Should routers send redirects by default?

Mark Smith nanog at
Tue Aug 24 22:11:32 UTC 2010

On Tue, 24 Aug 2010 13:25:01 -0700
"David W. Hankins" <David_Hankins at> wrote:

> On Sun, Aug 22, 2010 at 10:12:01AM +0930, Mark Smith wrote:
> > o  allow an IPv6 router to indicate to an end-node that the destination
> > it is attempting to send to is onlink. This situation occurs when the
> > router is more informed than the origin end-node about what prefixes
> > are onlink.
> > 
> > This shouldn't happen very often either, as multiple onlink IPv6 routers
> > should be announcing the same Prefix Information Options in their RAs,
> > and therefore end-nodes should be fully informed as to all the onlink
> > prefixes. ICMPv6 redirects in this scenario would only occur during the
> > introduction of that new prefix information i.e. the time gap between
> > when the first and second onlink routers are configured with new prefix
> > information.
> It may be true the situations where redirects are required for this
> are few in number, but I think it is not true that the use of
> redirects is limited solely to the configuration gap between
> introducing a new prefix.
> In NBMA networks, it is said that the nodes will have IPv6 addresses
> with no covering advertised prefixes ("IPv6 Core Protocols
> Implementation", page 393, just spotted while reading today).
> Additionally, the typical use of /128 "role addresses" for services
> aliased onto lo0 mean the router has a /128 route for the role address
> to an on-link device, but a covering prefix advertisement would be
> both futile and inappropriate.
> I don't necessarily want to say that your conclusion is false, but
> rather that it seems to me there are more enumerations in the set.

Before coming to any conclusions, we'd need to more strictly define
what the NBMA topology is. Is it fully transitive i.e. all nodes can
see all other nodes, and that the only property that makes it different
from a conventional LAN is the absence of a broadcast/multicast
capability? Or is there only partial visibility, such that end-nodes
only have permanent visibility to a router i.e hub-and-spoke? In the
latter case, another property is whether or not direct communcations
paths can be set up between the spoke nodes on demand, such as in the
case of Dynamic Multi-point VPNs.

Whether ICMPv6 redirects are necessary or useful, or whether other
somewhat similar mechanisms, such as Next Hop Resolution Protocol, are
used in an NBMA subnet very much depends on the sort of connectivity
it provides or can provide between the nodes on the subnet.


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