Cisco Cisco IOS Software Release 12.2(27)SBC

SSO refers to the implementation of Cisco IOS software that allows applications and features to maintain 
a defined state between an active and standby Route Processor (RP). 

In specific Cisco networking devices that support dual RPs, SSO takes advantage of RP redundancy to 
increase network availability. The SSO feature takes advantage of RP redundancy by establishing one of 
the RPs as the active RP while the other RP is designated as the standby RP, and then synchronizing 
critical state information between them. Following an initial synchronization between the two 
processors, SSO dynamically maintains RP state information between them. 

A switchover from the active to the standby processor occurs when the active RP fails, is removed from 
the networking device, or is manually taken down for maintenance. 

The DHCP server that is ISSU and SSO aware is able to detect when a router is failing over to the standby 
RP and preserve the DHCP lease across a switchover event. 

Each DHCP binding is synchronized and re-created from the active RP to the standby RP upon lease 
commit. 

 illustrates this process. The lease extension and release are also synchronized to the 

standby RP.

CPE

DSLAM

BRAS

(DHCP Server)

DHCP Offer

DHCP Request

DHCP Ack

DHCP Discover

BNDUPD

Lease created

Lease updated

Lease destroyed

Active

Standby

DHCP Request

DHCP Ack

BNDUPD

DHCP Release

BNDUPD

IP

170468