The 7705 SAR can be used in deployments where the uplink bandwidth capacity is considerably less than if the router is used for fixed or mobile backhaul applications. However, the 7705 SAR is optimized to operate in environments with megabits per second of uplink capacity for network operations. Therefore, many of the software timers are designed to ensure the fastest possible detection of failures, without considering bandwidth limitations. In deployments with very low bandwidth constraints, the system must also be optimized for effective operation of the routers without any interruption to mission-critical customer traffic.
In lower-bandwidth deployments, SGT can impact mission-critical user traffic such as TDM pseudowire traffic. To minimize the impact on this traffic, SGT can be redirected to a data queue rather than to the high-priority control queue on egress. All SGT applications can be redirected to a data queue, but the type of application must be considered because not all SGT is suitable to be scheduled at a lower priority. SGT applications such as FTP, TFTP, and syslog can be mapped to a lower-priority queue.
Care must be taken when determining which SGT applications should be moved to data queues, as interrupting traffic flow for applications such as routing protocols (for example, BGP, OSPF, and IS-IS) and MPLS can adversely affect router operation, services, and the network.
As an example, in a scenario where the uplink bandwidth is limited to a fractional E1 link with 2 x DS0 channel groups, downloading software for a new release can disrupt TDM pseudowire traffic, especially if SGT traffic is always serviced first over all other traffic flows. Having the option to map a subset of SGT to data queues will ensure that the mission-critical traffic flows effectively. For example, if FTP traffic is redirected to the best-effort forwarding queue, FTP traffic is then serviced only after all higher-priority traffic is serviced, including network control traffic and TDM pseudowire traffic. This redirection ensures the proper treatment of all traffic types matching the requirements of the network.
Timeouts for signaling and/or routing protocols can initiate a session teardown. The teardown will not only have local impacts as severe as losing the sole uplink of a node but could also be network-wide if loss of a node is propagated throughout the network. In this scenario, routing protocols, as an example, will rerun on all the nodes within the same area, generating a CPU load and extra control-plane traffic. Such a scenario could cause potential instability across the network or an area of the network. To avoid this scenario, the user must ensure that uplink capacity is enough to transmit all crucial SGT plus the mission-critical user traffic, and SGT redirection should only be used for the non-mission-critical traffic that can tolerate delay and jitter.
Redirection of SGT applications is done using the config>router>sgt-qos> application>fc-queue or config>service>vprn>sgt-qos>application>fc-queue command.
Redirection of the global ping application is not done through the sgt-qos menu hierarchy; this is configured using the fc-queue option in the ping command. Refer to the 7705 SAR OAM and Diagnostics Guide, ‟OAM and SAA Command Reference”, for details.
SGT redirection is supported on the base router and the virtual routers on ports with Ethernet or PPP/MLPPP encapsulation.