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New Member

static route question

Hi folks,

I have what I am sure is a simple routing problem, but I cant see where Im going wrong in my logic.

I have a Data E1 link (between 2 x 26xx's).The link provides the only connection between two small office ethernet LANs.

LANa---RouterA-----E1------RouterB----LANb

Problem:

LANa devices cant ping LANb devices and vice versa.

However, Router A can ping Router B via the E1 serial link.And LAN devices can ping their local 26xx router ethernet interface. (its their default gateway, as there is no other egress point).

Each router can ping any device on its respective local LAN.

From router A I can ping router B, both its serial and ethernet interfaces. Yet no further than that.

If I traceroute the same path, I only get as far as Router B's serial int, it doesnt even get to Router B's ethernet int.

Config is like this:

Router A:

serial 0/0:0 10.0.0.1 255.255.255.242

fa 0/0 192.168.150.9 255.255.255.0

ip route 172.16.0.0 255.255.0.0 serial 0/0:0 permanent

Router B:

serial 0:0/0 10.0.0.2 255.255.255.242

fa 0/0 172.16.1.5 255.255.0.0

Both routers are configured as follows:

ip classless

ip subnet-zero

no VLANs are setup

no ACL's are setup

As Router B knows about 172.16.0.0/16 via its local fa 0/0, why is it not forwarding packets from the serial interface, out that fa int?

Thanks for any help,

Merry Christmas and Happy New Year ;-)

  • Other Network Infrastructure Subjects
5 REPLIES
Bronze

Re: static route question

Try the following

a) Change the IP route statement on router A to be

"ip route 172.16.0.0 255.255.0.0 10.0.0.2".

It is not a good idea to configure an interface as the next hop of a route especially if you already know what the next hop IP address is. Router A will arp for every IP address in the 172.16.0.0/16 range with the current config. Not a good idea.

b) Is there a "ip route 192.168.150.0 255.255.255.0 10.0.0.1" on router b ? Router B needs to know how to get back to router's A lan.

Happy new year to you too :)

New Member

Re: static route question

Thanks for the advice. I will give it a try.

Your comments trigger a question from me, about the arp behaviour you mention.

with the current router A will ARP for all target 172.16.0.0/16 addresses, with ARP propogating only as far as the broadcast domain it originates in

is it correct that becuase the router treats static routes as directly connected (?) it will ARP for any address in the 172 range.

I assume if I modify the config as you suggest (swap the int for an ip) then the arp process will be skipped as the router only issues an arp for locally connected devices.

Bronze

Re: static route question

Yes you are correct.

Having a static route pointing to the next hop IP address instead of multicaccess interface would stop R1 from doing proxy ARP for all destinations that match the route statement. The packets would simply be routed and switched based on the next hop ip address.

See if the following documents helps you understand this issue better.

Title: Troubleshooting Cisco Express Forwarding Routing Loops

URL: http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a00800cdf2e.shtml

New Member

Re: static route question

Hi,

I tried modifying the ip route statement to use the next hop ip but the problem is still there.

Rather than fixing complete end to end connectivity, I want to take it step by step, so Im concentrating on why Router A, cant ping live hosts in LAN B, via router B.

I noticed though that since modifying the statement, from router A, I can ping the ethernet interface of the router B. So i can reach the ethernet int of the far router, and maybe the live hosts on the distant LAN, but the responses just cant comeback?

The traceroute from router A to a live host on LANB only gets as far as router B.

Based on this logic, the next things I tried were:

- Router B.Set gateway of last resort pointing to the WAN link. This was done on router B, in an attempt to see responses. Still the same problem.

Is there anything I need to do to enable static routing, or will it work by default?

Any help you could offer is appreicated,

thanks,

Silver

Re: static route question

I beleive the default gateways on the hosts on router b are set incorrectly .

from router B do an extended ping to a host on the ehternet segment of router b and source your ping from the serial interface , this will force the host to forward the packet to its default gateway. when pinging and not sourcing your ping you are just pinging from the same segment.

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