I might be being special here but...

Magic Johnson

...please can someone explain this to me before my brain melts, and if this is obvious I deserve a slap, but long day!

I was at a customer site replacing an old managed switch that was put in by another company with a new Cisco SF300.

Now, the router is a Cisco 881, which has 4 L2 switchports and an ethernet WAN port.

From what I saw, there were two links going in to the switch from F0 and F3. I thought this might have been an etherchannel or just STP in action.

Anyway I replaced the 3COM switch with our one and plugged F0 in to one of the gigabit ports on the primary SF300. I checked the router config and the switch and there was nothing special at all, no etherchannels, nada. Anyway I plugged F3 in to the second gigabit port same as on the 3COM and it...just worked. I checked STP, both were forwarding. It's like it was doing load balancing.

Now for all the world I thought that would produce a switching loop, and STP would kick in and block the second link. I checked the frame counters on both ports and they were increasing at about the same rate, load balancing.

What am I missing.

p.s I apologise if this is a stupid question in advance.

xnx

Understanding and Configuring the Unidirectional Link Detection Protocol Feature - Cisco

Possibly UDLD, don't take my word from it I've been on a 1 month break from networking...

Magic Johnson

Nope, UDLD not a feature on the switches I put out.

theodoxa

Are the 881 Switports in different VLANs or the same VLAN?

Magic Johnson

Same VLAN mate, 2.

martell1000

strange.

did you try to reboot the device to see if it isnt just a display glitch ?

Magic Johnson

No, it was during working hours and had to get the changoever done with minimum downtime, hang on I'll go and get the output from the router.

EDIT: Both ports were flashing though, suggesting activity.

Magic Johnson

CUSTOMERROUTER#show interfaces
FastEthernet0 is up, line protocol is up - Goes in to G0 in the Switch
Hardware is Fast Ethernet, address is 6073.5cc7.ee4e (bia 6073.5cc7.ee4e)
Description: LAN
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:26, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
1 minute input rate 14000 bits/sec, 6 packets/sec
1 minute output rate 8000 bits/sec, 4 packets/sec
51027378 packets input, 997394571 bytes, 0 no buffer
Received 3195024 broadcasts (2065449 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
59271824 packets output, 1512771606 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
5776 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
3 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out

---

REMOVED


FastEthernet3 is up, line protocol is up - Goes in to G1 on the same Switch
Hardware is Fast Ethernet, address is 6073.5cc7.ee51 (bia 6073.5cc7.ee51)
Description: LAN
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:27, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
1 minute input rate 0 bits/sec, 0 packets/sec
1 minute output rate 1000 bits/sec, 2 packets/sec
7064973 packets input, 763216367 bytes, 0 no buffer
Received 3130672 broadcasts (3876096 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
428488 packets output, 111373020 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
5694 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
1 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out

---

REMOVED


Loopback1 is up, line protocol is up
Hardware is Loopback
Internet address is X
MTU 1514 bytes, BW 8000000 Kbit/sec, DLY 5000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation LOOPBACK, loopback not set
Keepalive set (10 sec)
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out

NVI0 is up, line protocol is up
Hardware is NVI
Interface is unnumbered. Using address of FastEthernet4 (wan ip)
MTU 1514 bytes, BW 56 Kbit/sec, DLY 5000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation UNKNOWN, loopback not set
Keepalive set (10 sec)
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out

Vlan1 is up, line protocol is up
Hardware is EtherSVI, address is 6073.5cc7.ee4e (bia 6073.5cc7.ee4e)
Description: MANAGEMENT VLAN
Internet address is 10.10.1.1/29
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive not supported
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
34 packets output, 2040 bytes, 0 underruns
0 output errors, 3 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out

