Black holes caused by aggregation

Thu Oct 26 11:32:24 UTC 1995

My appologies if you have already seen this.  It appears to be an
operational/configuration issue that our clients might be interested
in..... :)  Perhaps its time to upgrade all that mbone/mrouted software
out there.  

> From: Bill Fenner <fenner at parc.xerox.com>
> Subject: Black holes caused by aggregation
> Date: Wed, 25 Oct 1995 18:52:46 PDT
> 
> Longest-match routing was introduced in mrouted version 3.5 .  Previous
> versions would only accept the least specific route that it heard, and
> if it heard routes that overlapped it would log a message and drop the
> new route.
> 
> People now appear to be using Cisco's route aggregation feature in
> places that it shouldn't be used, which will cause problems when
> forwarding through mrouted versions previous to 3.5 (which is about 75%
> of the MBONE).  A nice example of the breakage is net UIUC-CAMPUS-B,
> 128.174.  (Not to pick on UIUC in particular, but it's the easiest
> example for me.)  The three routes that I have in my routing tables for
> 128.174 are:
> 
> 128.174.212.0/25
> 128.174.240/24
> 128.174/16
> 
> Note that the last route covers the first two, and mrouted versions previous
> to 3.5 will only accept the last route.  In fact, if I traceroute towards
> 128.174.0.0, I have a nice short route:
> 
> Mtrace from 128.174.0.0 to 204.162.228.2 via group 224.2.0.1
> Querying full reverse path... 
>   0  dartvader (204.162.228.2)
>  -1  parc.dart.net (204.162.228.1)  DVMRP  thresh^ 1  200 s   
>  -2  ames.dart.net (140.173.144.1)  DVMRP  thresh^ 1  200 s   
>  -3  mbone2.nsi.nasa.gov (192.203.230.242)  DVMRP  thresh^ 64  200 s   
>  -4  mbone.nsi.nasa.gov (192.203.230.241)  DVMRP  thresh^ 1  200 s   
>  -5  oday.psc.edu (192.88.114.10)  DVMRP  thresh^ 64  176 s   
>  -6  mbone.merit.edu (198.108.2.20)  DVMRP  thresh^ 64  201 s   
>  -7  tibia.cic.net (192.217.65.100)  DVMRP  thresh^ 32  254 s   Next router no mtrace
>  -8  dcl2.gw.uiuc.edu (192.17.2.8) [proteon/mrouted 1.0] doesn't support mtrace
> 
> However, if I trace towards one of the specific routes, I end up taking
> the shortest mrouted-3.5-or-greater path:
> 
> Mtrace from 128.174.240.0 to 204.162.228.2 via group 224.2.0.1
> Querying full reverse path... 
>   0  dartvader (204.162.228.2)
>  -1  parc.dart.net (204.162.228.1)  DVMRP  thresh^ 1  200 s   
>  -2  ames.dart.net (140.173.144.1)  DVMRP  thresh^ 1  200 s   
>  -3  mbone2.nsi.nasa.gov (192.203.230.242)  DVMRP  thresh^ 64  200 s   
>  -4  mbone.nsi.nasa.gov (192.203.230.241)  DVMRP  thresh^ 1  200 s   
>  -5  mbone.sdsc.edu (198.17.47.39)  DVMRP  thresh^ 64  200 s   
>  -6  VINEGAR.SESQUI.NET (128.241.0.91)  DVMRP  thresh^ 64  201 s   
>  -7  mae-bone.psi.net (192.41.177.247)  DVMRP  thresh^ 64  128 s   
>  -8  MBONE1.UU.NET (137.39.43.34)  DVMRP  thresh^ 64  201 s   
>  -9  MBONE2.UU.NET (137.39.246.98)  DVMRP  thresh^ 64  201 s   
> -10  dec3800-2-fddi-0.SanFrancisco.mci.net (204.70.158.61)  DVMRP  thresh^ 64  2828 ms  
> -11  dec3800-2-fddi-0.Denver.mci.net (204.70.152.61)  DVMRP  thresh^ 1  41 s   
> -12  dec3800-1-fddi-0.WillowSprings.mci.net (204.70.104.29)  DVMRP  thresh^ 1  -24 s   
> -13  tibia.cic.net (192.217.65.100)  DVMRP  thresh^ 32  254 s   Next router no mtrace
> -14  dcl2.gw.uiuc.edu (192.17.2.8) [proteon/mrouted 1.0] doesn't support mtrace
> Round trip time 295 ms
> 
> Now, you might say "Well, that just means that traffic is taking a suboptimal
> path, no big deal."  But, because of the way DVMRP builds its forwarding
> trees, traffic that matches the specific routes (e.g. 128.174.240.1) will
> *not* get forwarded from a 3.5-and-up to a 3.4-and-down mrouter.  Here is
> a (slightly altered, since there is no active source in 128.184.240.x right
> now) mtrace showing what you would see on that boundary:
> 
> Mtrace from 128.174.240.0 to 141.210.188.1 via group 224.2.0.1
> Querying full reverse path... 
>   0  ? (141.210.188.1)
>  -1  ? (141.210.188.3)  DVMRP  thresh^ 1  208 s   
>  -2  mbone.wayne.edu (141.217.1.74)  DVMRP  thresh^ 32  -250 s   Not forwarding
>  -3  mbone.merit.edu (198.108.2.20)  DVMRP  thresh^ 32  201 s   
>  -4  tibia.cic.net (192.217.65.100)  DVMRP  thresh^ 32  254 s   Next router no mtrace
>  -5  dcl2.gw.uiuc.edu (192.17.2.8) [proteon/mrouted 1.0] doesn't support mtrace
> 
> Note the "Not forwarding" error code on the mbone.wayne.edu line.
> 141.210.188.3 is 3.4, mbone.wayne.edu is 3.5 .  (Again, not to pick
> on wayne.edu; this was the easiest test case that I could come up with.)
> 
> This means that if you are one of the people advertising this kind of
> specific and general routes, that you are isolating your traffic from
> the specifically routed subnets to the connected cloud of 3.5-and-above
> routers.  Of course, the long term solution is to get the whole MBONE
> able to accept this kind of routing; the short term solution, however,
> is not to advertise this kind of route.  Therefore, I will be periodically
> posting a list of routes that fall into this category; hopefully, the
> people responsible for those networks will reconfigure their routers to
> either advertise only a general route or only non-overlapping specific
> routes.
> 
> The list follows.  I will be posting this list (with a shorter explanation)
> periodically.
> 
>   Bill
> 
> 36.18/16 is covered by 36/8
> 36.53/16 is covered by 36/8
> 36.56/16 is covered by 36/8
> 36.103/16 is covered by 36/8
> 36.153/16 is covered by 36/8
> 36.224/16 is covered by 36/8
> 128.32.211/24 is covered by 128.32/16
> 128.32.226/24 is covered by 128.32/16
> 128.171.3/24 is covered by 128.171/16
> 128.171.10/24 is covered by 128.171/16
> 128.171.11/24 is covered by 128.171/16
> 128.171.45/24 is covered by 128.171/16
> 128.174.212.0/25 is covered by 128.174/16
> 128.174.240/24 is covered by 128.174/16
> 132.250.88.170/32 is covered by 132.250.88.160/28
> 137.194.2/23 is covered by 137.194/16
> 137.194.34/23 is covered by 137.194/16
> 137.194.98/23 is covered by 137.194/16
> 137.194.160/23 is covered by 137.194/16
> 137.194.224/23 is covered by 137.194/16
> 137.194.230/23 is covered by 137.194/16
> 137.194.232/23 is covered by 137.194/16
> 

-- 
--bill