Hi,
I have a simple question but I'd like to double check the answer.
AFAIK, ICMP redirect messages are only sent when a local gateway receives traffic from a source connected to a directly connected LAN, if the source is sending traffic towards the gateway, but the gateways knows that on the same LAN there's a better gateway toward the destination.
To make my question clear, i've attached a visio drawing to this post.
I would like to have 3 CPE on same interconnexion network (here 1.1.1.0/24) with a hub and spoke topology (CPE X being the hub) rather than a direct routing configuration between CPE A and CPE B (CPE A and B would the spokes).
I know this is not bandwidth efficient, but there are other reasons behind this design.
CPE-A :
10.1.0.0/16 => eth1
1.1.1.1/24 => eth0,
0.0.0.0/0=> 1.1.1.8 (provider), eth0
CPE-B :
192.168.0.0/23 => eth1
1.1.1.2/24 => eth0,
0.0.0.0/0=> 1.1.1.8 (provider), eth0
CPE-X (provider) :
192.168.0.0/23 => 1.1.1.2
10.1.0.0/16 => 1.1.1.1
0.0.0.0/0=> backbone
I want to be sure that configuration will not lead router X to generate a lot of icmp redirect messages.
As far as I know, routerX shouldn't sent any ICMP redirect when receiving traffic from source 192.168.0.x towards 10.1.x.x (or vice versa), as router X isn't directly attached to any of these networks.
It should only redirect traffic coming from 1.1.1.x toward 172.16.1.x or 192.168.1.x. But as the only hosts on 1.1.1.0 are routers or firewalls and will generate little to no traffic with their interfaces as source addresses, this shouldn't be an issue.
Can you confirm my understanding of this point ?
sources :
RFC 792, page 13:
The gateway sends a redirect message to a host in the following situation. A gateway, G1, receives an internet datagram from a host on a network to which the gateway is attached. The gateway, G1, checks its routing table and obtains the address of the next gateway, G2, on the route to the datagram's internet destination network, X. If G2 and the host identified by the internet source address of the datagram are on the same network, a redirect message is sent to the host. The redirect message advises the host to send its traffic for network X directly to gateway G2 as this is a shorter path to the destination. The gateway forwards the original datagram's data to its internet destination.
For datagrams with the IP source route options and the gateway address in the destination address field, a redirect message is not sent even if there is a better route to the ultimate destination than the next address in the source route.
from Cisco :http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13714-43.html
When Are ICMP Redirects Sent?
Cisco routers send ICMP redirects when all of these conditions are met:
The interface on which the packet comes into the router is the same interface on which the packet gets routed out.
The subnet or network of the source IP address is on the same subnet or network of the next-hop IP address of the routed packet. //not the case here
The datagram is not source-routed.
The kernel is configured to send redirects. (By default, Cisco routers send ICMP redirects.
