Steps for Sending a Packet Out

Topics: IP address, Routing, Internet Protocol Pages: 4 (832 words) Published: May 17, 2013
• Steps for sending a packet out -

Step 1 - (IPv4 Routing Lookup) --> Goal is to decide on the following entities first.

- Longest match / Lowest AD / Lowest Metric / Overlapping Routes(Multiple routes to the same destination in the routing table) / Policy Routing (Will override all the above steps) - Outgoing interface

- Next-hop

Step 2 - (IPv4 Switching Method) --> Goal is to move packets between interfaces. Load Balancing occurs at this stage. Even though we have multiple routes in the routing table, the load balancing will not occur if the switching method is not configured to do so. If the switching method is not configured for load balancing, the packets will follow the same path even though multiple paths exists in the routing table. Can be verified by "show ip cef exact-route ". The above command based on the inputs of source and destination, will tell us the next hop IP and outgoing interface the packet is taking.

Decide on the following entities in the second step.

- Process
- Fast
- Etc.

[Note -

1.) Once CEF is enabled, it will form the FIB and adjacency table with the help of Routing table and at the same moment and it will not check the routing table from then on unless there is change in the routing table and will forward all the packets based on CEF table only. CEF is ON by default on the routers.

2.) If on a local LAN, some users are saying that they are able to surf web without any issues and some are saying that they are having problems in surfing even though they all are on the sam LAN, then it's most probably issue with the underlying switching method. You can verify it using the command ip cef exact-route , by giving the different Users source IP address and the same destination IP address. You can see different next-hop IP address, in the output if two route exists in the routing table for the same destination...
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