There seems to be some strange things going on, when we connect a new device such as a laptop or server to our 3750 we cant ping it for a while from another device, but as soon as we try to ping it direct from the 3750 it starts to work.
I think it has something to do with an ARP table but not sure.
Yes so for example i have a 3750 switch and i connect a laptop to port 1 and another to port 2 and they cant ping each each other for a good while until i try to ping them from the 3750 directly and then they can communicate.
they are using 10.20.30.0/24 subnet on the same vlan.
I think its to do with the ARP table. once they start to ping each other its fine.
Im wondering if there is some kind of ARP update interval settings i need?
It can take around 5 minutes to start to ping or even longer until i entiate a ping from the switch
Attaches my config - nothing overly special - not using portfast because it does the same if i put LAPTOP-2 in a access switch.
Never seen this before.
Building configuration... Current configuration : 3102 bytes ! version 12.2 no service pad service timestamps debug uptime service timestamps log datetime no service password-encryption service sequence-numbers ! hostname 3750-12 ! ! no aaa new-model switch 1 provision ws-c3750g-24ts ip subnet-zero ! ! ! ! ! ! no errdisable detect cause loopback errdisable recovery cause loopback errdisable recovery interval 60 port-channel load-balance src-dst-mac no file verify auto ! spanning-tree mode rapid-pvst spanning-tree extend system-id ! vlan internal allocation policy ascending ! ! interface Port-channel1 description HSRP-3750-13 no switchport ip address 10.0.0.1 255.255.255.0 standby 1 ip 10.0.0.3 standby 1 priority 110 standby 1 preempt standby 2 ip 10.0.0.4 standby 2 preempt ! interface Port-channel2 description 3750-13 switchport trunk encapsulation dot1q switchport mode trunk spanning-tree portfast trunk ! interface GigabitEthernet1/0/1 description HSRP-3750-13-P1 no switchport no ip address channel-group 1 mode active ! interface GigabitEthernet1/0/2 description HSRP-3750-13-P2 no switchport no ip address channel-group 1 mode active ! interface GigabitEthernet1/0/3 description 3750-13-P3 switchport trunk encapsulation dot1q switchport mode trunk channel-group 2 mode active spanning-tree portfast trunk ! interface GigabitEthernet1/0/4 description 3750-13-P4 switchport trunk encapsulation dot1q switchport mode trunk channel-group 2 mode active spanning-tree portfast trunk ! interface GigabitEthernet1/0/5 description 2950-LAB-P1 switchport trunk encapsulation dot1q switchport mode trunk ! interface GigabitEthernet1/0/6 description LAPTOP-1 switchport access vlan 10 switchport mode access ! interface GigabitEthernet1/0/7 description LAPTOP-2 switchport access vlan 10 switchport mode access ! interface GigabitEthernet1/0/8 ! interface GigabitEthernet1/0/9 ! interface GigabitEthernet1/0/10 ! interface GigabitEthernet1/0/11 ! interface GigabitEthernet1/0/12 ! interface GigabitEthernet1/0/13 ! interface GigabitEthernet1/0/14 ! interface GigabitEthernet1/0/15 ! interface GigabitEthernet1/0/16 ! interface GigabitEthernet1/0/17 ! interface GigabitEthernet1/0/18 ! interface GigabitEthernet1/0/19 ! interface GigabitEthernet1/0/20 ! interface GigabitEthernet1/0/21 ! interface GigabitEthernet1/0/22 ! interface GigabitEthernet1/0/23 ! interface GigabitEthernet1/0/24 switchport access vlan 99 switchport trunk encapsulation dot1q switchport mode access ! interface GigabitEthernet1/0/25 ! interface GigabitEthernet1/0/26 ! interface GigabitEthernet1/0/27 ! interface GigabitEthernet1/0/28 ! interface Vlan1 no ip address ! interface Vlan10 ip address 10.20.40.252 255.255.255.0 secondary ip address 10.20.30.252 255.255.255.0 standby 10 ip 10.20.30.1 standby 10 timers msec 333 1 standby 10 priority 200 standby 10 preempt standby 40 ip 10.20.40.1 standby 40 timers msec 333 1 standby 40 priority 200 standby 40 preempt ! interface Vlan99 ip address 10.10.10.2 255.255.255.0 ! ip classless ip http server ip http secure-server ! ! ! control-plane
Can you please provide additional information:
1. In "broken" state, where laptops are connected, but ping doesn't work, can you check following:
a) do you see MAC addresses of both Laptops on respective interfaces on switch?
b) do you try to ping only from one laptop to another, or vice-versa as well?
c) do you see ARP entries for "peer laptop" on both LAPTOPS or not?
d) Can you check, where exactly packets are dropped (ICMP requests or replies)? By performing capture on laptops, or SPAN from switch and also by checking interface counters.
In general, if everything is configured correctly, this behavior looks weird. Could be a bug, or transient issue.
There was a bug CSCvb78700 with similar symptoms, but for 4500 platform, related to incorrect hardware unknown unicast floodset. If one of the Laptops is "silent" before ping - doesn't generate any traffic, than switch doesn't know it's MAC. If you initiate ping from another Laptop, switch should flood this packet to all the ports in this VLAN, except receiving port. This operation is done in hardware (ASIC). List of such ports is programmed into hardware (ASIC). If, for some reason, this list is not correct, that unknown unicast traffic will not be flooded.
When you initiate ping from switch, this traffic is generated in software (by CPU), and it uses unknown unicast floodset list on it's own, software layer. Once Laptop replies, MAC address is learned and subsequent pings between laptops work.