[B.A.T.M.A.N.] dublicate HNAs

Freifunk freifunk at ddmesh.de
Thu Dec 11 14:01:33 UTC 2008

Hi Axel,

thanks for your description. Would it be better to only ignore the HNA that
is already received from another node instead of ignoring the node completely?
I currently use one node that is connected to icvpn (inter-city vpn) that connects
different freifunk cities together. The network runs bgp and allows to setup
a second server that also initiates a connection to the icvpn. both server
would then announce the same ip addresses.
In this case the batman notes only should ignore the HNA if they are injected
by different icvpn servers and keep the server still in network.


Axel Neumann schrieb:
> Hi,
> On Dienstag 09 Dezember 2008, Stephan Enderlein (Freifunk Dresden) wrote:
>> -----------
>> Another question: in previous versions I have seen that if two batmand
>> announce the same HNA ip (-a) one of the batmand are ignored and complete
>> ignored any batmand traffic. As result the node was removed from any
>> batmand list and was not reachable anymore.
>> -----------
> And it should be also the current behavior of bmx. The background is:
> BMX currently does NOT support anycast routing. 
> Therefore in practical terms, if two nodes are announcing the same IP it is 
> effectively an IP doubler. Such a scenario triggers the dublicate-address 
> detection which results in ignoring the younger of the two nodes announcing 
> this IP.
> With the batman routing algorithm it is indeed difficult to do a consistent 
> anycast routing. Therefore I've decided to protect against casual duplicate 
> address announcements with the described behavior.
> Older versions (2007 and before) tended to have chaotic routing entries due to 
> duplicated entries or removals of HNA routing entries.
> When dynamically adding an HNA (using e.g. bmxd -ca the daemon 
> checks if other nodes are already announcing this specific HNA. If this is 
> the case the announcement is rejected and a warning should be given in 
> debuglevel 3. You can always inspect current announcements from other nodes 
> using debug-level 9.
> This debug-level also differentiates between announced interfaces (e.g. 
> and networks (e.g. ( the idea for the future was,  
> that network announcements should become anycast announcements )
> Indeed it is a problem when a daemon is started from the beginning with a 
> duplicate HNA announcement. In this situation the daemon can hardly check if 
> the network is already announced. Then, only his neighboring nodes are aware 
> of the IP doubler. They will ignore everything from this node and cause a 
> warning in debug level 0. 
> hope this clarifies a bit.
> /axel
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