Configuration Tricks - Catalyst 4900M with CVR-X2-SFP and 1Gb SFP

Cisco Catalyst 4900M

The Cisco Catalyst 4900M is a high performance, low latency, layer-3 switch suitable for Top of Rack 10Gb link aggregation or small data centers server connectivity. eWeek testing validated 320 Gbps throughput, or 16 10G ports running at full line speed with latency of ~2.6 microseconds.

The 4900M chassis includes 8 fixed wire-speed X2 ports. Two half-card module slots can be added for additional ports. Supported modules available are:
  • WS-X4920-GB-RJ45 - 20 port 10/100/1000 RJ45
  • WS-X4904-10GE - 4 port wire speed 10GE (X2)
  • WS-X4908-10GE - 8 PORT 2:1 over subscription 10GE (X2)
  • WS-X4908-10G-RJ45 -  8 PORT 2:1 over subscription 10GbaseT
I recently had a situation were we deployed a Catalyst 4900M with the WS-X4920-GB-RJ45 and WS-X4908-10GE modules. The switch needed to connect a 1GB metro Ethernet circuit. No problem, with the 20 port 10/100/1000 module we had it covered...So I thought.

Network Engineer #SMH
The service provider handed off 1GB multi-mode fiber.

How Do I Connect 1GB Multi-Mode Fiber To This Thing?

Cisco 4900M 8port 10Gb Module
The WS-X4908-10GE offers 8 10GE ports at a 2:1 over subscribed ratio. A feature of this card is the ability to convert 10GB ports to dual-1GB ports with SFP transceivers.


The CVR-X2-SFP Cisco TwinGig Converter is installed into an X2 port and 2 SFPs are plugged into the TwinGig

(Note: The CVR-X2-SFP10G can be used to convert a 10GB X2 port into a 10GB SFP+ port)

Pre-Sales - Cisco Dynamic Configuration Tool
The first step happens in the sales process. Using the Cisco Configuration tool, you choose the WS-X4908-10GE module.

Second, click the "+" next to the 4908 part number, click the "+" next to the Port Group 1, click "SFP options" then choose the GLC part needed.

Now you can't see it, but this adds the CVR-X2-SFP TwinGig Converter Module.

Using this method, the CVR-X2-SFP is included at no-cost. Otherwise the converter sells for $195 list price.

Installation - Good 'ol CLI
The half-card slot the WS-X4908-10GE is divided into 4 port-groups. 8 ports divided by 4 port groups = 2 ports per group. A command is required to convert a port-group into 1GB ports. Since an entire port-group is converted, two of the X2 ports are switched to 1GB.

The Catalyst 4900M is based on the Catalyst 4500 family of switches. the 4900M carries over the configuration concept of modules. Module 1 is the fixed 8 ports of 10GB, module 2 is the top left slot, and module 3 is the top right slot.

To see the current port-group can be seen with:

4900# show hw-module module 3 port-group
Module Port-group Active                         Inactive
   3        1     Te3/1-2                        Gi3/9-12
   3        2     Te3/3-4                        Gi3/13-16
   3        3     Te3/5-6                        Gi3/17-20
   3        4     Te3/7-8                   Gi3/21-24

To change the configuration the command is:
4900(config)# hw-module module 3 port-group 4 select gigabitethernet

Verify the configuration with:

4900# show hw-module module 3 port-group
Module Port-group Active                         Inactive
   3        1     Te3/1-2                        Gi3/9-12
   3        2     Te3/3-4                        Gi3/13-16
   3        3     Te3/5-6                        Gi3/17-20
   3        4     Gi3/21-24                      Te3/7-8

Next a reboot is required.

We ran into a small caveat. We created interfaces G3/23 and G3/24, connected the 1GB SFP into G3/24 but the interface would not connect. We had to move the 1GB SFP to interface G3/23.