BGP multihoming - best scenario for upstream failure?

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Jul 19th, 2010
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Hello everyone,

I’m currently configuring a bgp multihoming environment ( two ISP’s - one primary and the other as a backup) I'm advertising my public /24 with my own ASN via BGP to both ISPs.  I am only receiving a default route from both of my ISPs and running iBGP between my two edge routers. If the physical link fails between my ISP1(primary) the traffic switches to the secondary with no problems. 

Here is the question and my concern:  what would happen if ISP1 (primary) had a problem upstream beyond my physical link? Since am only getting their default route, would this mean that I will still be sending traffic since my edge routers are not seeing any issues? And what is the best way to avoid this scenario?

Thank you very much in advance.   

Cheers!

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Tharak Abraham Mon, 07/19/2010 - 10:26
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Hi,


If the ISP1 implements a default route withdrawal w.r.t the IP SLA then probably you would have a solution here.

You need to sort this out with the ISP1.


Otherwise i am afraid there is any other way.

fsebera Mon, 07/19/2010 - 12:28
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You probably do not need BGP for your setup since you are not receiving any prefixes from either ISP and you are using only as single inbound/outbound path at a time.


You could probably get away with IP SLA static route tracking and monitoring.


Pick a known good and pingable upstream destination. Destination should be passed your ISP and of course the ISP must allow icmp to/from this destination. When the upstream destination is no longer available, IP SLA tracking will automatically pull the "downed" static router from the routing table (in your case 0.0.0.0/0 to IPS A) and insert 0.0.0.0/0 to IPS B.


This process is completely dynamic.


http://www.cisco.com/en/US/docs/ios/12_4/ip_sla/configuration/guide/hsthresh.html


HTH

Frank

mguzman4158 Mon, 07/19/2010 - 12:36
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Frank,


The only issue I see is that I do need to advertise my /24 to the internet, hence the need to run BGP with both ISPs.


Thanks,

MG

Tharak Abraham Mon, 07/19/2010 - 19:37
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My humble opinion, you dont really need to worry about how your SP is mutihomed.

Certainly since they are your Primary SP they would be the Best, aint it ?


As someone cited, conditional advertisement of the default can help you.

Talk with the SP and get to know how they do it ?

Mohamed Sobair Mon, 07/19/2010 - 12:50
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Hi,


The Solution to this is to request your upstream service providers to conditionally advertise a default route to your edge routers.


with this, you will only recieve a default route if the upstream of the upstream is up.



HTH

Mohamed

Chetan Kumar Ress Mon, 07/19/2010 - 12:54
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Hi


You have question that if your ISP had a problem with upstream service provider : -


1] All SP have multihoming with different SP, So if they face issue with upstream service provider then it will switch to other SP.


So accourding to me you should not face any issue.


Regards

Chetan Kumar

Mohamed Sobair Mon, 07/19/2010 - 13:02
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Chetan,


Not all of the ISP are multihoming, some ISPs doesnt have multihoming connections.


He could easily ask the ISP to get a default route ONLY conditionaly advertised if the upstream is up.



HTH

Mohamed

Chetan Kumar Ress Mon, 07/19/2010 - 13:26
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Hi Sobair


If SP won't have any multihoming . Then i think you should not take any service from him.


If user are planning for multihoming then he should think always in broad way , he should take service from well known SP


Regards

Chetan Kumar

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