Nortel Optical Multiservice Edge 6130 Planning Manual page 205

Auto GRE tunnel
OME6130 supports auto GRE tunneling with iISIS routing protocol to provide
seamless integration of OSI only networks with Dual or IP only networks
elements.
The auto GRE tunnel is automatically configured and enabled when iISIS
routing protocol is enabled at the nodal level. There is no specific user
configuration required. The IP over OSI tunnel creation is done automatically
on a per packet basis over all the packets forwarded to the auto GRE interface
by IP. A default static route shall be provisioned automatically on the auto
GRE (called agre0) interface that is presented to the IP protocol stack. In this
scenario, the IP protocol stack shall forward packets that are not routable by
any other IP routing table entry to the auto GRE tunnel for further processing.
The default route shall have the last priority, as any user provisioned specific
static route shall have higher priority than default route when routing packets.
Note 1: If an NE contains 0.0.0.0/0 and 10.0.0.0/8 as two routes already
present, then auto-GRE creation can cause packets reaching through one
of the routes to be dropped. It is better to reconfigure the 0.0.0.0/0 and/or
10.0.0.0/8 routes to another suitable route so that the automatic creation of
auto GRE interface route functions seamlessly.
Note 2: Static routes can be added by the user other than the default route
that points to the Auto-GRE tunnel. Then, the packets that did not find a
match in the IP routing table will be routed to the Auto-GRE tunnel.
AGRE IP/OSI routing interactions
Figure 10-2 presents a diagram describing the IP packet routing when iISIS
routing protocol is enabled. The OME6130 supports an IP routing table, which
is updated with the least cost route for each IP destination from Static IP
routes, OSPF learnt routes and iISIS routes learnt over PPP interfaces, such as
connection to IP-capable network elements. IP routes learnt over CLNP are not
stored in the native IP routing table, but they are added to the OSI/GRE routing
table as part of the OSI routing function. The AGRE interface when
provisioned with appropriate static routes, allows for packets to be pushed
from the native IP routing domain to the iISIS routing domain (with IP routes
that communicate through OSI network elements). At each network element,
the AGRE interface must be properly provisioned with static routes covering
all destinations which will need to be auto-tunnelled. Once in the OSI routing
domain, the IP destinations of a received IP packet are looked up against the
OSI/GRE forwarding table to determine the NSAP (OSI address) that provides
most specific and least cost route to that IP destination. If a match is found then
GRE encapsulation of the IP packet into a CLNP (OSI) PDU with this NSAP
as the destination is performed. This CLNP PDU is then routed to this NSAP
by looking it up in the OSI routing table.
Planning Guide NT6Q92MA Rel 1.0 Iss 1 Standard September 2006
Appendix A: Data communications planning 10-19