Routing between the T1 and T3 Backbones

Thu Jan 9 04:27:59 UTC 1992

The T1 and T3 backbones share many potential interconnection
points.  The current routing plan is designed so that all
of the networks known to both backbones are announced to each
other at one interconnection point, Ann Arbor. Two additional
interconnection points have been established at San Deigo and
Houston.  Currently traffic between the two backbones traverse
the Ann Arbor interconnect.  Should the path between the T1
router and T3 router in Ann Arbor fail for whatever reason,
the interconnection at Houston becomes the path between
the T1 and the T3 backbones. If both the Ann Arbor and Houston
sessions should fail, the Merit Network Operations Center can
manually enable the third interconnection point at San Diego.

There are three categories of networks that are known to both
backbones (don't get excited, we're not talking com and edu):
	a) networks that are only known to the T1 backbone
	b) networks that are only known to the T3 backbone
	c) networks that are announced by mid-levels to both backbones
To accomodate more and more networks that will fit into category
"c", and to eliminate excessive traffic between the two backbones,
we have redesigned the way the two backbones will peer with each
other.  In fact, we have made some changes in the routing
configuration files on the T3 backbone to accomplish those
goals.  The purpose of this note is to describe our plans
and some of the actions that we have taken.

We plan to install another interconnect point at Princeton.  Therefore
there will be four established points of interconnection between
the T1 and T3 networks. Ann Arbor and Houston will be the active
interconection points with San Diego and Princeton providing
backup.  The primary and backup locations are paired.  Princeton
will act as backup for Ann Arbor, and San Diego will be the
backup for Houston.

The T1 routers will announce all of the networks known to
the T1 backbone to the T3 routers at both active interconnection
points. The T3 backbone routers are configured to assign different
BGP metrics to the announcements learned from the T1 routers, thereby
preferring one path to the T1 network.

As the two backbones are now configured, all traffic from the T3
network to the T1 network prefers the path via the Ann Arbor
interconnect.  If the Ann Arbor interconnect should fail, the
traffic will flow via the Houston interconnect.

The largest change is the routing announcements from the T3 to 
the T1 network.  For networks that are known to both the T1 and
T3 backbones via mid-level announcements, they are either
announced to the T1 at Ann Arbor or at Houston. Traffic to dual
homed networks should only use the path between the T1 network
and the T3 network if the session between the mid-level and 
the T1 or T3 router fails. 

For networks that are known only to the T3, the T3 routers announce
these networks at both interconnects with different metrics. If 
the interconnection with the lowest metric fails, the T3 only
networks will still be reachable via the other interconnect.

The following diagram may help to clarify the new routing design.

           C (T3 only)   E (T3 only) 
           |             |
           |             |
      +-------------------------+
      |                         |
     /|     T3 backbone         |\
    / +-------------------------+ \
 D    Houston |      | Ann Arbor    B
   \  +-------------------------+ /
    \ |                         |/
      |     T1 backbone         |
      +-------------------------+
                  |
                  |
                  A ( T1 only)

Network A is known only to the T1 backbone.
Networks C and E are known only to the T3 backbone.
Networks B and D are known to both backbones.

The T3 backbone prefers the Ann Arbor interconnect for traffic
to Network A.

The T1 backbone prefers the Houston interconnect for traffic
to Network C. The T1 backbone prefers the Ann Arbor interconnect
for traffic to Network E. 

Networks C and E  will use the T3 network to communicate with Networks
B and D.

Network A will use the T1 network to communicate with Networks
B and D.

Traffic between B and D will depend on some local routing decisions.

The interesting scenarios are those which represent traffic
flows between T1 only and T3 only known networks.

Scenario 1 : E is the source and A is the destination.

E ---> Ann Arbor Interconnect ---> A ---> Ann Arbor Interconnect ---> E

Scenario 2 : C is the source and A is the destination.

C ---> Ann Arbor Interconnect ---> A ---> Houston Interconnect ---> C

We have taken the first steps to implement this new routing
plan.  The routing announcements between the T3 routers and T1
routers at Ann Arbor and Houston reflect the split configuration.
T3 only networks are announced at both interconnects where one
is the primary path and the second is the secondary path.
Networks that are known independently to both the T3 and T1
networks are announced at either the Ann Arbor interconnect
or the Houston interconnect.

If you have any questions concerning the new routing configurations
between the T1 and T3 backbones, please feel free to send
a message to ie at merit.edu.

          --Elise