList Cluster Threshold = the exception list (X-list) occupancy threshold (in terms of percentage of space available). If this threshold is exceeded, the vSTP raises a minor alarm. The percentage of occupancy refers to the number of exception list entries (10000) as compared to the maximum number of entries the exception list can hold. For example: the exception list contains 5000 entries, the percentage of the exception list space used is 50%. If this threshold is exceeded, the vSTP raises a minor alarm.

The vSTP raises a major alarm when the exception list is completely full and the vSTP fails to create any more exception list entries.

An exception List entry’s timer is restarted when an exception list entry gets created, updated, or used for routing. This expiration timer (the "XList Expiry Timer Duration" parameter in the M3RL Options MO) can be set from a minimum of 20 minutes to a maximum of 24 hours. The default value upon vSTP startup is 60 minutes. If the timer expires before it is restarted, the exception list entry is removed. The expiration timer allows the vSTP to save resources, if the exception list entry is idle for a long time.

An exception list entry can be created for two distinct set of conditions:

The first set of conditions creates exception list entries based on the status of the route (allowed, restricted, or prohibited), and these entries are marked as "XList due to routing".
The vSTP creates an exception list entry to maintain the congestion status of a non-provisioned, cluster-routed destination point code. These entries are marked "XList due to congestion".

An exception list entry for a particular cluster can be removed from the exception list when the following conditions are met:

Status of all routes to member M change to less or equally restrictive than corresponding status of cluster C’s routes. This can happen due to network management message (TFA (Transfer Allowed Signal) or TFR (Transfer Restricted Signal)).
Expiry timer for "X-List due to routing" entry of member M expires.
When ELEI in Remote Signaling Point MO is changed to Yes for the cluster C, vSTP removes all X-List entries created for that cluster.
When congestion abates for "X-List due to congestion" entry.

Cluster Routing

When the vSTP receives an MSU to route, the routing function looks for the MSUs Destination Point Code as a FPC entry in the routing table. If found, the FPC entry is used to find the corresponding routeset and the outgoing route. If a FPC entry is not found, the routing function uses the DPCs network and cluster values to find a cluster entry to which the DPC belongs. If found, the cluster entry is used to find the corresponding routeset and the outgoing route. If neither a FPC entry or cluster point code entry is found, the vSTP generates existing "event mtp3RouteingFailEvent with reason DPC is not in routing table”.

Compatibility with Non-Cluster Routing Nodes

It is possible that not all nodes in the network in which the vSTP is operating in are cluster routing nodes. In such case, the nodes not performing cluster routing will interpret TCx messages, and apply them to each individual point code belonging to the concerned cluster. This may cause an inconsistency in the status records for exception listed point codes in different nodes. In order to avoid such cases, the vSTP performs the following steps:

After broadcasting a TCR message for cluster C, vSTP will stop any T8 timers running for X listed members of cluster C and enable response method TFPs for cluster’s X listed (prohibited) member point codes by stopping T8. This will allow sending of TFPs for prohibited members immediately after a TCR is broadcast.
After broadcasting a TCA message for cluster C, vSTP enables one-time response method TFR for cluster’s X listed (restricted) member point codes by stopping T18 and enables response method TFPs for cluster’s X listed (prohibited) member point codes by stopping T8. This allows TFPs for prohibited members and TFRs for restricted members to be sent immediately after a TCA is broadcast.

Cluster Management and the ITU Network

ITU SS7 networks lack concept of clusters of point codes and cluster network management messages. vSTP does not generate TCX messages towards ITU nodes. When vSTP is acting as gateway STP between a ITU network and ANSI network, during the broadcast phase of TCX messages, vSTP does not send TCX messages to adjacent ITU point codes. Message loss may occur in such cases. To reduce message loss and quickly notify the sender ITU node about the status, vSTP enables response method messages immediately (without T8 or T18) and relies on response method to convey the status information. While sending response method network management messages in response to a received MSU, vSTP will check MSUs OPC. If MSUs OPC is a ITU point code, always a TFX message will be returned.

Protocol features non preferred options:

vSTP will respond with a TFP (Transfer Prohibited Signal), when a message is received for a inaccessible member and corresponding cluster does not exist.
Upon receiving a TCR (Transfer Cluster Restricted Signal) message concerning cluster for which no cluster is provisioned, vSTP will mark all individually provisioned members as restricted and start RSR procedure for them.
Upon receiving a TCP (Transfer Cluster Prohibited Signal) message concerning cluster for which no cluster is provisioned, vSTP will mark all individually provisioned members as prohibited and start RSP (RouteSet Prohibited ) procedure for them.
vSTP will stop T8 and T18 timers for prohibited and restricted member of the clusters after broadcasting a TCA for cluster. These members can be X listed point codes or full point code.
vSTP will stop T8 timers for prohibited member of the clusters after broadcasting a TCR. These members can be X listed point codes or full point code.

