[c-nsp] multipath BGP not balancing equally.

[Dean Smith]

Exactly whats happening. On a couple of occasions when only 1 IP address at
the far end is active for downloads we see the traffic on just one of our
links because its all 1 IP to 1 IP (which was the point I was going to
make...and then forgot!) instead of all 3 links. In this case its 1 BGP
peering (eBGP multihop) that has 3 equal cost paths between but the
principle is the same.

(we cant go per packet CEF load balancing because the far end doesn't
support it - and the major traffic flow is inbound to us)

Dean

-----Original Message-----
From: Rodney Dunn [mailto:rodunn at cisco.com] 
Sent: 05 August 2009 21:23
To: Dean Smith
Cc: Mikael Abrahamsson; Cisco
Subject: Re: [c-nsp] multipath BGP not balancing equally.

Ah...good one. If the sources were not random enough and it's NAT'ed to 
one external ip you could really be multiplexing flows with NAT. ;)



Dean Smith wrote:
> Would agree that volume is rare between 2xIP addresses but we have 
> something similair although on not quite the scale.
> 
> We NAT a very large organisation to the Internet. They have a large 
> number of disparate sites that all do their own AV updates. All the PCs 
> download at the same time in the evening and we generate about .75 Gb/s 
> of traffic between our external PAT address and the AV download site for 
> a good couple of hours. If we had a bigger internet pipe it would be a 
> higher figure. (for less time of course).
> 
> Dean
> ----- Original Message ----- From: "Rodney Dunn" <rodunn at cisco.com>
> To: "Mikael Abrahamsson" <swmike at swm.pp.se>
> Cc: "Cisco" <cisco-nsp at puck.nether.net>
> Sent: Wednesday, August 05, 2009 2:19 PM
> Subject: Re: [c-nsp] multipath BGP not balancing equally.
> 
> 
>> For small flow combinations you are right. btw, it would be just L3 
>> src/dst flows by default unless the L4 port option is enabled.
>>
>> I thought about there being a single flow causing the difference that 
>> would be hashing down one of the paths. But 2G, while not impossible, 
>> typically isn't used between two ip addresses. It's something to check 
>> though for sure.
>>
>> Rodney
>>
>>
>>
>> Mikael Abrahamsson wrote:
>>> On Tue, 4 Aug 2009, Rodney Dunn wrote:
>>>
>>>> That's usually caused by routes not being the same on the paths.
>>>
>>> It was my understanding that this usually was caused by not having 
>>> enough L4 flows to loadshare on...? Ie if you have 100 TCP flows and 
>>> 4 paths, then it's not enough flows to get good load share on, but if 
>>> you instead have 10k flows and all of them are low-speed, then the 
>>> odds of them being equally load shared is much better?
>>>
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