The command: Switch# show ip cef displays the entire FIB contents of a switch running CEF (Cisco Express Forwarding). When CEF (Cisco Express Forwarding) is enabled on a switch, an FIB (Forwarding Information Base) is build that enables forwarding of arriving packets at wire speed. However, there are packets that may still need intervention by Layer 3 Engine. If an arriving packet is required to be forwarded to Layer 3 Engine, then the packet is marked as "CEF punt" and sent to Layer 3 engine for further processing.
The following are the occasions when the packet is marked as CEP punt and forwarded to Layer 3 engine:
1. An entry can not be found in the FIB
2. The FIB is full
3. The IP TTL has expired
4. The MTU is exceeded, and the packet needs to be fragmented.
5. The encapsulation type is not supported
6. Compression or encryption operation is needed etc.
The first 24 bits of a multicast MAC address always start with 01:00:5E. The Ethernet MAC Address Range used for IPv4 multicast traffic is from 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff. The low-order bit of the first byte is the individual/group bit (I/G) bit, also known as the unicast/multicast bit, and when it is set to 1, it indicates that the frame is a multicast frame, and the 25th bit is always 0. The lower 23 bits of the multicast MAC address are copied from the lower 23 bits of the multicast group IP address.
Administratively scoped block (239.0.0.0/8): These addresses are limited to a local group or organization. These addresses are similar to the reserved IP unicast ranges (such as 10.0.0.0/8) and will not be assigned by the IANA to any other group or protocol. In other words, network administrators are free to use multicast addresses in this range inside of their domain without worrying about conflicting with others elsewhere on the Internet.
In the multicast address space, multiple blocks of addressing are reserved for specific purposes, as shown in Table below.
| Designation | Multicast Address Range |
|---|---|
| Local network control block | 224.0.0.0 to 224.0.0.255 |
| Internetwork control block | 224.0.1.0 to 224.0.1.255 |
| Ad hoc block I | 224.0.2.0 to 224.0.255.255 |
| Reserved | 224.1.0.0 to 224.1.255.255 |
| SDP/SAP block 224.2.0.0 to | 224.2.0.0 to 224.2.255.255 |
| Ad hoc block II | 224.3.0.0 to 224.4.255.255 |
| Reserved | 224.5.0.0 to 224.255.255.255 |
| Reserved | 225.0.0.0 to 231.255.255.255 |
| Source Specific Multicast (SSM) block | 232.0.0.0 to 232.255.255.255 |
| GLOP block | 233.0.0.0 to 233.251.255.255 |
| Ad hoc block III | 233.252.0.0 to 233.255.255.255 |
| Reserved | 234.0.0.0 to 238.255.255.255 |
| Administratively scoped block | 239.0.0.0 to 239.255.255.255 |
Routing Information Base (RIB): The Routing Information Base RIB is where all IP Routing information is stored. It is not specific to any routing protocol, rather a repository where all the routing protocols place all of their routes. Routes are inserted into the RIB whenever a routing protocol running on the router learns a new route. When a destination becomes unreachable, the route is first marked unusable and later removed from the RIB as per the specifications of the routing protocol they were learned from. The RIB lives in the control plane. The RIB table does not directly guide the forwarding, but guides the generation of the FIB table. The FIB table provides guidance for forwarding, which is irrelevant to route generation.
Forwarding Information Base (FIB): Forwarding Information Base (FIB) is used to make IP destination prefix-based switching decisions. FIB contains the interface identifier and next hop information for each reachable destination network prefix. The FIB is conceptually similar to a routing table. It maintains a mirror image of the forwarding information contained in the IP routing table. The FIB lives in the data plane. FIB entries are used to guide the actual forwarding of packets on the device. The FIB entries specify the actual interface to which packets are forwarded and the interface on which packets are transmitted.