Re: COM21 is killing me with ARP

[Bryan Andersen <bryan@visi.com>]

Patrick Colbeck wrote:
> 
> It really doesnt matter that there is a whole class B address space as
> you should only get arped when someone om the same class B needs to
> know your mac address. Once the arping device has your mac address it
> should cache it so it doesn't have to arp for it again for a long
> time. All the other people on the calls B shouldn't be trying to find
> your MAC address as they theoretically should only be talking to your
> service providers DSLAM.

Actually it does matter.  When Joe user turns off their box it 
nolonger can answer requests for it's ethernet adderess.  This
means a bunch of requests for it's arp address.  So when 
someone scans the network you get bombarded by arp requests, 
and the caches naturally gets trashed durring this.  If you 
know a provider does this you can realy hose up their network 
by bombarding them with random addresses in their network space.  
To keep from having this trash a network the router really 
needs to have enough cache entries to store all hosts on the 
network.  Many routers just can't handle that for a class B
network.  They really should break their network up into 
class Cs.

> It sounds like somebody has screwed up at the service provider
> configuring their routers they have probably:-
> 
> i) Configured a really small arp cache timeout value so the service
>    provider router is permanatly having to re arp for the mac
>    addresses of all the DSL modems or
> 
> ii) Configured a static route via a broadcast interface (eg etherent)
> on the cental router. This is a really bad thing as instead of just
> arping for the next hop address the router will arp every time it
> needs to send a packet to any address on the network the route is for
> to try and determine the gateway to that address. This is a really
> good way to crucify network performance , static routes pointing at
> interfaces rather than next hop addresses should only be used on point
> t point networks (leased line etc).

-- 
|  Bryan Andersen   |   bryan@visi.com   |   http://www.nerdvest.com   |
| Buzzwords are like annoying little flies that deserve to be swatted. |
|   -Bryan Andersen                                                    |