Juniper JUNOSE 10.0.X PHYSICAL LAYER, Configuration Manual

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Consequently, if you reinstall a module that was formerly unavailable or removed,
the distribution of static tunnel-service interfaces over the modules might be uneven.
Because users create and remove static tunnels, the distribution might remain uneven
indefinitely.
Dynamic Tunnel-Service Interfaces
The router dynamically creates and deletes dynamic tunnel-service interfaces as
dictated by the operation of the relevant protocols. Currently, L2TP sessions are the
only dynamic tunnel-service interfaces available.
When the router creates a dynamic tunnel-service interface, it assigns that interface
to a particular module. If that module becomes unavailable, the router removes the
interface. If the initiator of the dynamic interface requests its reestablishment, the
router recreates the dynamic tunnel service interface and assigns it to an available
module.
Going forward, if you reinstall a module that was formerly unavailable or removed,
the router deletes unwanted dynamic tunnel-service interfaces and creates new ones
for applications on other modules. Gradually, the distribution of dynamic
tunnel-service interfaces on the modules becomes even.
Interface Allocation for Shared Tunnel-Server Modules
When determining how to distribute interfaces across tunnel-server ports, the E-series
router does not perform interface policing to prevent the access services of a shared
tunnel-server module from depriving the tunnel services of the requisite interface
resources (and vice-versa). We recommend that when provisioning shared
tunnel-server ports, you restrict the number of interfaces configured for both access
and tunnel services to prevent competition between them.
For example, when paired with the ES2-S1 OC3-8/STM1 IOA or the ES2-S1 GE-4 IOA,
the ES2 4G LM on the E320 router can support a maximum of 16,000 access interfaces
and 8,000 shared tunnel-server interfaces, both of which must compete for the overall
supported maximum of 16,000 interface columns.
For tunneling, PPP, and IP maximums, see JUNOSe Release Notes, Appendix A, System
Maximums .
ISMs
You can install multiple ISMs to provide redundancy. If you install multiple ISMs at
the same time, the router automatically distributes ISM interfaces over the modules
in proportion to the available ISM interfaces.
Even distribution of ISM interfaces is not critical to router performance. However,
the number of ISMs that you install must be able to support the extra tunnels if one
of the modules becomes unavailable.
When you configure a static IPSec interface, the router automatically assigns that
interface to a particular ISM. If that ISM becomes unavailable, the interface becomes
not present (operational state down).
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Redundancy and Interface Distribution of Tunnel-Service Interfaces
JUNOSe 10.0.x Physical Layer Configuration Guide