SBC Lost of connectivty to Canada?

Richard A Steenbergen ras at
Tue Jul 25 05:33:05 UTC 2006

On Mon, Jul 24, 2006 at 09:43:19PM -0400, Valdis.Kletnieks at wrote:
> On Mon, 24 Jul 2006 13:18:42 EDT, Richard A Steenbergen said:
> > Actually they just leaked some routes this morning, and blew out max 
> > prefixes to most peers. Pretty typical as far as accidental leaks go, 
> > those folks with sensible filtering and running routers which trip on max 
> > prefixes accepted not prefixes received probably weren't even affected. 
> > Another day on the internet.
> If I didn't know better, I'd say you were implying that your best bet for
> a survivable design is to plan as if your peers listened to Randy Bush on
> network design?

Not sure what you're referencing there. If Randy said something to the 
effect of "all peers will screw up and leak something to you eventually, 
no matter how large or generally well run, so plan accordingly", I would 
agree with it though.

I was suggesting that if even if you don't have the ability to fully 
prefix or as-path filter every peer (which, face it, most of us with large 
numbers of interesting peers don't have), you can still filter the really 
obvious stuff and prevent a large amount of the impact from common fat 
finger events. I can't tell you the number of problems that are prevented 
by applying a simple set of as-path filters matching large networks you 
know you should never hear from your peers (or most of your peers). 
Something simple like:

    deny _209_
    deny _701_
    deny _1239_
    deny _1299_
    deny _1668_
    deny _3320_
    deny _3356_
    deny _3549_
    deny _3561_
    deny _5511_
    deny _6453_
    deny _7018_

Catches a huge amount of "stupid stuff", especially when the event isn't a 
full blown leak which trips max prefixes, but is an isolated set of 
prefixes leaked by someone not directly connected to you. On a Cisco, the 
leaked 7018 routes from 7132 this morning would have been gone splash 
harmlessly with such a filter, and sessions wouldn't even trip max 
prefixes and bounce.

Unfortunately Juniper has a bit of a backwards take on the use of prefix 
limits. They believe that the reason to have a prefix limit is to protect 
a router against memory exhaustion (for example, someone sending you a few 
million routes that you are rejecting filling up your adj rib in), rather 
than as a policy tool (shutting down a wildly broken session that is 
sending you stuff it shouldn't). Juniper applies the max prefixs to routes 
received from a session rather than routes accepted, so even if the routes 
are filtered the prefix limit will still trip and shut down the session. 
This is particularly bad if for example you have a customer session which 
is fully prefix list filtered, and the customer accidentally leaks you a 

In reality, both types of limits are probabaly a "good idea". I've talked 
to a lot of people from a lot of companies who say they have requested 
Juniper add a seperate accepted-prefixes limit (or more likely, convert 
the existing limits to accepted-prefixes, and add a new received-prefixes 
knob), but so far it hasn't been implemented. If you are a Juniper 
customer who is reading this and you think having prefix limits which only 
count accepted prefixes is a good idea, please use this as an opportunity 
to submit an enhancement request so we can get this behavior improved. 
Feel free to throw in a request for "outbound prefix limits" as a last 
ditch safety net too. :)

Richard A Steenbergen <ras at>
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)

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