Redundancy & Summarization

Adam Greene maillist at
Sat Aug 22 08:52:13 CDT 2009

Another option could be to announce one /17 to each upstream provider 
and use conditional BGP to announce the other /17 to the provider that's 
still active in the event that one provider goes down.

On 8/21/2009 4:08 PM, Patrick W. Gilmore wrote:
> On Aug 21, 2009, at 3:47 PM, Brian Dickson wrote:
>>> My institution has a single /16 spread across 2 sites: the lower /17 is
>>> used at site A, the upper /17 at site B.  Sites A & B are connected
>>> internally.  Currently both sites have their own ISPs and only 
>>> advertise
>>> their own /17's.  For redundancy we proposed that each site advertise
>>> both their own /17 and the whole /16, so that an ISP failure at either
>>> site would trigger traffic from both /17s to reconverge towards the
>>> unaffected location.
>> There are two different ways to achieve almost-identical results.
> As much as I like Brian, I am going to have to respectfully disagree.
>> However, one is a 50%  more "green" than the other, i.e. friendly to 
>> other network operators.
>> These two choices are functionally equivalent, and possible, only 
>> because things currently work for both your /17's.
>> Here are the two ways to do this:
>> One is:
>> - announce /17 "A" and /16 from uplink ISP-A
>> - announce /17 "B" and /16 from uplink ISP-B
>> - This results in 3 prefixes globally: A, B, and /16.
>> The other is:
>> - announce /17 "A" and /17 "B", with different policies (i.e. prepend 
>> your AS once or twice), at *both* ISPs.
>> - This results in 2 prefixes globally: A and B.
>> In all cases, as long as one ISP link is up, there is a path to both 
>> A and B.
>> In most cases, the best path to A or B, is *mostly*, but not 
>> completely, under your influence.
> This is highly dependent on variables not in evidence.  If your 
> upstreams are, for instance, Sprint & Level 3, then a large percentage 
> of the Internet will be traveling through one or the other.  And once 
> it hits your upstream, prepends are irrelevant.  Every upstream (for 
> values of "every" == "100%" to at least one decimal place) localprefs 
> their downstreams' prefixes.
> In this case, anyone downstream of either L3 or Sprint will send _all_ 
> traffic through that upstream's link.  While not the whole Internet, 
> it's still quite a bit.  Moreover, many people do things like 
> localpref Sprint _down_ because they are more expensive.  So even 
> someone multi-homed to both may send all traffic through L3.  Etc., etc.
> A slight twist on Brian's idea would be to use communities and tell 
> Upstream A to localpref Prefix B below that of peer routes.  Then you 
> only need two prefixes, and each site only receives its own traffic 
> except when the other site fails.  If Upstream B goes down, Upstream A 
> will accept Prefix B and propagate it.
> Again, dependent upon your upstreams having communities able to do 
> this.  Or if they are "nimble", maybe a call to their operations 
> department?

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