Should routers send redirects by default?

Ricky Beam jfbeam at
Mon Aug 23 21:39:47 UTC 2010

On Sat, 21 Aug 2010 20:42:01 -0400, Mark Smith  
<nanog at> wrote:
> In IPv6, redirects serve two purposes, where as in IPv4 they only
> served one -

IPv4 redirects serve exactly the same two situations... both are  
situations where a router would be required to hairpin a packet -- either  
the destination is on-wire or the path to it is elsewhere on-wire.

Let's say host 1.100 has a default gw of 1.1 and no other routes.  A  
packet destined for 2.1 would go to 1.1, even if 2.1 is on the same wire.   
Following old rules, 1.1 sends a redirect and drops the packet.  Most  
hosts would ignore that redirect as it "makes no sense" (redirects are not  
a substitute for device routes.  I've seen this happen too many times.)

Now, if 2.1 was behind 1.2, the redirect would tell 1.100 to go there  

(This isn't as much of a problem these days since routers don't to the  
"drop packet" part -- which used to be mandated by RFC.  And "secondary"  
networks have given way to inter-VLAN routing.)


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