Devices fail, network fails, disaster occurs. But still, passing traffic is our goal. SRX provides high availability feature and is different from traditional HA devices. When one router fails, the other router must know about the device configuration and traffic sessions that first router was dealing with. Otherwise, the second router needs to relearn all the routes and sessions making unnecessary traffic congestion in the network. The interaction between two HA device is unique compared to other vendors. To get into more detail about High Availability let’s learn about Juniper SRX High Availability basics.
Juniper SRX High Availability Basics
In HA mode the SRX devices act like a single device creating a chassis cluster. In chassis cluster the two devices acts like one. The flexible PIC concentrator (FPC) starts from zero (0) in one device and ends at other device’s last FPC number. For example in the given figure, the FPC is starting from zero in device A and ends with FPC nine in device B.
The control plane and data plane in SRX is separated. In HA, there can be only one RE no matter what. If the primary RE fails, then only secondary device takes the initiative of primary RE. The control plane synchronizes the state between the routers by exchanging the Hello messages. On RE the process called JSRPD and KSYNCD. JSRPD stands for Junos Stateful redundancy protocol daemon. This process is responsible for exchanging messages and doing failover between devices. Similarly, KSYNCD stands for kernel state synchronization daemon. This process is responsible for synchronizing the kernel state between the two devices.
Remember when we talked about traffic sessions and device configuration that second device must know when first device fails. This information is exchanged between devices by the data plane. Data plane simply synchronizes the sessions and services between the devices. Sessions are current information about the traffic flow. For example if a user is browsing Google’s mail then the session is maintained by the router. This session information is synchronized between devices. Now, to dive more deeper let’s learn about some JunOS High Availability concepts.