Do not chain L3 devices

Never ever chain-connect L3 devices. In situation when you need to add additional device to a segment which exists between two routers only you will have to redo wiring and reconnect everything to switch fabric.





All L3 interfaces of all segments should be connected to a switch fabric directly. The only valid reason to connect L3 interfaces back-to-back directly is keeping segments isolated. However, the isolation can be achieved with SVI absence in segment in question.



Frame, packet and segment sizes or TCP/IP without fragmentation

It appeared to be really complicated to find relative information about sizes of transmission units in different levels of TCP/IP stack and completely impossible to find a united chart with their comparison. So, here it is.

Layer Protocol Header Size Recommended size with header included without protocol extensions, ensuring no segmentation on all underlying layers Maximum size of transmission unit with protocol extensions
Transport TCP 20-60B 1480 Bit 1 GB
UDP 8 B 1480 Bit 65535 B
Network IPv4 20 B - 60 B 1500 Bit* 65535 B
IPv6 40 B - infinite 1500 Bit 4 GB
Data link 10/100 Ethernet 18 B 1542 bit 1542 bit
1/10 GB Ethernet 18 B 1542 bit 9000 bit**
10/100 GB Ethernet 18 B 1542 bit 64000 bit***
802.11 (WIFI) 34 B 2312 bit 2312 bit

* Fragmentation is widely used in IP protocols

** Jumbo frames are used on high-rate data link protocols, like Gigabit Ethernet or higher

*** is called Super jumbo frame. Not practically used and may not be profitable on a links lower than 10Gbit Ethernet