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Cisco router sizing - BGP routing table

Posted on 2010-08-28
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Last Modified: 2012-05-10
I'm looking for some guidance in sizing Cisco routers for different levels of BGP support.  I'm aware that it requires 1 GB of RAM to handle the full BGP routing table, but the processing required to maintain neighbors/sessions, etc., would be too much for a 1941 ISR, for example.  I'd like recommended solutions for the following scenarios:

1) A router that will be dual-homed to two different ISP's (same speed lnks - let's say 100-Mb each), and will be maintaining a full routing table to choose the best path based on BGP path attributes.  So, in this case, the router will not be offering Internet transit services.

2) Same router, but it will only be storing a select number of Internet routes (for key sites/businesses - let's say 1,000 routes.)  Still, no transit traffic.

3) Lastly, an ISP router maintaining full Internet routing table (322K routes as of today), and three OC3 connections to other ISP neighbors.

Thank you – as always, supporting links/reference documentation is appreciated.

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Question by:cfan73
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by:rfc1180
ID: 33550685
Not sure what you mean here: "Internet transit services" however,  you state 100Mbps links to each provider and dual homed, so you would have to offer some type of transit (Maybe not a transit between your ISP Peers, but at least a transit for your customers), can you please elaborate.


3) Lastly, an ISP router maintaining full Internet routing table (322K routes as of today), and three OC3 connections to other ISP neighbors.

do you plan on receiving full routes from all three peers?

Billy
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by:cfan73
ID: 33550699
Thank you for the response, and sorry for the confusion.  

To clarify on 1) and 2) - there will be dual ISP connections, and routes would be received from both.  By "non-transit", I meant that Internet traffic would not flow through the router (ex: in from ISP1 and out to ISP2), so the 2nd link is just for redundancy and best outbound path selection.

For 3), llet's assume that full routes are being received.

Thank you!
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by:rfc1180
ID: 33550735
Here you go:

1. Cisco 7201 (MAX RAM)
2. Cisco 3925E (1GB Should be sufficient)
3. ASR 1002 - fixed and a Cisco 4-Port OC-3c/STM-1 POS SPA

Billy
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Author Comment

by:cfan73
ID: 33550738
Thanks, Billy - I appreciate the answers, but you could also provide supporting reference links/doc?  I trust your answers, but want to be able to research these types of questions myself in the future.

Thanks again.
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by:rfc1180
ID: 33550746
you have a point, I will post
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by:rfc1180
ID: 33550769
Note: I always dedicate 1 GB of ram just for a full table (Others have a full table on 512MB), with CEF and even IPv6, 1GB is my rule of thumb.

The 7201 is cheaper than a ASR 1002, and since you are only peering with 2 ISPs, the 7201 with a NPE-G2 with max ram (2GB) and the on board processor, with 100Mbps from both peers, I thought a 7201 was adequate enough based on comparisons:


Cisco 7201 (MAX RAM)
http://www.cisco.com/en/US/products/hw/routers/ps341/prod_models_comparison.html

The 3925 is a multi-core process route with MAX ram of 2 GB and though should be sufficient with a full routing table and more, these have been designed for branch office use, but since you will be filtering routes and not expecting a full table, these should suite your needs just fine

Cisco 3925E (1GB Should be sufficient)
http://www.cisco.com/en/US/prod/collateral/routers/ps10536/data_sheet_c78_553924.html

Since your requirements were 3 peers with full tables, this puts you into the 3GB range, something is need to handle the BGP process for 3 peers, so I thought the ASR was best low end cost effective solution

ASR 1002 - fixed and a Cisco 4-Port OC-3c/STM-1 POS SPA
http://www.cisco.com/en/US/products/ps9343/prod_models_comparison.html
http://www.cisco.com/en/US/prod/collateral/routers/ps167/product_data_sheet0900aecd804dfd03.html


Billy
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by:cfan73
ID: 33550841
Your comments are helpful, but I've been through the datasheets before, and they don't seem to indicate scaling capabilities. The datasheet for the 3925, for example, indicates that BGP support is there (of course), but the datasheets for the 2800 would indicate the same thing.  When looking at the range of routers that support BGP, can you pinpoint a doc somewhere (or your thought process) in choosing this particular platform?

Sorry if I wasn't being clear, but the datasheets don't really provide these guidelines (at least that I can derive).

Thanks again - really appreciate the help.
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by:rfc1180
ID: 33550912
>I've been through the datasheets before, and they don't seem to indicate scaling capabilities.
Correct, and I do not know of any datasheets or documents that show you any scaling capabilities; this is something that is part of the design process (Business model, requirements, etc). Any router can scale, how well it can scale is based on the requirements, resources and your business model. This is something none of us or any document can tell/explain to you.

