Subject: Re: Question on NSSA ABRs

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Mon, 23 Sep 2002 19:27:18 -0700

Mani,

Regarding this modified diagram of yours

                                   ASBR
      -----     ------     -----    -------     -----
     |     |   |      |   |     |  |       |   |     |
     | R1  |   | R2   |   |  R3 |  |  R5   |   | R6  |
     |     |   |      |   |     |  |       |   |     |
      -----     ------     -----    -------     -----
          |I1 I2|  | |I4  I5|  |I6 I9| |  |I11  |   |
           -----   |  ------    -----  |   -----    |
           AREA1   |  AREA 0    AREA2  |            |
                   |I3                 |I10        N11
                   |      -----        |       RIP learned
                   |   I7|     |I8     |         network
                    -----|  R4 |-------
                   AREA0 |     | AREA2
                          -----

It implies R5 learns an external route to N11 from R6 via
RIP across I11. R5 should pick a single forwarding address
for the RIP learned route to N11 and stick with it provided
the interface associated with that address remains in an Up
state. R5 makes no assumptions about whether R3 or R4 is
translating when picking this address. Its best to make R5's
I11 interface a part of the OSPF cloud (i.e. make it a stub
network). Then the forwarding address can simply be R6's
address on I11. Barring that choice, a local address (i.e.
loopback address) is chosen next since it is always in an Up
state. Stub interface network addresses are chosen after
local addresses. Addresses, such as those assigned to I9 and
I10, are the last choice.

Thus when R3 and R4 toggle their elected translator duties,
the path from R2 to N11 should not change. R2's path to N11
depends only on what forwarding address is picked by R5 for
N11 and the interface costs of I3, I4, I6 and I8. It has
nothing to do with who is translating (unless address ranges
are a factor). Note that an address that is part of I11
might caused both paths to be used.

Finally, in the new NSSA draft, Type 7 LSAs that are
translated into Type 5 LSAs by R3 and R4 with a non-zero
forwarding address (such as the one for N11) are never
flushed by an NSSA elected translator, such as R3, when it
is deposed by another NSSA translator, such as R4, via the
NSSA translator election vehicle. Instead these LSAs are
allowed to simply age out. This is done to prevent the
flushing and reorigination of large amounts of translated
Type 7 LSAs during periods of NSSA border instability. Both
LSAs represent the same path with the same forwarding
address. They just have different originators.

Pat