L2TPv3: Transport Header Overhead Used By Various Transport Types

In yesterday’s post, I have explained how PMTU help to stop the reassembly of l2tpv3 packets over IP cloud consequence saves router CPU processes. In this post, I have transport header size with respect to the transport type which help to calculate the exact MTU size for CE.

MTU= 1500- (20 Bytes (IPv4 Header) + 4 Bytes (L2TPv3 Overhead) + 8 Bytes (Option Cookie Overhead) + Transport Header Size)

Transport Type Transport Header Size
Ethernet Port 14 Bytes
Ethernet Vlan 18 Bytes
HDLC 2 Bytes
PPP 4 Bytes
Frame Relay DLCI,CISCO 2 Bytes
Frame Relay DLCI,IETF 8 Bytes

E.g. If we want to calcute the overhead occured in case of Ethernet where dot1q is used. The calculation is given below

1500-(20+4+18) = 1462

1462 Bytes is the maximum MTU avail by CE. The same is depicted in previous post.


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