>Sorry if I wasn't being clear, but the datasheets don't really provide these guidelines
Correct, they do not; However, you need a set of requirements before you can go and spec out datasheets.

Example:
I want to run BGP (This is very general), do you plan to run iBGP and eBGP? peering with an ISP is one thing, but peering with other customers and internal BGP is another.
What about IGP (OSPF, IS-IS, etc)
Other processes (http, ipsec, mpls, etc)
All these and others take up resources, and this is what you have to take into consideration

>The datasheet for the 3925, for example, indicates that BGP support is there (of course), but the datasheets for the 2800

Yes, but you are not going to run a full BGP table on a 2800 either;at least not reliably. Granted the 2821 and 2851 does have the capability of 1GB of ram, but with other services (such as IGP/BGP) running on single threaded processor that is designed as a branch router is not a scalable solution; as I said, my rule of thumb requirement is 1 GB of ram for one full table from a peer (This will include that I have enough memory for iBGP, eBGP, CEF, and any other features I want to enable that requires more resources and memory).

Billy
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Author Comment

by:cfan73
ID: 33550931
Cool, but let's say I have spec'd out everything that I need in detail - again, for example:

- eBGP w/ two ISP's (no iBGP)
- let's say partial BGP routes this time (let's go with approximately 1,000)
- OSPF locally, but minimal OSPF routes (let's say less than 20)
- no other services or processes

Armed w/ this, how does one use a datasheet (or whatever else) to scope the router model/config that can handle this?

Again, sorry for all of the follow-up.  I want to award points...   :)

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rfc1180 earned 500 total points
ID: 33551065
Hardset requirements:

BGP Full table, Cisco recommends 1GB of RAM
CEF Requirements (if enabled [enabled on most by default])256-512MB of RAM
RIB Requirements (256-512MB)
iBGP = Branch routers
eBGP = WAN/Edge Routers


http://www.cisco.com/en/US/products/hw/routers/ps359/products_tech_note09186a0080110d68.shtml#ts1
http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a00800949e8.shtml
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a83.shtml

Branch routers have rinky dink processors, very low throughput and not intended for any type of service as an edge device for production traffic. Running eBGP, a full table, I would not even consider looking at any ISR routers. I focus on anything that is for enterprise and service provider edge applications, in your case: Internet Edge Routers

armed with new information

- eBGP w/ two ISP's (no iBGP)
- let's say partial BGP routes this time (let's go with approximately 1,000)
- OSPF locally, but minimal OSPF routes (let's say less than 20)
- no other services or processes

You also have to take cost into consideration:

This does not justify the need for a 7200 or a ASR as your requirement does not include a full table, however, with multiple peers, eBGP, a beefier processor would be needed. The only router that supports anything above 200Mbps (100Mbps plus 100Mbps downstream to your network ) is the 3900 series based on packets per second. Note: The ISR routers feature an ASIC bus, replacing the PCI bus found in older routers. Please note that unlike ASIC driven switches and routers, the ASIC found in the ISR line of routers is just for bus operation only. Routing functions are still fully handled by the CPU

http://www.cisco.com/web/partners/downloads/765/tools/quickreference/routerperformance.pdf

The 2800 routes do not have the performance to meet 200Mbps

Billy

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Author Comment

by:cfan73
ID: 33551067
Perfect - that will give me enough to really dig in and start to understand this stuff.

Thank you very much, and I appreciate your patience!!
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Author Closing Comment

by:cfan73
ID: 33551069
Extremely helpful!
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by:rfc1180
ID: 33551072
you bet
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Expert Comment

by:bsdnazz
ID: 34676175
Hi,

Did anyone try a 3925 with full BGP tables? Are there any real world stats available?

At work we're looking to multi-home our network which provides a number of on-line services. The services are  not high bandwidth (the current link is only 2Mb) but they're very important to us. Having experienced a number of network outages we want to multi-home our network. We've experienced outages due to line faults, exchange faults and even ISP core network problems.

Our plan is

get a second internet connection - 4Mb/s from different telco and ISP - ordered
get an ASN and PI IP address space - done
select two routers - hence this post!
advertise our ASN/PI IP address space via BGP
run full eBGP between routers and ISPs and iBGP between routers

Doing all this would mean that we would receive incoming traffic via both ISP links and would use the ISP with the shortest (measured by AS path lenth) route for outgoing traffic to any particular destination.

The big question being how powerful do the routers need to be?

I see lots of people  say 7201 but I can't find any stats as to how busy a pair of 7201s would be in this sort of configuration. Does anyone have any figures for the CPU usage on 7201s or any other routers operating in this sort of configuration?
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