Should routers send redirects by default?
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
(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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