Re: BGP Trick: Peering Double Default Route

The really fun part is the rule about using the default-route to hit
the BGP next-hop has changed over time between different IOS releases.
 I believe the current releases work as everybody has stated whereby
the default route will not be used for next hop reachability, but I
have actually seen it both ways.

On Thu, Jul 19, 2012 at 12:37 PM, Marko Milivojevic <markom_at_ipexpert.com> wrote:
> Sud,
>
> If the only route to reach the neighbor is the default route, a router
> will not attempt to connect to it. I wrote a blog about it few years
> back:
>
> http://blog.ipexpert.com/2010/11/08/bgp-peering-and-default-routes/
>
> By the end of the article, I address the double-default problem in a
> slightly different fashion than posted here. My solution would work
> after a reload though ;-).
>
> --
> Marko Milivojevic - CCIE #18427 (SP R&S)
> Senior CCIE Instructor - IPexpert
>
> On Thu, Jul 19, 2012 at 5:30 PM, Sud <mitsuie2_at_gmail.com> wrote:
>> Hi Jose,
>>
>> I don't understand why it is not working with default routes at each end.
>>
>> Sud
>>
>> Sent from my iPad
>>
>> On Jul 18, 2012, at 5:29 PM, Raghava Rao <raghava.rao85_at_gmail.com> wrote:
>>
>>> Good article to further read on
>>>
>>> http://gandikitchen.net/post/2011/06/20/When-Null0-and-BGP-May-Cause-Problems
>>>
>>>
>>>
>>> //RR
>>>
>>> On 18 July 2012 17:14, Jose Jara <jjarafiz_at_gmail.com> wrote:
>>>
>>>> Hi mates,
>>>>
>>>> I am just writing to put here a trick that I have found out after doing
>>>> some testing in BGP peering. Let4s say we have two routers (R1&R2) directly
>>>> connected and they're going to establish an eBGP session between their
>>>> loopbacks.They will use update-source with the loopback and
>>>> disable-connected check. However, the reachability is via a default route
>>>> in each router, therefore, the BGP session can not be established:
>>>>
>>>> BGP: *2.2.2.2 active open failed - no route to peer*, open active delayed
>>>> 32091ms (35000ms max, 28% jitter)
>>>>
>>>> What I thought was, how could they go ahead and establish the session
>>>> without changing the routing? What if I install a route to the loopback
>>>> pointing to null 0 to bypass the BGP check and then, remove it ?
>>>> I tested it and surprisingly it worked! Look at that:
>>>>
>>>> *R1(config)#ip route 2.2.2.2 255.255.255.255 null 0
>>>> *R1(config)#
>>>> *Mar 1 00:05:11.751: BGP: 2.2.2.2 open active, local address 1.1.1.1
>>>> *R1(config)#no ip route 2.2.2.2 255.255.255.255 null 0
>>>> *R1(config)#
>>>> *Mar 1 00:05:17.843: BGP: 2.2.2.2 went from Active to OpenSent
>>>> *Mar 1 00:05:17.847: BGP: 2.2.2.2 sending OPEN, version 4, my as: 100,
>>>> holdtime 180 seconds
>>>> *Mar 1 00:05:17.851: BGP: 2.2.2.2 send message type 1, length (incl.
>>>> header) 45
>>>> *Mar 1 00:05:17.903: BGP: 2.2.2.2 rcv message type 1, length (excl.
>>>> header) 26
>>>> *Mar 1 00:05:17.907: BGP: 2.2.2.2 rcv OPEN, version 4, holdtime 180
>>>> seconds
>>>> *Mar 1 00:05:17.907: BGP: 2.2.2.2 rcv OPEN w/ OPTION parameter len: 16
>>>> *Mar 1 00:05:17.907: BGP: 2.2.2.2 rcvd OPEN w/ optional parameter type 2
>>>> (Capability) len 6
>>>> *Mar 1 00:05:17.907: BGP: 2.2.2.2 OPEN has CAPABILITY code: 1, length 4
>>>> *Mar 1 00:05:17.907: BGP: 2.2.2.2 OPEN has MP_EXT CAP for afi/safi: 1/1
>>>> *Mar 1 00:05:17.911: BGP: 2.2.2.2 rcvd OPEN w/ optional parameter type 2
>>>> (Capability) len 2
>>>> *Mar 1 00:05:17.911: BGP: 2.2.2.2 OPEN has CAPABILITY code: 128, length 0
>>>> *Mar 1 00:05:17.911: BGP: 2.2.2.2 OPEN has ROUTE-REFRESH capability(old)
>>>> for all address-families
>>>> *Mar 1 00:05:17.911: BGP: 2.2.2.2 rcvd OPEN w/ optional parameter type 2
>>>> (Capability) len 2
>>>> *Mar 1 00:05:17.911: BGP: 2.2.2.2 OPEN has CAPABILITY code: 2, length 0
>>>> *Mar 1 00:05:17.915: BGP: 2.2.2.2 OPEN has ROUTE-REFRESH capability(new)
>>>> for all address-families
>>>> BGP: 2.2.2.2 rcvd OPEN w/ remote AS 200
>>>> *Mar 1 00:05:17.915: BGP: 2.2.2.2 went from OpenSent to OpenConfirm
>>>> *Mar 1 00:05:17.915: BGP: 2.2.2.2 went from OpenConfirm to Established
>>>> *Mar 1 00:05:17.915: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Up
>>>>
>>>> Just for fun!
>>>>
>>>>
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>>>>
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-- 
Regards,
Joe Astorino
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http://astorinonetworks.com
"He not busy being born is busy dying" - Dylan
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