Re: bgp 2 isp question

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
>>>
>>>
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>>
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Received on Wed Sep 01 2010 - 13:58:09 ART