Cluster Routing Rules

These rules apply to the Cluster Routing feature:

If the provisioned number of exception list entries (10000) are already created, the vSTP will not create any more exception list entries. The vSTP raises an alarm in advance of such an occurrence, and logs each occurrence of failure to create an exception list entry.
All adjacent point codes for linksets must be full point codes.
When vSTP is used as an ITU ANSI gateway STP:
- vSTP does not broadcast phase TCX procedures towards ITU nodes. This introduces possibility of message loss until response method kicks in. It is recommended that cluster routing is not used when acting as a gateway STP.
- Cluster destination will not be allowed to have ITU alias point codes.
All ANSI alias point codes specified for real ITU point codes are required to be full point codes.
Point code specified in SCCP MapSet/MrnSet must have a Full point code entry in the routing table.
Point code specified in GTA (Global Title Address) must be a full point code only. It may have FPC or cluster route.
To configure unprovisioned member of provisioned cluster in GTA RSP, select cluster Rsp in RSP and then give the unprovisioned point code in newly added parameter “FullRspPc”.
Point code specified in GTT Action must be full point code only. It may have FPC or cluster route. To configure unprovisioned member of provisioned cluster in Gtt Action RSP, select cluster RSP in RSP and then give unprovisioned point code in newly added parameter “FullRspPc”.
In GTA MO, OPC and Calling Party Point Code can be Cluster Point Code.
vSTP allows cluster routing for subsequent global title (GTT) messages.
vSTP sends subsystem status messages to the concerned point codes using cluster routes.
vSTP does not generate MTP status message for point codes using cluster routing. Hence all MAP table point codes must be full point code entries.
The FiltPC in a PCT translation can be a cluster point code for ANSI domain.
If a point code is in the routing table as an exception-listed (X-listed) member and the user provisions the same point code as a full point code, the X-listed entry is deleted. A new full point code entry is created, inheriting cluster’s route status.
When a cluster destination point code is removed from the vSTPs database, all related exception-listed(XList) point codes of that cluster are removed.
Due to the requirement of same routeset for cluster and its members, once a cluster and FPC members are provisioned in vSTPs Remote SP and Route MO, all routes must be deleted before a full point code member or a cluster can be removed. otherwise, entire cluster will be inaccessible before a member can be removed from Remote Signaling Point MO.
The route assigned to a full point code DPC cannot be deleted directly from the Route Table if that DPC is a member of a cluster point code defined in Remote Signaling Point Table.
if a route assigned to a cluster point is deleted from the route table, the same or corresponding inherited routes of all members of that cluster are also deleted from the Route Table.
The cluster and its provisioned members must have the same routeset. Hence, if a cluster exists only cluster routes can be added or modified.
If member routes exist, a newly added cluster inherits member routes. If cluster route exists, a newly added full point code member inherits all cluster routes.

Protocol features non preferred options:

vSTP responds with a TFP (Transfer Prohibited Signal), when a message is received for an inaccessible member and the corresponding cluster does not exist.
Upon receiving a TCR (Transfer Cluster Restricted Signal) message concerning cluster for which no cluster is provisioned, vSTP marks all individually provisioned members as restricted and start RSR procedure for them.
Upon receiving a TCP (Transfer Cluster Prohibited Signal) message concerning cluster for which no cluster is provisioned, vSTP will mark all individually provisioned members as prohibited and start RSP (RouteSet Prohibited ) procedure for them.
vSTP stops T8 and T18 timers for prohibited and restricted member of the clusters after broadcasting a TCA for the cluster. These members can be X listed point codes or full point code.
vSTP stops T8 timers for prohibited member of the clusters after broadcasting a TCR. These members can be X-listed point codes or full point code.

Home Cluster

A home cluster is a provisioned cluster point code to which vSTP itself is a member. example, if vSTPs true point code or any capability point code is 1-1-1 and a cluster 1-1-* is provisioned then 1-1-* is considered a home cluster.

Provisioning a home cluster causes a profound impact on network management regarding the home cluster and its members. These impacts are identified below:

Ignore messages from X concerning cluster to which X belongs to.
Send only TCA for Home Cluster for RCX.
Send only individual TFP/TFR for the home cluster in response for message received for routing to inaccessible or restricted destination.
As vSTP is an accessible member of the home cluster, it will not transmit TCP or TCR messages regarding the home cluster, except for the following::
- Broadcast TCP: when cluster (except vSTP itself) becomes inaccessible.
- Broadcast TCR: vSTP will stop T8 for all the members.
- Preventive TCP: when starting to route to the cluster through an adjacent node.
If individual full point code members are provisioned for the home cluster, vSTP will generate network management messages for these full point code.
When the home cluster is inaccessible, vSTP will generate one response TFP per T8 timer for members of the home cluster. If vSTP continues to receive traffic for the home cluster, eit will eventually send TFP responses for all members of the cluster.
All above impacts may compromise network management reliability for the home cluster and its members.