Vlan2 is up, line protocol is up
Hardware is EtherSVI, address is 6073.5cc7.ee4e (bia 6073.5cc7.ee4e)
Description: VLAN2
Internet address is 192.168.1.254/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive not supported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:01, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 3
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 8000 bits/sec, 5 packets/sec
5 minute output rate 9000 bits/sec, 2 packets/sec
50938802 packets input, 791369999 bytes, 0 no buffer
Received 3200253 broadcasts (14381 IP multicasts)
0 runts, 0 giants, 80 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
57255644 packets output, 1163885923 bytes, 0 underruns
0 output errors, 1 interface resets
511159 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out


CUSTOMERROUTER#sh span
CUSTOMERROUTER#sh spanning-tree


VLAN2 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 6073.5cc7.ee4e
Configured hello time 2, max age 20, forward delay 15
We are the root of the spanning tree
Topology change flag not set, detected flag not set
Number of topology changes 4 last change occurred 3d22h ago
from FastEthernet0
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 1, topology change 0, notification 0, aging 300


Port 1 (FastEthernet0) of VLAN2 is forwarding
Port path cost 19, Port priority 128, Port Identifier 128.1.
Designated root has priority 32768, address 6073.5cc7.ee4e
Designated bridge has priority 32768, address 6073.5cc7.ee4e
Designated port id is 128.1, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 170626, received 2


Port 4 (FastEthernet3) of VLAN2 is forwarding
Port path cost 19, Port priority 128, Port Identifier 128.4.
Designated root has priority 32768, address 6073.5cc7.ee4e
Designated bridge has priority 32768, address 6073.5cc7.ee4e
Designated port id is 128.4, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 170562, received 0

theodoxa

Is the SF300 forwarding on both links? The 881 is the STP Root Bridge.

Magic Johnson

ARGH I don't know because I can't telnet in to it. Was support that set it up. If it wasn't though surely the frame count for one of the links would be nothing?

tomtom1

The root bridge normally has all its ports in FWD mode, so that is normal behaviour. I'm curious to see what the switch reports about spanning-tree operations.

Magic Johnson

tomtom1 wrote: »

The root bridge normally has all its ports in FWD mode, so that is normal behaviour. I'm curious to see what the switch reports about spanning-tree operations.

Is the frame count normal though?

I can't satisfy your curiosity because it looks like support haven't properly set up the VTY lines on the switch!

Dieg0M

Packets input/ouput don't mean nothing when you see this: Last clearing of "show interface" counters never, check the 5 min input/output rate instead. Notice that F3: 1 minute input rate 0 bits/sec, 0 packets/sec and that it did not receive any BPDU's: BPDU: sent 170562, received 0

tomtom1

Magic Johnson wrote: »

Is the frame count normal though?

I can't satisfy your curiosity because it looks like support haven't properly set up the VTY lines on the switch!

Well, the show interface mentions:

Last clearing of "show interface" counters never

So what you could do, to get a more realistic view of the counters is do a clear counters <interface name>​ first.

Magic Johnson

Once again guys, you've saved my little brain.

I cleared the counters and F3 is doing nothing basically, it's received 18 packets (?) and is outputting 543 in the last 5 minutes, is this BDPUs it is forwarding on to the blocked port on the SF300?

Clearly this is STP in action, I missed I think because:

a) I didn't see the router as being the RB
b) I couldn't get access to the switch and didn't think about clearing the counters

Top work guys, repped.

xnx

Nice to know it's cleared up now, STP is on by default on Cisco Switches so it'd always have been on unless you turned it off.

I've also learnt a new command thanks to this thread :P

tomtom1

Hello BPDU's are normally sent out in 802.1d (STP) every 2 seconds by the root bridge, so at least 150 of your packets should be BDPU's. Some other traffic might be sent over that link too, stuff like CDP / VTP advertisements. So it's normal that at least a few packets are traversing the link.

show spanning-tree root might also come in handy next time. It shows you an overview of STP processes running (2 in my case - PVST with 2 vlans). It shows you the root BID and root ports (if applicable). A root bridge obviously has no root ports.

Root    Hello Max FwdVlan                   Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---  ------------
VLAN0001         32768 000e.58e4.c98e    200019    2    6   4  Fa0/1           
VLAN0002         32768 000e.58e4.c98e    200038    2    6   4  Fa0/9