Should routers send redirects by default?

Ricky Beam jfbeam at gmail.com
Mon Aug 23 16:39:47 CDT 2010


On Sat, 21 Aug 2010 20:42:01 -0400, Mark Smith  
<nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org> 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  
instead.

(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.)

--Ricky




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