How does ICMP redirect consult the routing table on the router?

[ayaz.akhtar]

We have three routers, R1, R2, and R3, connected to the same Ethernet segment as host H1. H1 is configured to use R3 as its default gateway and is also learning the following route:

10.0.0.0 via R1

We are trying to connect to host 10.4.245.1 via R2 from H1. If we manually delete the 10.0.0.0 route from H1, H1 will connect to 10.4.245.1, but only for a short time (a minute or two) before it learns the 10.0.0.0 route again.

Even if we enter an explicit route on H1 to 10.4.245.0 via R2, it seems that H1 still uses the 10.0.0.0 route it has learnt.

It would appear that H1 is convinced that the whole of network 10 is reached via R1 and does not send traffic destined for 10.4.245.0 to R2 at all.

Checking on R3 it would appear that an ICMP redirect is being generated for the 10.0.0.0 subnet, but what we find puzzling is that R3 has the following static routes:

10.0.0.0 255.0.0.0 via R1

10.4.245.0 255.255.255.0 via R2

So, theoretically, the ICMP redirect message to H1 would be to inform H1 that the best route to reach 10.4.245.1 is via R2 and not R1.

But, it would seem that H1 just learns the one general 10.0.0.0 route via R1, rather than the following two specific routes:

10.0.0.0 via R1

10.2.245.0 via R2

What could be causing this…???

And, more importantly, what can we do to resolve this problem?

Thanks in advance for your help.

[rjackson]

I cant explain whats going on but as a workaround try changing the admin distance on the 10 route.

ip route 10.0.0.0 255.0.0.0 5

[jamey]

What kind of OS is the host running? What is the ethernet segment IP range? Just taking a stab in the dark here, but it could be something with the host OS. It almost seems the host OS doesn't like classless routing. Maybe check out the docs on classless routing for the host. I know some older versions of Unix had classless issues. Also maybe add the global config command "ip classless" on the ciscos. Maybe also disable ICMP redirects with the no ip redirects interface command.

My $.02

[ayaz.akhtar]

This is a VAX host.

It appears that the VAX learns the 10.0.0.0 route via ICMP redirects, but despite having a more specific route to 10.4.245.1, prefers to use the 10.0.0.0 route.

I think your thoughts about this being a classless routing issue on the host OS may be right.

Please let me know if you have any further suggestions.

Thanks.

[jamey · Accepted solution]

I don't know much about VAX but here is a URL I found about Digital TCP/IP services for VAX v5.0 and it mentions ip classless. Maybe check the version of TCP/IP services you're running?

http://216.239.39.104/search?q=cache:CObcWtOQdRQJ:acornsw.com:8080/ovmsdoc072/72final/6529/6529profile.html+vax+ip+classless&hl=en&ie=UTF-8

Maybe also, disable ICMP redirects on the routers so the VAX will never learn about the 10.0.0.0 route.

-HTH

[ayaz.akhtar]

Resolved. Thanks for your help.

Yes, it turned out that this was related to the way the VAX was doing classful routing based on the routes it was learning via the ICMP redirects.

The simplest option was to disable the ICMP redirects, pending a VAX upgrade.

Once again, thanks to all who participated in this conversation.

[Kwang Jin Ho]

What does R2 routing table look like?

[ayaz.akhtar]

On R2 the route for the 10.4.245.0 subnet is learnt through EIGRP from the service providers MPLS edge router.

For all other subnets on the 10 network that are not found off the MPLS network there is a static route on R2 for 0.0.0.0 0.0.0.0 via R1.

Equally, R1 learns 10.0.0.0 subnet routes, which have not been migarted to the MPLS backbone, through EIGRP from the BT Frame-Rely edge router.

[Kwang Jin Ho]

This sounds odd...

You're positive R1,R2,R3, and H1 are all on the same vlan and subnet? What are the specific IPs of all R# and H1 on that subnet.

Can you provide output of 'show ip route 10.4.245.0' on R3

You mentioned H1 learning the 10.0.0.0 route? Is it running a routing protocol? If so which protocol and to which router?

Do you have HSRP running on any of the routers?

If so which ones?