Question

BGP looking glass results differ for multi-homing tests

Asked by: kbws1

Good day.
I have just setup multi-homing on my network for Internet redundancy using BGP on a Cisco 3745 router.  My primary ISP (ISP1) provides a class C netblock that I use.  ISP2 is my backup, I receive IP's from them but am not concerned about them as far as redudnancy.  I have established default route BGP sessions with each ISP and annouced ISP1's netblock over both.    I have done the proper route-maps to restrict annoucement of my isp1 netblock to allow only that netblock to be annouced over each ISP.  I have added a route-map to prepend my ASN 2 extra times on the ISP2 connection so that it is less preferrable when the ISP1 route is available.  I also have a local pref statement in BGP for ISP 1 so my router likes it better as well

To test I have used several BGP looking glass sites to see the BGP tables to reflect my annoucements.  My concern is that they drastically differ.  Some only see anoucement from ISP1, a few only see ISP2 annoucement.  Most see multiple instances of ISP1's annoucement, a few see both.  The ones that see both, some prefer ISP1 others prefer ISP2 because of a shorter AS Path.  I imagine I can fix that by increasing my prepend statement to make the AS path longer.

My concern is that is it normal to have such variation across the Internet?  I have not actually caused a failure on my network to actually test the redundancy as I'd like to have a better feeling on the configuration is proper before I actually down the ISP1 port.

Thank you!

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Asked On
2007-12-11 at 07:49:33ID23015746
Tags

bgp

,

glass

,

looking

Topics

Network Routers

,

Network Design & Methodology

,

Network Operations

Participating Experts
2
Points
500
Comments
4

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Answers

 

by: lrmoorePosted on 2007-12-11 at 15:23:01ID: 20453847

If ISP1 does not specifically give you a 'portable' ip subnet, then they are typically sending summary routes to their peers and many other ISP's will simply not accept an individual advertisement of these networks that belong to the larger summary network, or the summary has precidence.
Does ISP1 know that you are trying to advertise their network out through ISP2?

 

by: td_milesPosted on 2007-12-11 at 20:20:24ID: 20455007

Another thing that I have come across is that some ISP's ignore the prepending of AS paths and local preference settings you send them, so it could be that ISP is doing this. lrmoore is correct, the first point of call would be to talk to your two ISP's and see what they have to say.

An alternative way of doing things is to use a conditional BGP advertisement, whereby you only advertise the route via ISP2 when the link to ISP1 goes down (or more specifically when you lose BGP connectivity to ISP1).

http://www.ciscopress.com/articles/article.asp?p=169556&seqNum=2&rl=1

 

by: kbws1Posted on 2007-12-12 at 06:04:55ID: 20457029

Thanks lrmoore and td_miles, I'll contact both ISP's and see what they say.  But yes both ISP's are aware of my netlbock annoucements and my goal for redundancy.

 

by: kbws1Posted on 2008-01-03 at 06:21:11ID: 20572968

I checked with both ISP's they say its setup properly and working.  I did more testing and actually cut the primary circuit.  They Internet remained up, I was able to surf, and could hit my servers off every remote machine/looking glass site I tried.  Thanks

20120131-EE-VQP-002

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