MPLS Ethertype

Lessons Discussion
1

Hey,

here’s a question about MPLS:

Scenario: I have an MPLS Domain with some Edge Routers. The Layer 3 Protocol can be anything, IPv4, IPv6, IPX or a custom protocol.
There are hosts connected to the Label Edge Routers, for them the MPLS domain should be completely transparent.

Therefore Label Edge Ingress Routers has to squeeze the MPLS-Label between Ethernet header and Layer 3 Protocol Header and the Label Edge Egress Router has to remove the label.

But for the LSRs (label switch routers) within the domain the Ethertype field must be modified to MPLS type so they now that an MPLS-Label is coming. Therefor Ethertype is modified by the ingress router to 0x8847.

Where is the information about the protocol carried by Ethernet stored?
The former Ethertype field is overwritten with MPLS type.

TNX
Jan

2

Hi Jan,

We won’t need this information anymore. The router before the destination PE router will pop the transport label because of PHP.

The PE router will then receive a packet that only has the VPN label left. It does a lookup for the VPN label and finds the VRF that it belongs to. The VPN label is then popped and we do an IP lookup for the IP packet which is then forwarded to the CE router.

Rene