Pete's Packet

ref: http://forum.internetworkexpert.com/ubbthreads.php/ubb/showflat/Number/14757/page/1#Post14757

If you use a 3550 as SW3 or SW4 it doesn’t like traffic being routed out a layer 3 port and then back across the switch in layer 2. 3560′s are fine with this behaviour. I think this is because a 3550 will send traffic for it’s own L3 port to the CPU:

My SW4 is a 3550 and my SW3 is a 3560:

Rack1SW4#sh run int fa0/20
Building configuration…

Current configuration : 87 bytes
!
interface FastEthernet0/20
no switchport
ip address 204.12.1.10 255.255.255.0
end

Rack1SW4#sh int fa0/20 | i bia
Hardware is Fast Ethernet, address is 000c.ced3.c580 (bia 000c.ced3.c580)

Rack1SW4#sh mac-address-table address 000c.ced3.c580
Mac Address Table
——————————————-

Vlan Mac Address Type Ports
—- ———– ——– —–
All 000c.ced3.c580 STATIC CPU
Total Mac Addresses for this criterion: 1

Rack1SW3#sh run int fa0/21
Building configuration…

Current configuration : 86 bytes
!
interface FastEthernet0/21
no switchport
ip address 204.12.1.9 255.255.255.0
end

Rack1SW3#sh int fa0/21 | i bia
Hardware is Fast Ethernet, address is 001a.6c81.15c1 (bia 001a.6c81.15c1)

Rack1SW3#sh mac-address-table address 001a.6c81.15c1
Mac Address Table
——————————————-

Vlan Mac Address Type Ports
—- ———– ——– —–
53 001a.6c81.15c1 DYNAMIC Fa0/19
Total Mac Addresses for this criterion: 1

So you can see that SW3 knows at layer 2 to get to the layer 3 port you need to go out via the trunk to SW4 and back over the access-vlan to the layer 3 interface.

SW4 thinks that to get to this mac address you go to the CPU, not out the trunk to SW3 and back over the access-vlan to the layer 3 interface.

For this reason SW4 cannot ping SW3 or R5, only BB3 (which is connected to SW3 so the traffic needn’t transit back over SW4 at layer 2).

Rack1SW4#ping 204.12.1.255

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 204.12.1.255, timeout is 2 seconds:

Reply to request 0 from 204.12.1.254, 28 ms
Reply to request 1 from 204.12.1.254, 28 ms
Reply to request 2 from 204.12.1.254, 8 ms
Reply to request 3 from 204.12.1.254, 28 ms
Reply to request 4 from 204.12.1.254, 20 ms

This makes this lab impossible to complete on 3550′s as SW3 or SW4 without modification. Just make the Fa0/20 into a vlan interface so that it doesn’t have to double hop SW4.

Rack1SW4(config)#do sh run int fa0/20
Building configuration…

Current configuration : 87 bytes
!
interface FastEthernet0/20
no switchport
ip address 204.12.1.10 255.255.255.0
end

Rack1SW4(config)#int fa0/20
Rack1SW4(config-if)#no ip add
Rack1SW4(config-if)#int vlan 53
11:23:51: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan53, changed state to up
Rack1SW4(config-if)#ip add 204.12.1.10 255.255.255.0
Rack1SW4(config-if)#end
11:24:00: %SYS-5-CONFIG_I: Configured from console by console
Rack1SW4#ping 204.12.1.255

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 204.12.1.255, timeout is 2 seconds:

Reply to request 0 from 204.12.1.9, 1 ms
Reply to request 0 from 204.12.1.254, 32 ms
Reply to request 0 from 204.12.1.5, 4 ms
Reply to request 1 from 204.12.1.9, 1 ms
Reply to request 1 from 204.12.1.254, 24 ms
Reply to request 1 from 204.12.1.5, 4 ms
Reply to request 2 from 204.12.1.9, 1 ms
Reply to request 2 from 204.12.1.254, 20 ms
Reply to request 2 from 204.12.1.5, 12 ms
Reply to request 3 from 204.12.1.9, 1 ms
Reply to request 3 from 204.12.1.5, 1 ms
Rack1SW4#