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New Member

Strange routing issue - only one IP affected

I've encountered a routing problem to a single IP address within a global IP range.

Two traceroutes to a pair of addresses in the same subnet range are attached; the first traceroute, to 150.176.6.139, works, while the second traceroute, to 150.176.6.134, goes into a loop one hop out from the firewall of the destination network.

If these addresses are part of the same subnet 150.176.6.128/29, how can one work and the other one fail?

My ip route for this subnet is:

'ip route 150.176.6.128 255.255.255.240 Vlanxx'

Thanks,

Marc

10 REPLIES

Re: Strange routing issue - only one IP affected

this is interesting trace route, your static route is good too. If you are in firn2-doe-turlington-ce.firn2.net [150.176.4.138] router can you get to host 134? I would look into this router configuration.. but this is weird ! it seems as though 150.176.4.138 does not know about 150.176.6.134/29

Jorge

New Member

Re: Strange routing issue - only one IP affected

That's why I posted this - it goes over my head; I'm trained in PIX/ASA and LAN protocols - I'm not a CCNP yet ;-P

I admin the 150.176.4.138 hop - the 150.176.8.240 router is a connected VPN router that points somewhere else completely; why the trace would go there at all is a mystery, much less for a single address but no others...

Even from the 150.176.4.138 device (an L3 3750 with VLANs configured), traceroutes fail to that IP.

I already ruled out the target server as a cause by replacing it with a laptop and making a static NAT in the firewall to the global IP.

I've scheduled a reboot for both of those devices for tonight, to rule out 'wierd IOS error'. Other than that, I have no idea what this might be.

Marc

Re: Strange routing issue - only one IP affected

It looks to me like firn2-doe-turlington-ce.firn2.net is routing differently for 150.176.6.139 than for 150.176.6.134. THey may be in the same subnet as far as you are concerned, but firn2-doe-turlington-ce.firn2.net may think differently. It could, for example have a host route as a result of ip mobility, or a frame-relay p2mp, or a PPP link, or something like that.

You really need show ip route on firn2-doe-turlington-ce.firn2.net

Kevin Dorrell

Luxembourg

Purple

Re: Strange routing issue - only one IP affected

Maybe you can clarify a little about the 150.176.6.128./29 . If its a /29 then .134 and .139 are not in the same subnet , /29 ends at the .135 address and .139 is in the next subnet so one could fail and the other not because they would be in different subnets. You have the static route at the /28 boundary so you would have to clarify that .

Re: Strange routing issue - only one IP affected

Kevin posted good though ..and Glen nailed it, and lessson learn at my end to look closer.. if this is the case that is a good catch by Glen, if you take a look at /28 and /29.

/28 hosts addresses

150.176.6.128 subnet address

150.176.6.129

150.176.6.130

150.176.6.131

150.176.6.132

150.176.6.133

150.176.6.134

150.176.6.135

150.176.6.136

150.176.6.137

150.176.6.138

150.176.6.139

150.176.6.140

150.176.6.141

150.176.6.142

150.176.6.143 Broadcast

/29 hosts addresses

150.176.6.128 Subnet address

150.176.6.129

150.176.6.130

150.176.6.131

150.176.6.132

150.176.6.133

150.176.6.134

150.176.6.135 broadcast

so as posted by Glen, you would have to take a look at if indeed you have one /28 and one /29 network somewhere or both to taylor your static route.. if you do have both networks going same destination then two staic routes will be needed one for /28 and /29.

New Member

Re: Strange routing issue - only one IP affected

Apologies to all - that mask was a typo; it used to be a /29 but was opened up to a /28 earlier this year.

A 'sh ip route from that 3750:

xxxxx3750#sh ip route

Codes: C - connect*snip*

Gateway of last resort is 150.176.4.137 to network 0.0.0.0

*** Private ranges removed ***

C 150.176.6.32/29 is directly connected, Vlan600

S 150.176.47.0/24 [1/0] via 10.10.245.132

C 150.176.6.60/30 is directly connected, Vlan7

S 150.176.6.0/28 [1/0] via 10.10.245.133

C 150.176.8.0/22 is directly connected, Vlan21

S 150.176.6.16/28 [1/0] via 10.10.245.131

S 150.176.6.64/26 [1/0] via 150.176.6.62

S 150.176.6.160/27 is directly connected, Null0

C 150.176.6.128/28 is directly connected, Vlan593

C 150.176.4.136/30 is directly connected, Vlan591

C 150.176.6.144/30 is directly connected, Vlan8

S 150.176.6.144/29 is directly connected, Null0

C 150.176.6.148/30 is directly connected, Vlan9

S* 0.0.0.0/0 [1/0] via 150.176.4.137

xxxxx3750#

The global route commands taken off of the 'show running-config':

ip route 150.176.6.0 255.255.255.240 10.10.245.133

ip route 150.176.6.16 255.255.255.240 10.10.245.131

ip route 150.176.6.64 255.255.255.192 150.176.6.62

ip route 150.176.6.128 255.255.255.240 Vlan21

ip route 150.176.6.144 255.255.255.248 Null0

ip route 150.176.6.160 255.255.255.224 Null0

ip route 150.176.47.0 255.255.255.0 10.10.245.132

A reboot of both this switch and the 150.176.8.240 router had no effect.

Marc

Re: Strange routing issue - only one IP affected

first what is 150.176.6.134 is it a switch router or PC?

can you post just show ip route 150.176.6.0 from 3570.

New Member

Re: Strange routing issue - only one IP affected

The problem has been fixed;

I got a solution from a contractor consultant that my Agency uses; he identified a proxy ARP statement in the 150.176.8.240 1700 VPN router for the affected IP address.

Apparently, the 1700 was responding first to ARP requests sent out by the 3750, since both of its interfaces connect to it (on different VLANs) and was thus was giving the 3750 an erroneous ARP entry for that IP; since the 3750 is the default route out of the 1700, a loop was being created.

I would've never found this on my own.

Thanks all for inquiring and trying to help me out.

Marc

Re: Strange routing issue - only one IP affected

Proxy arp respond if the 3750 is trying to reach a address outside the configure network.

Did you forget to change the netmask on the 1700 when you changed your network from a /29 to /28 ?

New Member

Re: Strange routing issue - only one IP affected

I don't manage the 1700 router, so I just went on what he told me; all I know is that whatever tweak they made in the configuration worked...

Marc

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