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New Member

Routing anomaly in Catalyst 3750 network

I have a problem I can't quite figure out.

If you look at the attached diagram:

If I run a traceroute from MRUCOLOPRODSW1 to a server on VLAN 20 off of switch MRUCOLOCORPSW1 It routes the packet through MRUMADCORPSW1 on VLAN 1 before returning to MRUCOLOCORPSW1.

Here's the traceroute:

MRUCOLOPRODSW1#traceroute 10.30.20.41

Type escape sequence to abort.

Tracing the route to 10.30.20.41

1 10.30.24.5 0 msec

10.30.19.101 0 msec

10.30.19.1 0 msec

2 *

10.30.20.41 0 msec 0 msec

Am I running too many instances of RIP?

Any guidance you could give me would be much appreciated.

Thanks,

-Rob Locke

I just ran traceroute again and got:

MRUCOLOPRODSW1>traceroute 10.30.20.41

Type escape sequence to abort.

Tracing the route to 10.30.20.41

1 10.30.19.1 0 msec 0 msec

10.30.24.5 0 msec

2 10.30.20.41 0 msec 0 msec 0 msec

MRUCOLOPRODSW1>

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Bronze

Re: Routing anomaly in Catalyst 3750 network

In order to get to the destination host, it needs to go via the switch hosting the destination host, right ?

If you look at the mac-address-table on the sourcing switch, which Vlan has the MAC address of the destination switch first ? The table is read in order, first match wins.

7 REPLIES
New Member

Re: Routing anomaly in Catalyst 3750 network

I would run RIP version 2 without auto summarization and repost the results.

router rip

version 2

no auto

Regards

New Member

Re: Routing anomaly in Catalyst 3750 network

I ran RIP version 2 as per your instructions and still get routed through the other switch:

MRUCOLOPRODSW1>traceroute 10.30.20.41

Type escape sequence to abort.

Tracing the route to 10.30.20.41

1 10.30.19.101 8 msec 0 msec

10.30.19.1 0 msec

2 10.30.20.41 0 msec 0 msec 0 msec

MRUCOLOPRODSW1>

New Member

Re: Routing anomaly in Catalyst 3750 network

Per you drawing, this behaviour is OK since you have layer 2 links between the MRUCOLOPRODSW1 switch and MRUMADCORPSW1 switch. Try sourcing your trace from your vlan 24 interface on switch MRUCOLOPRODSW1. This trace should go through MRUCOLOCORPSW1.

Hall of Fame Super Bronze

Re: Routing anomaly in Catalyst 3750 network

What you are seeing is normal. It's simply a behavior of the ARP and CAM entries in the participating switches.

Let's do a packet walk:

1) Packet is sent to the default gateway on that segment 10.30.24.5

2) The packet is Layer3 switched over to Vlan1. You have a trunk between the 2 switches and it will list all remote MAC addresses in order. I'm pretty sure Vlan1 will be seen first. (show mac-address-table)

3) On running the traceroute again, 10.30.19.101 won't respond because your gateway knows the egress port for the destination device. Do a show arp and it will be listed in your switch. If you wait a few minutes for the ARP to timeout, you will see the same first traceroute again.

HTH,

New Member

Re: Routing anomaly in Catalyst 3750 network

OK, I get that. But why is it Layer 3 switched to Vlan 1 instead of Vlan 20, which is the destination network?

Hall of Fame Super Bronze

Re: Routing anomaly in Catalyst 3750 network

In order to get to the destination host, it needs to go via the switch hosting the destination host, right ?

If you look at the mac-address-table on the sourcing switch, which Vlan has the MAC address of the destination switch first ? The table is read in order, first match wins.

New Member

Re: Routing anomaly in Catalyst 3750 network

Thank you very much.

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