Packet Tracer 3.2 Usage

Quirks

Packet Tracer v3.2 attempts to simulate the behavior of real networks and devices. As with all simulations, the program is inherently limited by certain discrepancies, assumptions, and modeling decisions. These "quirks" are listed as follows:

Real World Devices

- In Packet Tracer v3.2, each slot can only have one interface. In the real world, modular interface cards (for slots) can have multiple interfaces.
- With real devices, interfaces are displayed in this convention: <type> <slot number>/<interface number>. In Packet Tracer v3.2, the interface number is always 0 because each slot has only one interface. Thus you see descriptions such as "Ethernet 0/0", "FastEthernet 1/0", and "Serial 2/0".

Access Lists

- Extended IP: In the real Cisco IOS, the options for access lists are endless, but only the first one takes effect. Packet Tracer v3.2 allows for only one option in a command line.

Network Address Translation

- ip nat inside source list: In the real IOS, once it matches the first list, it gets the translation from pool, but if the pool fails to give translation because it ran out of addresses or ports, it will stop looking for other lists. In Packet Tracer, if a pool fails, it looks for other lists.
- Two pools:

1. ip nat pool pool1 <startIP> <endIP>
2. ip nat pool pool2 <same startIP> <same endIP>
3. ip nat inside source list 1 pool pool1
4. ip nat inside source list 2 pool pool2

In the real IOS, both pools would register the same IPs. The same IPs are registered to both pools. Then the NAT table would have one inside global IP for two inside local IPs, and it would load-balance between the two. Pools keep track of their own IP registrations, and increases sequentially. In Packet Tracer, IP registration is kept by the NAT table. When a pool requests to register for a IP, it will check the NAT table for existing registration first. So no two pools register the same IPs in Packet Tracer.

VLANs

- In Packet Tracer v3.2, trunking VLANs in switches uses 802.1q (tagging frames), not ISL (encapsulating frames).

Unavailable Commands

- Certain Cisco IOS commands are only available in simulation mode because certain information are not built/generated in topology mode. These include all user and enable mode commands (except "copy"), such as "show ip nat", and "show ip route".

Simulation Model

- Packet Tracer utilizes a discrete-time model in Simulation Mode so that the user can step through and examine the events that happen during a simulation run. To simplify the time model used, Packet Tracer does not use specific units of time. Each step in the simulation timeline is one arbitrary unit of time, where one unit is long enough to have any packet (regardless of size) to transmit over any link (regardless of speed). In other words, a packet takes one unit of time to transmit over a link. Also, because there is no physical dimensions in Packet Tracer, there is no propagation time of transmitting packets over links.
- Packets in Simulation Mode are assumed to be 1000 Bytes in size. There is no fragmenting of packets.