The SSO/ISSU feature
supports only the Cisco ASR 1000 Series Aggregation Services Routers
intrachassis (RP-to-RP) SSO, but not the interchassis (Cisco ASR1K-to-Cisco
ASR1K) SSO. The First Sign Of Life (FSOL) triggers that are supported on SSO
include DHCP proxy (where the iWAG acts as the DHCP proxy server) and DHCP
proxy plus unclassified MAC.
For more information
about ISSU, see the “Overview of ISSU on the Cisco ASR 1000 Series Routers”
section of the
Cisco ASR 1000 Series Aggregation Services
Routers Software Configuration Guide.
The process as part
of iWAG SSO handling GTP checkpoints to the standby RP the information that is
necessary to create a copy of the session on the standby RP. Such an inactive
copy of the session becomes active when the standby RP becomes active.
When an iWAG
mobility session with GTP tunneling is enabled using the SSO/ISSU feature, the
Cluster Control Manager on the active RP needs to wait for a few more
components, including the GTP, to become ready before checkpoint data
collection, and polls these additional components for checkpoint data during
data collection. A very similar operation is performed on the standby RP as
well. Although such additional CPU consumption is per session, it is not
expected to be too heavy since processing in each of these components should
include the time spent on a few data structure lookups and memory-copying
operations.
During ISSU SIP and
SPA upgrade, there is traffic interruption. To avoid session disconnect because
of dropped echo messages during such traffic interruption, a user has the
following options:
- Disable the echo messages
on the iWAG and GGSN for the duration of the ISSU.
- Re-enable the echo messages
after ISSU is completed on the iWAG and GGSN.
- Option 2: Extend the t3 and
n3 configurations to exceed the expected traffic interruption. The traffic
interruption characterized in the Cisco IOS XE Release 3.10S is 127 seconds.
Hence, we recommend the following t3 and n3 settings (t3_response: 1 and
n3_request: 7, resulting in 127 seconds on both the iWAG and GGSN) but the
duration of the traffic interruption may depend on the types of SIPs and SPAs
and how loaded the router is. If traffic interruption exceeds the configured t3
and n3 limits, the session is disconnected.
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