Ip forwarding -Routing

1. IP Forwarding (Routing)
1. A router receives the frame and checks the received frame check sequence (FCS); if errors
occurred, the frame is discarded. The router makes no attempt to recover the lost packet.
2. If no errors occurred, the router checks the Ethernet Type field for the packet type and extracts
the packet. The Data Link header and trailer can now be discarded.
3. Assuming an IPv4 packet, its header checksum is first verified. In case of mismatch, the packet is
discarded. With IPv6 packets, this check is skipped, as IPv6 headers do not contain a checksum.
4. If the header checksum passed, the router checks whether the destination IP address is one of
the addresses configured on the router itself. If it does, the packet has just arrived at its
destination. The router analyzes the Protocol field in the IP header, identifying the upper-layer
protocol, and hands the packet’s payload over to the appropriate upper-protocol driver.
5. If the destination IP address does not match any of the router’s configured addresses, the packet
must be routed. The router first verifies whether the TTL of the packet is greater than 1. If not, the
packet is dropped and an ICMP Time Exceeded message is sent to the packet’s sender.
6. The router checks its IP routing table for the most specific prefix match of the packet’s destination
IP address.
7. The matched routing table entry includes the outgoing interface and next-hop router. This
information is used by the router to look up the next-hop router’s Layer 2 address in the
appropriate mapping table, such as ARP, IP/DLCI, IP/VPI-VCI, dialer maps, and so on. This
lookup is needed to build a new Data Link frame and optionally dial the proper number.
8. Before creating a new frame, the router updates the IP header TTL or Hop Count field, requiring a
recomputation of the IPv4 header checksum.
9. The router encapsulates the IP packet in a new Data Link header (including the destination
address) and trailer (including a new FCS) to create a new frame.

2. A Layer 3 switch router performs the following three major functions:
 Packet switching
 Route processing
 Intelligent network services