R4 sends an HTTP GET to R1, and the first data packet sent by R1 is an unfragmented 292 byte packet. The rest of the first part of the exchange looks as shown below Wireshark. Both R1 and R4 have specified a TCP MSS of 536 bytes in their first packet: R2 S0/1 MTU is lowered to 300, causing fragmentation to occur at R2 for return traffic: R3 will inspect TFTP and HTTP inbound on S0/1. R1 is set up as a TFTP and HTTP server and has a 250 KB file in flash, which R4 will download. The original fragments are then forwarded based on the information in the reassembled packet, and the reassmbled packet is discarded.įirst we will look at how this can be useful in CBAC, also known as IOS classic firewall. The router can then make decisions based on the L4 and other information available in the reassembled packet that would otherwise not be available in the non-initial fragments. With virtual fragmentation reassembly enabled, a router holds fragments received on an interface until all fragments of a packet have been received and the packet can be reassembled. The topology and configurations for this example are shown below: In this post we will look at virtual fragmentation reassembly and a few examples of where it is useful.
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