RFC 1326 (rfc1326) - Page 2 of 5

Mutual Encapsulation Considered Dangerous

RFC 1326                Encapsulation Dangerous                 May 1992


   protocol of Y.  Likewise, the right and left S(Y) stubs use protocol
   Y, and the right and left S(X) stubs use protocol X.

          :::  :::::          :::::   :::          :::
 +------+ :Y   :X:Y  +------+ :X:Y    :Y  +------+ :Y   +------+
 |      | :::  ::::: |      | :::::   ::: |      | :::  |      |
 | S(Y) |-----Ra-----|      |-------Rb----|      |------| S(Y) |
 |      |            |      |             |      |      |      |
 +------+            |      |             |      |      +------+
                     | B(X) |             | B(Y) |
                     |      |             |      |
                :::  |      | :::   ::::: |      | :::::  :::
       +------+  X:  |      |  X:    X:Y: |      |  X:Y:   X: +------+
       |      | :::  |      | :::   ::::: |      | :::::  ::: |      |
       | S(X) |------|      |-----Rc------|      |------Rd----| S(X) |
       |      |      |      |             |      |            |      |
       +------+      |      |-----Re------|      |            +------+
                     +------+             +------+


   LEGEND:

        :::::
         X:Y:  A packet with protocol X encapsulated in protocol
        :::::  Y, moving left to right

           Rx  Router x

         S(Y)  A stub network whose native protocol is protocol Y

         B(X)  A backbone network whose native protocol is protocol X


             FIGURE 1:  MUTUAL ENCAPSULATION

   Figure 1 shows how packets would travel from left S(X) to right S(X),
   and from right S(Y) to left S(Y).  Consider a packet from left S(X)
   to right S(X).  The packet from left S(X) has just a header of X up
   to the point where it reaches router Rc.  Since B(Y) cannot forward
   header X, Rc encapsulates the packet into a Y header with a
   destination address of Rd.  When Rd receives the packet from B(Y), it
   strips off the Y header and forwards the X header packet to right
   S(X).  The reverse situation exists for packets from right S(Y) to
   left S(Y).

   In this example Rc and Rd treat B(Y) as a lower-level subnetwork in
   exactly the same way that an IP router currently treats an Ethernet
   as a lower-level subnetwork.  Note that Rc considers Rd to be the



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