BGP with 2 routers to 2 ISP's

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Dec 19th, 2007
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Getting ready to add a 2nd ISP to our external connectivity. We own a /24 public subnet. I want to have all traffic to/from internet traverse my current ISP, and just use the 2nd ISP (for pricing reasons) as my to/from path when I loose BGP neighbor with my primary ISP. The following link has been referenced on several netpro discussions, but doesn't totally solve my needs:

Can someone assist?

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e-dennington Wed, 12/19/2007 - 07:29
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I assume you already have applied for and recieved a public ASN from ARIN. You will need to advertise your /24 to both your ISPs via BGP. On your backup link, you can do an AS prepend on your backup router to artificially increase the AS path to your site via your backup link so all traffic will prefer your main link (due to a shorter AS path) unless it goes down.

You can run HSRP on the inside interfaces of your routers so traffic will failover when one of your internet links goes down as well.

gerheauserm Wed, 12/19/2007 - 08:00
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I own a /24, but do not yet have an ASN, will get that going now. As for the rest, I have a ton of experience with EIGRP, but no useful experience with BGP. I understand the premise of your suggestion, but not the specifics. How would I also insure that ISP #1 is the preferred path for all outbound traffic? Do I need to host the complete internet routing table as well?

Danilo Dy Wed, 12/19/2007 - 08:12
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Its simple to use ISP #1 as preferred path for outgoing traffic.

For a single router connected to two eBGP upstream, use WEIGHT.

For two routers (iBGP with each other) each one connected to one eBGP upstream use LOCAL PREFERENCE. Since default LOCAL PREFERENCE is 100 create a route-map to set a LOCAL PREFERENCE of 120 for all routes coming from ISP #1, this will ensure all outgoing will use ISP #1.

For incoming, whether you use MED or PREPEND, you will still received incoming traffic from ISP #2.



Danilo Dy Wed, 12/19/2007 - 07:39
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You want the second ISP to use only when primary is down? None of the document will satisfy that. Whatever you do, there will still be incoming traffic.

There are people who do the following.

1. Configure and established (UAT) eBGP with second ISP and make sure it works during failover.

2. Shutdown eBGP with second ISP

3. Setup a monitoring (i.e. WhatsUp) for link to primary ISP

4. Create a script to login to router and enable eBGP with second ISP

5. Configure the monitoring that when link to primary ISP is down to execute the script to enable eBGP with second ISP. You can manually shut it down when link to primary ISP is back online or you can create another script.

You can Google for Perl Script use to login to Cisco Router and execute commands. Just edit them to execute a command to enable eBGP with second ISP.




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