Cisco – Initial ping failure in Packet Tracer

ciscoicmppacket-tracerping

In Packet Tracer 7.0:

After directly connecting two switches or a router and switch, pinging the neighboring device always fails on the first one or 2 attempts (i.e. only 4/5 or 3/5 ping attempts are successful.)

Is this an artifact of the PT simulation or is this an expected behavior on a "new" network whose hosts have never communicated with the switch before (ARP resolution?)

Best Answer

This actually happens in the real world, too. On ethernet, and other protocols that use MAC addresses, it is due to ARP. You will drop one or two initial pings. It can happen, too, with frame relay, depending on how you have it configured.

Related Solutions

Cisco – Will STP Portfast Prevent a Loop?

Port fast bypasses the usual STP phases and goes straight into forwarding. This is useful for ports connected to end-devices which use DHCP. It does not stop BPDUs, and there are those who advocate using it on all ports, although Cisco has a different take on it:

Caution: Never use the PortFast feature on switch ports that connect to other switches, hubs, or routers. These connections can cause physical loops, and spanning tree must go through the full initialization procedure in these situations. A spanning tree loop can bring your network down. If you turn on PortFast for a port that is part of a physical loop, there can be a window of time when packets are continuously forwarded (and can even multiply) in such a way that the network cannot recover.

BPDU guard will disable (errdisable) a port which receives BPDUs. The helps to prevent rogue switches and STP loops. Cisco has a document which explains BPDU guard:

Feature Description

STP configures meshed topology into a loop-free, tree-like topology. When the link on a bridge port goes up, STP calculation occurs on that port. The result of the calculation is the transition of the port into forwarding or blocking state. The result depends on the position of the port in the network and the STP parameters. This calculation and transition period usually takes about 30 to 50 seconds. At that time, no user data pass via the port. Some user applications can time out during the period.

In order to allow immediate transition of the port into forwarding state, enable the STP PortFast feature. PortFast immediately transitions the port into STP forwarding mode upon linkup. The port still participates in STP. So if the port is to be a part of the loop, the port eventually transitions into STP blocking mode.

As long as the port participates in STP, some device can assume the root bridge function and affect active STP topology. To assume the root bridge function, the device would be attached to the port and would run STP with a lower bridge priority than that of the current root bridge. If another device assumes the root bridge function in this way, it renders the network suboptimal. This is a simple form of a denial of service (DoS) attack on the network. The temporary introduction and subsequent removal of STP devices with low (0) bridge priority cause a permanent STP recalculation.

The STP PortFast BPDU guard enhancement allows network designers to enforce the STP domain borders and keep the active topology predictable. The devices behind the ports that have STP PortFast enabled are not able to influence the STP topology. At the reception of BPDUs, the BPDU guard operation disables the port that has PortFast configured. The BPDU guard transitions the port into errdisable state, and a message appears on the console.

Continual Ping in Cisco Packet Tracer

The -t has to go before the address, like this:

ping -t 192.168.4.100

Best Answer

Related Solutions

Cisco – Will STP Portfast Prevent a Loop?

Continual Ping in Cisco Packet Tracer

Related Topic