Default routes on BGP routers with full feeds

Owen DeLong owen at
Wed Nov 5 17:48:00 UTC 2014

> On Nov 4, 2014, at 10:49 PM, Andreas Larsen <andreas.larsen at> wrote:
> There is one setup where you would need default route from your provider. 

That may be true, but this isn’t it…

> If you have no IBGP between two sites and your prefix is a large /16 on side and maybe a /18 from that /16 on another site. These site would not be able to talk to each other if you orginate from the same AS. 

1.	Don’t do this. No, really, this is like the old joke about “Doctor, Doctor, it hurts when I do this!”. Just get a second AS.
	Supposed definition of an AS: “A collection of prefixes with a common routing policy”.
	If you have a /18 advertised from group A and a /17 and a /18 advertised from group B (even if you’re pretending it’s a /16
		and including the covered separate /18), then you have 3 (or pretending 2) prefixes which have different routing

2.	If you are going to do this, then you’re better off building a tunnel between the sites and setting up iBGP across the tunnel.

3.	Another option is to coerce your BGP into accepting routes with your own AS in the AS PATH. This circumvents BGP loop
	detection, but if you’re two sites are stub sites (and I can’t imagine a scenario where you would do this with transit sites),
	then that is a pretty low risk. Further, you can filter out the potential loop routes pretty easily since you know which ones
	are local to each site, making that particular loop detection irrelevant).

> Other than that I see not harm in having both default and a full table since longest prefix match will always win even if you have 2 or more transits.

The harm is that instead of dropping traffic that can’t go anywhere, you’re passing it to someone else to drop for you. I suppose as long as you’re paying for the bandwidth used, it’s not a big deal, but it also breaks your ability to implement things like BCP38.


> // Andreas
> Med vänlig hälsning
> Andreas Larsen
> IP-Only Telecommunication AB| Postadress: 753 81 UPPSALA | Besöksadress: S:t Persgatan 6, Uppsala |
> Telefon: +46 (0)18 843 10 00 | Direkt: +46 (0)18 843 10 56
> <>
>> 5 nov 2014 kl. 02:41 skrev Chris Rogers <crogers at <mailto:crogers at>>:
>> We don't accept a default from anyone, but will send one to a customer when
>> specifically requested.
>> We heavily filter all incoming routes (bogon, 1918, and many others). We
>> don't want data resorting to 0/0 and ::/0 when we specifically rejected the
>> matching route at the import policy.
>> Additionally, if your upstream isn't announcing a route to you, where are
>> they going to send your traffic anyway?
>> Regards,
>> Chris Rogers
>> +1.302.357.3696 x2110
>> <>
>> On Tue, Nov 4, 2014 at 5:42 PM, Owen DeLong <owen at> wrote:
>>> It seems in such a case, the traffic still doesn’t know where to go, but
>>> you don’t realize it because you have a default.
>>> Then you pass the traffic to one of the providers who doesn’t have a route
>>> for it and they drop it instead of you.
>>> If you see something different, then, by definition, said provider is not
>>> feeding you a full set of their tables, or, they, too, are depending on a
>>> default and are not receiving a full set of tables.
>>> Owen
>>>> On Nov 4, 2014, at 10:25 AM, Mike Walter <mwalter at> wrote:
>>>> I have 5 providers and we get the default from all of them and full
>>> routing tables.
>>>> I have seen cases where if there is no default route, the traffic didn't
>>> know where to go, even with full routes from all my providers.
>>>> -Mike
>>>> -----Original Message-----
>>>> From: NANOG [mailto:nanog-bounces at] On Behalf Of Berry Mobley
>>>> Sent: Tuesday, November 04, 2014 12:47 PM
>>>> To: nanog at
>>>> Subject: Default routes on BGP routers with full feeds
>>>> I'm wondering how many of you who are multihomed also add default
>>>> routes pointing to your providers from whom you are receiving full feeds.
>>>> If so, why? If not, why not?
>>>> Thanks,
>>>> Berry

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