Soft reset is an advanced high availability feature that greatly reduces the impact of IOM/IMM resets either during a software upgrade or during other maintenance or debug operations. The combination of In Service Software Upgrade (ISSU) and Soft reset maximizes service availability in an operational network.
A soft reset re-initializes the control plane while the data plane continues operation with only very minimal impact to data forwarding. During the soft reset some processes that rely on the IOM control plane do not run for a duration that is similar to the duration of an IOM Hard reset. These processes include the updating of the IP forwarding table on the IOM (IP FIB downloads from the CPM), Layer 2 learning of new MAC addresses on the IOM, updating of the MAC forwarding table (for MAC addresses learned from other IOMs), ARP, Ethernet OAM 802.3ah, LLDP and handling for specific ICMP functions such as Can’t Fragment, Redirect, Host Unreachable, Network Unreachable and TTL Expired. Note that protocols and processes on the CPM continue to operate during a Soft Reset (BGP continues to learn new routes from peers, and the new routes are downloaded to the IOM after the Soft Reset has completed).
The combination of the very small data plane impact and special soft reset enhancements for protocols ensures that most protocols do not go down and no visible impacts to most protocols are detected externally to the SR/ESS platforms. BFD timers are temporarily increased for the duration of a soft reset to keep BFD sessions up. Protocols such as BGP, OSPF, IS-IS, PIM, and so on with default timers remain up. A protocol using aggressive timers may go down momentarily during a soft reset.
Although the majority of protocols stay up during a Soft Reset, there are some limitations for a few protocols. See Known Limitations in the Release Notes for the relevant release for details.
Configuration changes are not allowed while any card is in the process of a soft reset.
The soft IOM reset procedure is applicable during the ISSU process and for a manual soft reset procedure.
To manually perform a soft IOM reset, enter the clear card slot-number soft command.
Soft Reset is supported on Ethernet IMMs and on IOMs that have Ethernet MDAs provisioned. The operator can optionally force a Soft Reset on an IOM that contains at least one MDA that supports Soft Reset but also has an MDA that does not support Soft Reset or is operationally down. To force Soft Reset in this case the hard-reset-unsupported-mdas is used and the supported MDAs and the card itself are soft reset while the MDAs that do not support soft reset (or are operationally down) are hard reset.
The show card and show mda commands indicate that a soft IOM reset is occurring during the soft reset process.