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.