How Routers Route IP Packets Using Ethernet Emulation
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WANs, by their very nature, give IP routers a way to forward IP packets from a LAN at one
site, over the WAN, and to another LAN at another site. Routing over an EoMPLS WAN
link still uses the WAN like a WAN, as a way to forward IP packets from one site to another.
However, the WAN link happens to use the same Ethernet protocols as the Ethernet
LAN links at each site.
The EoMPLS link uses Ethernet for both Layer 1 and Layer 2 functions. That means the link
uses the same familiar Ethernet header and trailer, as shown in the middle of Figure 3-11.
Figure 3-11 Routing over an EoMPLS Link
NOTE This book shows EoMPLS connections as a familiar single black line, like other
Ethernet links, but with a small cloud overlaid to note that this particular Ethernet link is
through an Ethernet WAN service.
The figure shows the same three routing steps as shown with the serial link in the earlier
Figure 3-8. In this case, all three routing steps use the same Ethernet (802.3) protocol.
However, note that each frame’s data-link header and trailer are different. Each router discards
the old data-link header/trailer and adds a new set, as described in these steps. Focus
mainly on Step 2, because compared to the similar example shown in Figure 3-8, Steps 1
and 3 are unchanged:
1. To send the IP packet to Router R1 next, PC1 encapsulates the IP packet in an
Ethernet frame that has the destination MAC address of R1.
2. Router R1 de-encapsulates (removes) the IP packet from the Ethernet frame and
encapsulates the packet into a new Ethernet frame, with a new Ethernet header and
trailer. The destination MAC address is R2’s G0/0 MAC address, and the source MAC
address is R1’s G0/1 MAC address. R1 forwards this frame over the EoMPLS service
to R2 next.
3. Router R2 de-encapsulates (removes) the IP packet from the Ethernet frame, encapsulates
the packet into an Ethernet frame that has the destination MAC address of PC2,
and forwards the Ethernet frame to PC2.