I'm wondering about packet size since the label imposition process increase the size of an IP packet.I would like to know, we need to apply tag-switching mtu <1500> or above in P and PE routers.Of course in P routers but in PE routers?
What is the current best practise the determine the actually or exact size of an IP packet? Is there any method or calculation etc?
If LDP is used in the MPLS network, the label stack size is 2 (the LDP label and the VC label).
If a TE tunnel instead of LDP is used between PE routers in the MPLS network, the label stack size is 2 (the TE label and the VC label).
If a TE tunnel and LDP are used in the MPLS network (for example, a TE tunnel between P routers or between P and PE routers, with LDP on the tunnel), the label stack is 3 (TE label, LDP label, VC label).
If you use MPLS Fast Reroute in the MPLS network, you add a label to the stack. The maximum MPLS label stack in this case is 4 (FRR label, TE label, LDP label, VC label).
Most documents (and therefore people) use a "tag mtu" of 1524. This is enough to carry 6 labels, which is plenty given the currently available applications. A tag MTU of 1524 is usually not an issue on WAN links but for LAN links make sure that the switches between your routers support baby giant or giant frames (Ethernet frame larger than 1500).
Hope this helps,
Harold Ritter Sr. Technical Leader CCIE 4168 (R&S, SP) email@example.com México móvil: +52 1 55 8312 4915 Cisco México Paseo de la Reforma 222 Piso 19 Cuauhtémoc, Juárez Ciudad de México, 06600 México
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