CBTS
Class-of-service based tunnel selection (CBTS) is a method of selecting a TE tunnel. Unlike the traditional method of load-balancing services on TE tunnels, CBTS selects tunnels based on services' priorities so that high quality resources can be provided for services with higher priority. In addition, FRR and HSB can be configured for TE tunnels selected by CBTS. For more information about FRR and HSB, see the section Configuration - MPLS - MPLS TE Configuration - Configuring MPLS TE Manual FRR and Configuration - MPLS - MPLS TE Configuration - Configuring CR-LSP Backup.
Background
Existing networks face a challenge that they may fail to provide exclusive high-quality transmission resources for higher-priority services. This is because the policy for selecting TE tunnels is based on public network routes or VPN routes, which causes a node to select the same tunnels for services with the same destination IP or VPN address but with different priorities.
Traffic classification can be configured on CBTS-capable devices to match incoming services on the ingress's inbound interface against a specific match rule and map matching services to configured priorities. A rule can be enforced based on traffic characteristics. Alternatively, a QoS Policy Propagation Through the Border Gateway Protocol (QPPB) rule can be used based on BGP community attributes in BGP routes.
Service class attributes can be configured on a tunnel to which services recurse so that the tunnel can transmit services with one or more priorities. Services with specified priorities can only be transmitted on such tunnels, not be load-balanced by all tunnels to which they may recurse. The service class attribute of a tunnel can also be set to "default" so that the tunnel transmits mismatching services with other priorities that are not specified.
Implementation
Figure 4-48 illustrates CBTS principles. TE tunnels between LSRA and LSRB balance services, including high-priority voice services, medium-priority Ethernet data services, and common data services. The implementation of transmitting services of each priority on a specific tunnel is as follows:
Service classes EF, AF1+AF2, and default are configured for the three TE tunnels, respectively.
Multi-field classification is configured on the PE to map voice services to EF and map Ethernet services to AF1 or AF2.
The configuration is repeated. Voice services are transmitted along the TE tunnel that is assigned the EF service class, Ethernet services along the TE tunnel that is assigned the AF1+AF2 service class, and other services along the TE tunnel that is assigned the default service class.
The default service class is not a mandatory setting. If it is not configured, mismatching services will be transmitted along a tunnel that is assigned no service class. If every tunnel is configured with a service class, these services will be transmitted along a tunnel that is assigned a service class mapped to the lowest priority. The following service classes are prioritized in ascending order: BE, AF1, AF2, AF3, AF4, EF, CS6, and CS7.
Usage Scenarios
TE tunnels or TE tunnels on an LDP over TE scenario are configured on a PE to load-balance services.
L3VPN, VLL and VPLS services are configured on a PE. Inter-AS VPN services are not supported.
LDP over TE is configured, and TE tunnels are established to load-balance services on a P.
- The TE tunnel includes two types: RSVP-TE tunnel and SR-MPLS TE tunnel.
