Basic 2821 question

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This may seem a little bit of a basic question, but I can't find this anywhere on the product page nor in the plethora of product documentation.

I simply want to route between two gigabit networks. The 2821 has two gigabit ethernet ports. Is this all I need or do I need to laos obtain one of the (various) network addin cards as well?

I'm also looking at using two of these together with HSRP (or similar) - again, the implication from the documentation is that it *is* supported but there's nothing written on the main product page spelling it out.

Thanks in advance,


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Edison Ortiz Wed, 04/29/2009 - 06:56
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Yes, the 2821 will allow you to route between 2 Giga networks but I wouldn't buy a 2821 for that purpose. I suggest getting a switch to route a wire speed as the router won't be able to route at wire speed.

For HSRP to function, both interfaces must be on the same subnet. You are unable to assign IP addresses from the same subnet on the same router. You need 2 physical devices for HSRP to function.




"I suggest getting a switch to route a wire speed as the router won't be able to route at wire speed."

Could you expand on this? If the interfaces in the router are GE and it's plugging into a GE network, what speed is lost? Are packets still pumped at GE speeds, but there's a measureable latency introduced passing the data from source to dest, or are packets pumped at slower than GE speeds?

Joseph W. Doherty Wed, 04/29/2009 - 08:57
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The 2821's gig Ethernet interface hardware should be capable of sending/receiving a single frame at gig rate. To process the frame/packet requires CPU interaction, and the CPU on the 2821 isn't fast enough to process multiple frames, back-to-back, at gig rate for minimum size packets. If the router is overdriven, packets are lost.

For minimum size packets (highest required packets per second), gig Ethernet requires 1,488 Kpps (double for duplex). The 2821 is documented at only 170 Kpps.

As packet size increases, Ethernet needed pps decreases. For instance, 1500 packets would only need 81.3 Kpps. On the face of it, the 2821 should be capable of gig Ethernet if all packets were full size but often the pps rating of the hardware doesn't hold for larger packets.


Ethernet gig required pps rates:

Packet Size (Bytes) 64 128 256 512 1024 1518

Theoretical Maximum Kpps 1488 845 453 235 120 81


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