Re: bgp 2 isp question

I second that prepending is not 100% secure.
And it's not only your ISP local preference, is any ISP local preference
that gets to know both paths. Been there.

Holes (longer prefixes) may help, but some ISPs do not (rightfuly)
accept them. And you are doing no good to the Internet at large.
Conditional advertising, on the other hand, is bullet proof and
goes easy on the global BGP table size. The downside is convergence
time.

-Carlos

Tech Guy @ 1/09/2010 0:58 -0300 dixit:
> I have to disagree with both Paul & Tyson.
>
> My real word answer is that AS-prepend will not always do the job for
> you (no matter how many you prepend), when you want to use the second
> ISP connection purely as the backup.
>
> This is because the ISP may by default always prefer the routes
> advertise from their customers, over the one learnt from other ISP and
> the Internet, disregard of the AS path. The end result is that they
> will always send traffic back to you via the direct peering. This is
> particular true if the ISP config follows the RFC1998. In this case,
> if you want to use the second ISP connection as backup you need to
> send them an agreed community to lower their local preference. You'd
> better check with both ISP for their specific design guideline.
>
> My best practice (without having to worry about the ISP design) is to
> send more specific (/24 subnets) to the primary ISP, and only the
> summary (/23) to the second ISP. The config you proposed is OK, but
> you need to make sure that you have outbound prefix-list or route-map
> to stop /24 from being advertised to the second ISP. By default,
> without those filter-list (assuming your two routers have iBGP peering
> with each other) both /24 and /23 will be advertised to both ISP, and
> you will not get the desired goal.
>
>
> More info on RFC1998 below
>
> http://tools.ietf.org/html/rfc1998
>
> 4. A Real-World Implementation Example
>
> MCI currently makes heavy use of the BGP "LOCAL_PREF" attribute value
> as part of its routing policy configuration process. Different BGP
> "LOCAL_PREF" values are assigned for routes from different sources.
> Table 1 details these values:
>
>
> +-------------------------+------------+
> | Category | LOCAL_PREF |
> +-------------------------+------------+
> |Customer Routes | 100 |
> |Customer backup Routes | 90 |
> |Other ISP Routes | 80 |
> |Customer-Provided backup | 70 |
> +-------------------------+------------+
>
> Table 1: Defined LOCAL_PREF Values
>
>
>
> On Wed, Sep 1, 2010 at 10:24 AM, Paul Negron <negron.paul_at_gmail.com> wrote:
>> In the real world, you would start with about 3 prepends and add one at a
>> time until you get the desired result. Once you get what you need, you would
>> maybe add another for good measure. This method has been proven to be quite
>> successful for me over the last 12 years in the Service Provider
>> Environment. Just my experience and 2 cents worth. :-)
>> --
>> Paul Negron
>> CCIE# 14856 CCSI# 22752
>> Senior Technical Instructor
>> www.micronicstraining.com
>>
>>
>>
>>> From: Fake Name <fname84_at_gmail.com>
>>> Reply-To: Fake Name <fname84_at_gmail.com>
>>> Date: Tue, 31 Aug 2010 13:53:22 -0400
>>> To: Tyson Scott <tscott_at_ipexpert.com>
>>> Cc: Cisco certification <ccielab_at_groupstudy.com>
>>> Subject: Re: bgp 2 isp question
>>>
>>> Tyson
>>>
>>> Thanks for the response. Let me ask you in the real world how many times
>>> would you prepend your as onto it to make it less perfered?
>>>
>>>
>>>
>>>
>>> On Tue, Aug 31, 2010 at 12:26 PM, Tyson Scott <tscott_at_ipexpert.com> wrote:
>>>
>>>> Yes that will work or you can use path prepending to make the AS-PATH
>>>> shorter for the one you want. Both are typical scenario's used on the
>>>> internet.
>>>>
>>>> Regards,
>>>>
>>>> Tyson Scott - CCIE #13513 R&S, Security, and SP
>>>> Managing Partner / Sr. Instructor - IPexpert, Inc.
>>>> Mailto: tscott_at_ipexpert.com
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
>>>> Fake
>>>> Name
>>>> Sent: Tuesday, August 31, 2010 12:06 PM
>>>> To: Cisco certification
>>>> Subject: bgp 2 isp question
>>>>
>>>> If i have a /23 block and I have two isps and 2 different routers. I want
>>>> to perfer traffic to come into 1 router for the block opposed to the other.
>>>> From what I understand /24 is the highest mask that wont be summarized.
>>>>
>>>> All traffic should come into router1 untill it is down then traffic will
>>>> come into router 2 because they have more specific routes correct?
>>>>
>>>>
>>>> Router1 most perfered
>>>> ip route x.x.x.x x.x.x.x (next hop) 0
>>>> ip route x.x.x.x x.x.x.x (next hop) 0
>>>> router bgp x
>>>> network x.x.xx.x 255.255.255.0
>>>> network x.x.x.x 255.255.255.0
>>>>
>>>> Router2 less perfered
>>>> ip route x.x.x.x 255.255.254.0 (next hop) 0
>>>> router bgp x
>>>> network x.x.x.x 255.255.254.0
>>>>
>>>>
>>>> Blogs and organic groups at http://www.ccie.net
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-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net