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Implementing Cisco Nexus 5500 as a core switch: Design considerations

The Cisco Nexus 5500 may be a better replacement for the Catalyst 6500 as a network core switch than the Nexus 7000, but there are design challenges to consider before investing.

In part one of this series on using the Cisco Nexus 5500 as a core switch, we explained why the Cisco Nexus 5500,...

in particular, the Nexus 5596UP,  may be a better choice than upgrading the Catalyst 6509 with a Supervisor 2T or a Nexus 7000. In part two, we look at Nexus 5500 design considerations that must be made prior to investing.

For engineers that need better performance than a Catalyst 6509 can handle, the Cisco Nexus 5596UP may be a less expensive alternative to the Nexus 7000. However, before you call up your local Cisco partner for a quote, there are some design considerations to note:

  • If a Cisco 6509 densely populated with 10/100/1000 copper line cards is being replaced, the Cisco Nexus 5596UP does not offer a migration path for all of those 1GbE copper ports by itself. Shops migrating from physical 1 GbE servers to 10GbE blade centers with virtual hosts will probably not see this as an obstacle, as the transition from 6509 to 5596UP can move in lock-step with the virtualization project.
  • If 1GbE copper ports are required for the long term, Cisco offers inexpensive Fabric Extenders (FEX) that can be plumbed to either a Nexus 5K or 7K, and are bundled with optics. A FEX is not a standalone switch; it would be more correct to think of a FEX as an external line card. Note that there are differences in the FEX architectures supported between the 5K and the 7K. The 7K has been behind the 5K in FEX features and support, so check with your Cisco partner as the state of this changes as new releases of NX-OS emerge. Also note that the Nexus 7K does have a 48 port 10/100/1000 line card option, but a 7K slot is an awfully expensive bit of real estate to fill with 1GbE ports.
  • If FEX units are in use for top-of-rack, note that only eight of them can hang off of a Nexus 5596UP with the L3 module. A 5596UP without the L3 module can handle 24 FEX. A Nexus 7K series with compatible line cards can cope with 32 FEX. This impacts shops using FEX as a part of the overall data center design, and is a unique consideration. Standalone top of rack switches (for example, the Catalyst 3750X) have no such uplink limitations.
  • There is no redundant supervisor engine option in a Cisco Nexus 5596UP. This means that if the switch experiences a catastrophic fault, it becomes a failure point. However, a network core consisting of dual 5596UPs will presumably also have dual-homed FEX units (if they are in use), properly configured first-hop redundancy, a dynamic routing topology, and a functioning virtual port channel domain with dual-homed uplinks. These features (among others) are all recommended by Cisco in its documentation and design whitepapers as best practices to mitigate the risk of a single 5596UP going down and to make code upgrades non-disruptive.
  • The 5596UP can’t expand port density like the 7009, which has finite expansion capabilities. Therefore, long-term demand for additional 10GbE ports will need to be considered -- not that a 7K is the only choice for scaling 10GbE port density.
  • The Nexus 5596UP switch only supports interface speeds up to 10GbE. There is no migration path to 40GbE or 100GbE in a fixed-configuration switch like this. Today, 40GbE is being used for uplinks from densely populated 10GbE switches, and is not expected to be a mainstream fixture in smaller data centers for many years. In fact, there is still some debate in the industry as to whether enterprises will skip 40GbE and go straight to 100GbE when the time comes. Therefore, 40GbE capability should probably not be part of the purchase decision, except for shops with IT hardware depreciation schedules greater than 5 years.
This was last published in March 2012

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The Layer 3 daughter card FEX limitations have changed. The new maximum FEXs per Cisco Nexus 5500 Series switch is 16 with the L3 daughter card installed:
Not to forget: the N5K will never support MPLS (if you need that inside your core)