Subject: [Isis-wg] IP reachability information and IP interface address

From: "Ju, Yu" <[email-masked]>
Date: Thu, 10 May 2001 10:52:03 -0400

Hi,

I have the following question regarding rfc1195.txt. 

1). Suppose  L1 A and L1 B are directly connected as the following. x and y
are IP interface addresses on A and B respectively.

          --------   x                      y ---------
         |  A    |-----------------------------|  B   |
         ---------                              --------

     x will be in the IP interface address field of  A's L1 LSP. y will be
in IP interface address field of B's L1 LSP. These information is enough for
other IS in the area to calculate route to x and y. Then does A have to put
y in the IP internal reachability information field of its L1 LSP?  The same
question for B.

2) Why L1 LSP can not have IP external reachability information?

Thanks,

Yu

_______________________________________________
Isis-wg mailing list  -  [email-masked]
http://external.juniper.net/mailman/listinfo/isis-wg

---

From: Jeff Learman <[email-masked]>
Date: Thu, 10 May 2001 11:19:15 -0400

>         |  A    |-----------------------------|  B   |

This is enough information to calculate the routes ONLY if these
routers are actually connected.  There are too many situations where
these routers might not actually be connected (e.g., switch failure).
Routes must be known to be available by IIH handshake in order to
use them for transit routing.

>2) Why L1 LSP can not have IP external reachability information?

IS-IS is a hierarchical system to reduce the amount of information
that an L1 router needs to know to do its job.  If the rules were
changed to permit L1 routers to perform L2 functions (i.e., forwarding
L2 traffic anywhere but to the nearest L2 router), this would
obliterate the hierarchical distinction and would be a fairly
radical change to IS-IS.