[AusNOG] Best practice BGP and wan links

[Mark Smith]

On 12 July 2015 at 19:09, Alex Samad - Yieldbroker
<Alex.Samad at yieldbroker.com> wrote:
> Hi
>
> Yes more info.  Multiple connections to multiple ISP's.  Currently they are terminated into switches

Presumably two separate switches so that a single switch isn't the
single point of failure?

> and then L3 terminated into RouterOS VM's.  I am planning on replacing the VM's with  some MT CCR's. My thought had been to leave the termination into the switches and then  L3 terminate onto the phy MT boxes.  As I can't HSRP / stack the routers my only option was VRRP. But BGP VRRP didn't seem like a good thing,

So originally it seemed that you were thinking of doing VRRP with a
single upstream provider, which would have meant that your single BGP
session would only be active on one of your VRRP routers, and then if
VRRP switched to the other router for some reason, the BGP session
would go down and then come back up. That would have added BGP
neighbor discovery and BGP session initialisation time to your fail
over period, which could be a significant time increase.

VRRP to two different providers would be both unusual and possibly
quite confusing to both of them. One of them would have a BGP neighbor
address that doesn't fall within the IP subnet/prefix that they've
assigned to their end of the link, which would be different from most
if not all of their other customers. The other confusing thing would
be that the BGP session for the non-active provider would be down
while you're not using that provider for traffic. That would be
confusing for them, because providers will consider a BGP session that
is up to mean the customer intends to use the link even if there is no
traffic flowing over it i.e., a link over which a BGP session is up
with no traffic is clearly a backup link. However, if the BGP session
is down and stays down for long periods, it would be considered a sign
that something is broken rather than an intentional but unusual setup
like using VRRP with a single BGP session to two different providers.

Another issue would have been if your segment became partitioned such
that both of your VRRP routers became active, meaning that you had two
active BGP sessions with the same IP address at your end to both of
your providers. While odd, at first impression I can't see how that
would cause a problem, but to be sure of no issues, you'd have to very
thoroughly work through all the possible failure modes of this
scenario. I think there is a high likelihood it would cause problems
if this happened and you were using the same upstream upstream
provider for the two links.

Regards,
Mark.



> better to get the extra IP and have  2 links.
>
> Interestingly I have BFD running on some of those links and reduced timers on the BGP session for the other links as some ISP didn't/wouldn't run BFD..
>
>
> Thanks
> Alex
>
> -----Original Message-----
> From: Mark Smith [mailto:markzzzsmith at gmail.com]
> Sent: Sunday, 12 July 2015 5:54 PM
> To: Alex Samad - Yieldbroker
> Cc: Benoit Page-Guitard; ausnog at lists.ausnog.net
> Subject: Re: [AusNOG] Best practice BGP and wan links
>
> On 12 July 2015 at 15:14, Alex Samad - Yieldbroker <Alex.Samad at yieldbroker.com> wrote:
>> Yeah that was sort of my thought, I guess I have to start the process of asking for the extra IP..
>>
>
> More details of your scenario would be better.
>
> VRRP being an option means that you only have a single link to your upstream. Since in general links fail more often than devices, the redundancy value of having two routers at your end and two BGP sessions over a single link to a single upstream router is a bit questionable, because you haven't eliminated all single points of failure. You have partial but not complete redundancy, and you need to consider whether not having complete redundancy is acceptable to either or both you or your network's users.
>
>
>
>> A
>>
>> -----Original Message-----
>> From: Benoit Page-Guitard [mailto:benoit at anchor.net.au]
>> Sent: Saturday, 11 July 2015 11:13 PM
>> To: Alex Samad - Yieldbroker
>> Cc: ausnog at lists.ausnog.net
>> Subject: Re: [AusNOG] Best practice BGP and wan links
>>
>> Hi Alex,
>>
>> I assume the use case here is having redundant routers at the branch end and using VRRP on the WAN link as a signalling mechanism for deciding which router should "own" the WAN IP + speak BGP with the upstream router?
>>
>> If so, I'd definitely opt for an extra WAN IP if you can swing it. It'll make the whole failover scenario a lot smoother, and would also have the indirect benefit of giving you free load balancing for your downstream-facing LAN interfaces.
>>
>> Regards,
>> Benoit
>>
>> On Sat Jul 11, 2015 at 08:03:10 +0000, Alex Samad - Yieldbroker wrote:
>>>
>>>What I was looking at doing was setting up bgp over vrrp on some mikrotik boxes, seems like it's possible, but it also seem easier to get an extra WAN ip.
>>>
>>>Any one doing this ?
>>>
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