RE: BGP multihoming issue

Jack,

Several things that you can do here to fix this.

First double check that you are actually advertising the route.

show ip bgp neighbor x.x.x.x advertised routes

If you are properly advertising both paths then possibly a device in ISP1 is
not properly flushing the BGP table when you lose connectivity. you can
either use conditional advertisement or OER to change your advertisements.

So you could use AS-PATH pre-pending to make it preferred via ISP1 when it
is up and then change the AS-PATH prepending to be less in the event the
first path is down so that even though IPS2 is receiving it from ISP1 the
lower AS-PATH will be preferred. You could use kron to change this in the
event of a IP SLA failure. And the list goes on and on.

You have a lot of options to fix this. The question is what do you want to
do?

Regards,
 
Tyson Scott - CCIE #13513 R&S, Security, and SP
Technical Instructor - IPexpert, Inc.
Mailto: tscott_at_ipexpert.com
Telephone: +1.810.326.1444, ext. 208
Live Assistance, Please visit: www.ipexpert.com/chat
eFax: +1.810.454.0130

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of jack
daniels
Sent: Sunday, December 06, 2009 1:34 AM
To: Joe Astorino
Cc: Cisco certification
Subject: Re: BGP multihoming issue

yup , I seee in bgp neighbour advertised routes 1.1.0.0/23 to ISP2 even when
my link to ISP1 is down

On Sun, Dec 6, 2009 at 11:56 AM, Joe Astorino <jastorino_at_ipexpert.com>wrote:

> Are you sure you are actually advertising the right networks and that they
> are both getting to the other side on ISP2 when things are working
> properly? If you try to advertise "network 1.1.1.0 mask 255.255.254.0" I
> don't believe that will work because the actual route in your RIB would be
> 1.1.0.0/23 correct? I would start with a phone call with ISP2 when BOTH
> links are up. They should be seeing both routes in their BGP table, one
> from you and one from ISP1. One of course will be preferred over the
other,
> but you need to see if it is getting there at all first.
>
> On Sun, Dec 6, 2009 at 12:55 AM, jack daniels
<jckdaniels12_at_gmail.com>wrote:
>
>> Hi guys,
>>
>> I have a scenario where customer connects to 2 diffrent ISP. Customer has
>> taken a block of IP ADDRESS from APNIC ( lets says 1.1.0.0/23)
>>
>> Now via ISP1 I advertise 1.1.0.0/24 and 1.1.1.0/23<<<<<<Primary for
>> 1.1.0.0/24 and secondary for 1.1.1.0/24
>>
>> Now via ISP2 I advertise 1.1.1.0/24 and 1.1.1.0/23<<<<<<Primary for
>> 1.1.1.0/24 and secondary for 1.1.0.0/24
>>
>> ISSUE - when I shut down the link of ISP1 then my 1.1.1.0/24 works but MY
>> 1.1.0.0/24 SUBNET IS NOT GOING THROUGH ISP.
>> WHEN I CALLED ISP2 they say we see 1.1.0.0/23 VIA ISP1 STILL.
>> <<<<<<<<<<POINT 1
>>
>>
>> when we no shut the link to ISP1 then ISP2 say THEY RECIVE 1.1.0.0/23FROM
>> customer connected to ISP2.<<<<<<POINT 2
>>
>>
>> Can some one help me with this issue or if some one has seen this issue
in
>> past ,,,,, why ISP2 is still recieving route from ISP1 when customer LINK
>> to
>> ISP1 is shutdown and
>> why we are not preffering route for 1.1.0.0/23 from customer conneted to
>> ISP2/
>>
>> Regards
>>
>>
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>>
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>>
>>
>
>
> --
> Regards,
>
> Joe Astorino CCIE #24347 (R&S)
> Sr. Technical Instructor - IPexpert
> Mailto: jastorino_at_ipexpert.com
> Telephone: +1.810.326.1444
> Live Assistance, Please visit: www.ipexpert.com/chat
> eFax: +1.810.454.0130
>
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Received on Sun Dec 06 2009 - 01:58:24 ART