Cisco Express Forwarding, Part 2 - router and switch capacity

Since there are two type of Cisco Express Forwarding ( CEF), which are the software-cef and hardware-cef, Cisco router and switcher can be categorized base on the type of CEF.

The below Cisco routers use software-cef:

• Cisco 1800
• Cisco 1900
• Cisco 2800
• Cisco 2900
• Cisoc 7200

The below cisco switches use hardware-cef

• Cisco 3550
• Cisco 3560
• Cisco 3650
• Cisco 3750
• Cisco 4500
• Cisco 6500
• Cisco 6800
• Cisco ME3600
• Cisco ME3800
• Cisco ASR family

It is importance to know it when you choose the device for your network.  for instance, if your network has over 3000 routes, you may like to know whether you can use cisco3750 to be layer3 router in the network.  To answer this question, let's read through the below paragraph:

How to check the maximal ipv4 unicast routes which the Cisco multi-layer switch can support in hardware CEF?

Cisco multi-layer switch uses hardware CEF to forward the layer3 packets in hardware speed.  The hardware CEF is special memory chip which usually is TCAM or ASIC. Since the hardware CEF is very expensive, the Capacity of TCAM is very limited. For cisco3750, the total capacity of TCAM is around 18K. Cisco3750 uses Switch Database Management (SDM) templates to configure the TCAM resource to support features.  By default of  Cisco3750 SDM template, total 2K ipv4 unicast indirect routes can be held in TCAM. what  it means is there are total 2k routes which related packets can be forwarded in hardware speed. If there are more than 2K routes, the related packet to the exceed routes will be processed by generic CPU, which will be forwarded way slower. large amounts of packet loss will happen. if changing cisco3750 SDM template from Default to Routing, then 8K unicast indirect routes in total can be held in TCAM.

Except the hardware CEF capacity, there is other thing needed to be considered is the RAM of switch. For example, if the multi-layer switch runs OSPF, the RAM of multi-layer switch will hold the routing table ( RIB ) and OSPF  database.  Each entry in RIB will consume between approximately 200 and 280 bytes plus 44 bytes per extra path. Each LSA of OSPF  database will consume a 100 byte overhead plus the size of the actual link state advertisement, possibly another 60 to 100 bytes.

Normally, a routing table with less than 500K bytes could be accommodated with 2 to 4 MB RAM; Large networks with greater than 500K may need 8 to 16 MB, or 32 to 64 MB if full routes are injected from the Internet.

So base the routing protocol and size of routing table ( RIB ), you can count whether the multi-layer switch has enough RAM or not to support your network.

For multi-layer switch, the capacity of hardware CEF is the key to determine how many routes it can handle.

You can check the SDM template to see the below switches capacity:

• Cisco 3550
• Cisco 3750
• Cisco 3650
• Cisco 3850
• Cisco Me3600
• Cisoc Me3800

Use the “show sdm prefer” to display the active template and see the capacity.

This is an example of output from the show sdm prefer command, displaying the template in use.

Switch# show sdm prefer

The current template is "desktop default" template.

The selected template optimizes the resources in

the switch to support this level of features for

8 routed interfaces and 1024 VLANs.

number of unicast mac addresses: 6K

number of igmp groups + multicast routes: 1K

number of unicast routes: 8K

number of directly connected hosts: 6K

number of indirect routes: 2K

number of policy based routing aces: 0

number of qos aces: 512

number of security aces: 1K

Using the “sdm prefer {default | routing | routing-pbr| vlan}” to change the TCAM resource allocation.

For high end multi-layer switch, for instance cisco4500 and Cisco 6500, it use FPC in supervisor engine performing the hardware CEF. Check the Cisco web site for capacity data-sheet

Use the below command to display the hardware forwarding capacity of ipv4 routes for Cisco 6500,  :

Switch6500#sh platform hardware cef   hardware

  CEF TCAM REV: 0x4E4C0501, type: CAM3CR

  Size: 262144 entries

        262144 rows/device, 1 device(s)

        32 entries/mask-block

        8192 total blocks (32b wide)

        1212416 s/w table memory

List of the maximal ipv4 unicast routes which Cisco multi-layer switch can hold in hardware CEF

Switch type	Maximal ipv4 routes with default setup
Cisco3550 FE	8K
Cisco3550 GE	12K
Cisco 3650	8K
Cisco 3750	2K
Cisco Me3600 metro	20K
Cisco Me3800 metro	20K
Cisco Catalyst 4500E Supervisor Engine 7L-E	64K
Cisco 6500 vs-s720-10G-3C	256K
Cisco 6500 vs-s720-10G-3CXL	1024K

 

How many routes the software-CEF base router can support?

Since it is software CEF, generic-purpose CPU actually forward all the packets.  how many routes which can supported by router is solely determined by how many RAM the router has. The more RAM router has, the more routes it can handle in software CEF.  How fast the packet can be forwarded is depended on the how powerful the CPU is.  So there is no hard limitation for software CEF, that is why the small Cisco2811 with enough RAM (memory) can handle the full bgp table, but it may crash or very slow due to the CPU is too busy to forward the packets.

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