Choosing a Branch Router – What Cisco Won’t Tell You | Curvature
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Choosing a Branch Router – What Cisco Won’t Tell You

Recently, Cisco has been pushing customers to adopt the new ISR 4000 series routers, and move away from the legacy ISR G1 or previous generation ISR G2 routers.  While the ISR 4000 series is a high-performance and very feature rich platform, the realities of what’s happening at the network edge do not always require the use of the most current routing platforms.  So, what differentiates these generations of routers, and what router will be the best fit for your environment?


The ISR G1, or 1800, 2800, and 3800, family of routers, introduced in 2006, is still a popular and well performing router series.  Performance ranges from 40Mbps from the 1841 up to 250Mbps for the 3845.  The ISR G1, even now, is still a full-featured router platform, and whose capabilities are still relevant and useful in a modern network.


The ISR G2, or 1900, 2900, and 3900, family of routers is an evolution of the designs and concepts of the ISR G1.  A faster CPU, gigabit Ethernet interfaces on all models, and a larger memory capacity all provide improved scalability compared to the previous generation ISR G1 family.  Performance ranges from 150Mbps for the 1941 up to 1Gbps for the 3945E router.

ISR 4000

The ISR 4000 represents a major change in system architecture compared to Cisco’s previous generation router offerings.  Instead of a single CPU handling both forwarding and system management duties, the ISR 4300 routers use separate CPU cores for forwarding and management, while the ISR 4400 is a dual CPU system, with an entire processor dedicated for traffic forwarding.  This allows for much more consistent performance compared to earlier router generations.  Performance ranges from 50Mbps for the ISR 4321 up to 2Gbps for the ISR 4451.

There are certainly some valid reasons to move to the ISR 4000s.  The following is a transcript of an interview with one of our lead engineers that really dives into this decision from a technical standpoint:

Q: These are deployed as branch routers.  What are common WAN speeds in a modern branch office?

A: The average WAN performance for the US in 2015 was a mere 50Mbps1.  While much higher speeds – 10Gbps is now readily available to businesses in major metro areas – are available for environments that demand it, such as datacenters or headquarters offices, the majority of sites will have limited WAN needs, and will fall around this 50Mbps average.

Q: What criteria should an organization use when determining what branch router to deploy?

A: First of all, performance is the main criteria – if the router can’t physically handle the connection, then there isn’t any point to considering it.

Beyond this, we look at interface needs and interoperability concerns.  For example, if the router needs 4G/LTE connectivity, then the ISR G2 is the minimum router to use, as that’s where the first 4G interface cards are introduced.  As for interoperability, some environments, especially where Cisco voice products are in use, require specific router families or software versions to ensure that all components of the IT infrastructure operate in harmony.

Finally, we look at what features we need to deploy on the routers.  As Cisco’s branch router families have evolved, so have the software features on these routers.  Looking at what the business needs are for the WAN environment will steer us to certain feature needs, which will then dictate what router families are best suited for your environment.

Q: The ISR 4000 routers promise a major performance improvement as well as a host of new features.  I can even run a Linux container or VM on one.  Why shouldn’t I upgrade right now?

A: There are a number of reasons to avoid or postpone an upgrade to the ISR 4000.

First of all, it comes down to if it’s even needed.  Without an increase in WAN performance, the ISR 4000 won’t actually contribute anything else unless the existing router is underpowered.  Even if the WAN connection is upgraded as well, if the existing WAN connection isn’t being regularly saturated, the end users won’t see any performance improvements.

Second is the newness of the ISR 4000 platform.  As a much newer platform that’s undergoing rapid software development, software defects are a fact of life.  A great deal of effort is expended by every IT organization in troubleshooting, identifying, and remediating software defects across all components of the IT infrastructure.  Why create needless work in this area?

Finally, consider the labor and disruption involved in changing out branch office routers.  Configurations need to be converted and vetted for proper operation on the new hardware, IT staff or 3rd party labor needs to be deployed to each site, and maintenance windows and rollback strategies need to be created.  With the importance of networking to modern business, change isn’t done lightly anymore.

Q: The ISR G1 routers are no longer supported by Cisco.  Why should they be considered at all for use in an enterprise environment?

A: The ISR G1, from a performance, featureset and reliability perspective, is still a relevant product, and can serve well in existing environments for some time to come.  Because of this, third party maintenance is an established option in the marketplace to extend the lifecycle of these routers.  Consider exploring this option to keep working hardware in service and postpone a major hardware upgrade until it is truly necessary.

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