1.1.b High availability techniques such as redundancy, FHRP, and SSO

What this objective means

High availability means the network keeps forwarding during expected failures. It does not mean every packet survives every event. ENCOR expects you to understand the common tools: redundant links, redundant devices, FHRP, supervisor failover, and stateful switchover.

Redundancy

Redundancy gives traffic another path. The common forms are dual uplinks, EtherChannel, dual supervisors, switch stacks, redundant power, redundant WAN circuits, and diverse providers.

Redundancy is only real when the backup path is usable. A second uplink blocked by STP may be fine. A second uplink in the same conduit to the same failed upstream is not real diversity.

Failure domain map:

Host gateway     FHRP (Dist1/Dist2 virtual IP)
Access uplink    STP + EtherChannel
Distribution     dual supervisors / SSO
WAN edge         dual circuits + IP SLA/track

Each layer covers different failures; none fixes all problems alone.

FHRP

First Hop Redundancy Protocols give hosts a virtual default gateway. The host points at one IP address. Two routers or multilayer switches decide which physical device is active.

HSRP example:

interface Vlan10
 description USERS gateway on DIST1
 ip address 10.10.10.2 255.255.255.0
 standby version 2
 standby 10 ip 10.10.10.1
 standby 10 priority 110
 standby 10 preempt
 standby 10 track GigabitEthernet1/0/48 30

Matching standby device:

interface Vlan10
 description USERS gateway on DIST2
 ip address 10.10.10.3 255.255.255.0
 standby version 2
 standby 10 ip 10.10.10.1
 standby 10 priority 100
 standby 10 preempt

Verify it:

show standby brief
show standby vlan 10
show arp | include 10.10.10.1

The exam trap: FHRP protects the default gateway. It does not fix a bad routing table, an upstream outage, or asymmetric filtering by itself.

SSO

Stateful Switchover keeps enough state on a standby supervisor or route processor that failover is fast. With SSO, the standby is hot, not starting from zero. In routing environments, Nonstop Forwarding can keep packets moving while routing protocols reconverge.

Useful verification:

show redundancy states
show platform
show issu state
show logging | include REDUNDANCY|SSO|SWITCHOVER

Lab: prove gateway failover

Build two distribution switches and one access VLAN. Configure HSRP. Start a continuous ping from a host to an upstream loopback. Shut down the active distribution uplink. Watch three things: HSRP state, routing reachability, and packet loss.

The lab is successful when you can explain why failover happened, how long it took, and which timer or tracking event caused it.

Pass check

You should be able to read an HA diagram and say which failures are covered, which failures are not covered, and which command proves the current active path.