Re: "Neighbor maximum-prefix" option on routers

[Jeff Aitken <jaitken () aitken com>]

On Mon, Nov 20, 2006 at 09:53:06AM +0100, Alexander Koch wrote:
> Ain't that a bit over- engineered? If you have a prefix list
> (presumably to a customer) do you want to seriously shutdown
> the session when he sends you random prefixes because it is
> easy to break when you are new to it? Would create unneeded
> tickets I'd say. Prefix lists (exact match) with no max-prefix
> (as those are then rejected anyway) work fine here...


There are several direct applications in provider networks that Joe 
alluded to, such as customer-triggered blackholes.  However, if you're
going to let your customers deaggregate, you need some other way of 
protecting your network from stupidity/maliciousness on your
customers' part.  You might do something like this, in cisco-speak:


    router bgp xxx
     neighbor 1.2.3.4 remote-as yyy
     neighbor 1.2.3.4 description my customer with ASN yyy
     neighbor 1.2.3.4 prefix-list FROM-ASyyy in
     neighbor 1.2.3.4 route-map from-cust in
     neighbor 1.2.3.4 maximum-prefix 100
     [...]
    !
    ip prefix-list FROM-ASyyy sequence 5 permit 10.0.0.0/8 le 32
    ip prefix-list FROM-ASyyy sequence 10 permit 172.16.0.0/12 le 32
    !
    route-map from-cust permit 10
     match community customer-blackhole
     set ip next-hop <null0, or whatever>
     set community no-export
    !
    route-map from-cust permit 20
     match ip address prefix-list longer-than-/24
     set community no-export
    !
    route-map from-cust permit 30
     set community my-customer-route-community
    !


Now, in addition to being able to announce his two aggregates, your
customer can announce more-specific routes (all the way up to /32)
to be blackholed or just for traffic-engineering purposes within your
network.  However, you're still protected if he announces more than
his configured maximum number of routes (100 in this example).

To answer your question, yes I absolutely want to shut down a peering
session with a misbehaving ASN *regardless* of whether or not that ASN
is a peer or customer.  The stability of my network (and therefore, my
ability to provide service to my other customers) depends on it.


--Jeff