IPv4 Router Alert Option

Sean Donelan sean at donelan.com
Fri May 23 16:59:01 CDT 2008

On Fri, 23 May 2008, Ron Bonica wrote:
> It is my belief that many ISPs, will not accept datagrams containing the
> Router Alert IP option from customers. Do I have that right?
> I am asking so that I might better evaluate Internet drafts that would
> require ISPs to accept such packets.

Depends on what you mean by the word "accept."

Transit backbone operators have been changing to the position of 
protecting their router CPU's from user packets being punted up the 
control plane.

If they can forward the packet without going up the control plane, I think
most transit backbones will "accept" the packet and ignore IP options like
Router Alert.

If someone writes a standard to require ISPs to do something besides 
ignore an IP option and forward the packet, then you may see ISPs drop 
packets instead of punting them to the control plane.  For example, 
packets with IP Source Route options.

Router# conf t
Router(config)# ip options ignore
Router(config)# exit
Router# write mem

As Chris mentions, packets with IP options are likely to have more 
problems crossing firewalls/security devices or even simple 

I don't remember who, but someone once suggested if we could go back
in time to the late 1970's and redo the Internet Protocol we would
get rid of all IP options and made IP addresses 64 bits and classless
from the beginning.

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