weird trace route on router - 3 IP's in one hop

[NetOpsTech]

I am doing a trace from switchA to a user ip.

User ip: 9.1.5.19

switchA: 9.1.6.16
switchB: 9.1.6.12

trace from router to user ip 9.1.5.19

Type escape sequence to abort.
Tracing the route to 9.1.5.19

1 9.1.6.16 0 msec
9.1.6.12 0 msec
9.1.6.16 4 msec
2 9.1.5.19 0 msec 0 msec 0 msec

It goes to switchA then to switchB then back to switchA and then it's sent to the destination.

Why are there three IP's in the first hop of the trace route?

 

[burtsbees]

How is it all hooked up? What kind of switches? VLANs? Layer 2 or 3 switches?

Burt

 

[brianinms]

1. Why are you using a public address scheme on an internal network?

2. You have a routing loop.

 

[jneiberger]

If I remember correctly, Cisco devices always send out three probes for traceroute. In your case, you only have two potential paths available, so the third probe takes the first path again.

 

[NetOpsTech]

router A connects to swithcA & switchB
router B connects to switchA only.

Someone told me that they see three hops in the first hop of the trace route because of load balancing.

I am tracing from router A.

hop 1 - why are there three IP's listed here.
1 9.1.6.16 0 msec
9.1.6.12 0 msec
9.1.6.16 4 msec

hop 2
2 9.1.5.19 0 msec 0 msec 0 msec

If there was a loop I would see something like this:
1 9.1.6.16 0 msec
2 9.1.6.12 0 msec
3 9.1.6.16 4 msec
4 9.1.6.12 0 msec
5 9.1.6.16 4 msec
etc...


If I trace from router B then it looks normal.

1 9.1.6.16 0 msec
2 9.1.5.19 0 msec 0 msec 0 msec

 

[jneiberger]

Router A apparently thinks Router B is a valid path to that destination. Is Router B the default gateway for that LAN?

 

[NetOpsTech]

router A's default route points to the internet.
router B's default route points to switch2.

routerA f0/0 = switch A
routerA f1/0 = switch B


routerB f0/0 = switch A

switchA default route is router A
switchB default route is router A

 

[Minue]

Hello
From what I see the switch A & B are doing layer 3 routing.Then they are both connected to the same layer 2 swicth that give access to the "user's PC user ip 9.1.5.19".
The two switches are advertising the 9.1.5.0 subnet to "router A" with the same metric's.So the router is doing per packet load balancing to the subnet in question.To verify this try setting the bandwith to a lower value or drop the link to 10Mb on Router's A interface to Switch A.You can also test it be turning on per destination load balancing.
Regards