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routing
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... also may be pure technnical reasons to renumber, such as changes in
technology (e.g., from bridging to routing).
While this document is titled "Router ...
... bridges.
Other considerations include compliance with routing outside the
organization. Routing issues here are primarily those of the global
...
... Other considerations include compliance with routing outside the
organization. Routing issues here are primarily those of the global
Internet, but may also involve bilateral private links ...
... using modems or ISDN, also may have new routing and numbering
requirements. Switched virtual circuit ...
... 1900, "Unless and until viable alternatives are
developed, extended deployment of Classless Inter-Domain Routing
(CIDR) is vital to keep the Internet ...
... 1900, "Unless and until viable alternatives are
developed, extended deployment of Classless Inter-Domain Routing
(CIDR) is vital to keep the Internet ...
... (CIDR) is vital to keep the Internet routing system alive and to
maintain continuous uninterrupted growth of the Internet....To
...
... maintain continuous uninterrupted growth of the Internet....To
contain the growth of routing information, whenever such an
organization changes to a new service provider, the organization's
...
... service providers themselves may have to change to a
new and larger block of address space. In either of these cases, to
contain the growth of routing information, the organizations
concerned would need to renumber.... If the organization does not
renumber, then some of the potential consequences may include (a)
...
... IP connectivity, or (b) extra cost
to offset the overhead associated with the organization's routing
information that Internet Service Providers have to maintain, or
both."
...
... switching has a "flat" address space that will need to be renumbered
into a hierarchical, subnetted space consistent with routing.
Traditional bridged networks ...
... subnets. This
typically means the addressing and routing plan must support multiple
prefix lengths, establishing one or more prefix ...
... address loss.
There are alternative ways to configure routing over NBMA, using
special mechanisms to exploit or simulate point-to-multipoint ...
... hosts to a data center). In such cases, some of the
limitations are due to the dynamic routing protocol in use.
In such "star" applications, static routing may actually be
...
... limitations are due to the dynamic routing protocol in use.
In such "star" applications, static routing may actually be
preferable from performance and flexibility standpoints,
...
... preferable from performance and flexibility standpoints,
since it does not produce routing traffic and is unaffected
by split horizon.
...
... subnet.
In a routing protocol, such as OSPF, that has a concept of designated
routers, explicit configuration usually is needed. Other problems in
...
... The router looks up the prefix in its routing table (formally called
a Forwarding Information Base). If the prefix ...
... a Forwarding Information Base). If the prefix is in the routing
table, the router then selects an outgoing interface that will take
...
... route.
If the prefix cannot be found in the routing table, the router
returns an ICMP Destination ...
... datagram through the indicated outgoing interface. If
multicast routing is in effect, the datagram may be copied and sent
out multiple outgoing interfaces ...
... be greatest in highly connected parts of "backbones," and least in
"stub" parts of the routing domain that have a single route to the
...
... subnets." It is critical to realize that routers make routing
decisions based on the total prefix of interest, regardless of who
...
... If a prefix is totally outside the control of the enterprise, it is
external, and will be minimally affected by routing. Potential
interactions of external prefixes with enterprise renumbering
...
... The old prefix may have been assigned to the enterprise but will no
longer be used for global routing, or the old prefix may have been
assigned to the previous provider ...
...
Low-order-part-only renumbering can occur when an enterprise modifies
its internal routing structure, and the changes only affect the
internal subnet structure of the enterprise network ...
... "boxes," and the overall system of routers in a routing domain. The
emphasis of this section is on making the current enterprise more
...
... Default routes are most appropriate for stub routers inside a routing
domain, and for boundary routers ...
... prefixes.
Not all routing mechanisms support general summarization. Interior
routing mechanisms that do include RIPv2 ...
... Not all routing mechanisms support general summarization. Interior
routing mechanisms that do include RIPv2, OSPF, EIGRP, IS-IS ...
... bits being renumbered are to the right of the summarized
prefix length, the change can be transparent to the overall routing
system.
Even when effective summarization is possible to hide the details of
...
...
Even when effective summarization is possible to hide the details of
routing, DNS, filters, and other services ...
... During this transition, old and new addresses may coexist in the
routing system. Coexistence over a significant period of time is
especially likely for DNS references to addresses ...
...
In the classic definition of NAT, certain parts of the routing system
are designated as stub domains, and connect to the global domain ...
... One way to categorize potential pitfalls is to look at those
associated with the overall numbering plan itself and routing
advertisement, and those associated with protocol behavior. In
general, the former case is static and the latter is dynamic.
...
...
Problems can be implicit to the address/routing structure itself.
These can include failures of components to understand arbitrary
prefix ...
... prefix addressing (i.e., classless routing), reachability due to
inappropriate default or aggregated routes, etc.
...
... Classless Routing Considerations ...
... address
classes. Classless Inter-Domain Routing (CIDR) is the administrative
realization of prefix ...
... of routers (and terminal access servers that participate in routing),
and an even more significant number of host IP ...
... maintain future compatibility by moving to a CIDR-supportive routing
environment. While this is usually thought to mean introducing a
classless dynamic routing protocol ...
... routing
environment. While this is usually thought to mean introducing a
classless dynamic routing protocol, this need not mean that routing
become much more complex. In a RIPv1 environment, moving to RIPv2 ...
... environment. While this is usually thought to mean introducing a
classless dynamic routing protocol, this need not mean that routing
become much more complex. In a RIPv1 environment, moving to RIPv2
...
...
Some routers support classless routing without further
configuration, other routers support classless routing ...
... routing without further
configuration, other routers support classless routing but require
specific configuration steps to enable it, while other routers only
...
... specific configuration steps to enable it, while other routers only
understand classful routing. In general, most renumbering will
eventually require classless routing support. It is essential to
...
... understand classful routing. In general, most renumbering will
eventually require classless routing support. It is essential to
know if a given router can support classless routing ...
... routing support. It is essential to
know if a given router can support classless routing. If it does
not, workarounds may be possible. Workarounds are likely to be
necessary.
...
... RFC 1897(-> 2471hist(-> 3701)) demonstrated problems with some existing equipment,
especially "equipment that depends on use of a classful routing
protocol, such as RIPv1 are prone to misconfiguration. Tested
examples are current Ascend and Livingston gear, which continue to
use RIPv1 as the default/only routing protocol ...
... routing
protocol, such as RIPv1 are prone to misconfiguration. Tested
examples are current Ascend and Livingston gear, which continue to
use RIPv1 as the default/only routing protocol. RIPv1 use will
create an aggregate announcement.... The Ascend was told to announce
...
...
Another problem that can occur with routers or routing mechanisms
that do not understand arbitrary length prefix addressing ...
... subnets inside 10.0.0.0. Router 2 has an
ambiguous routing table in terms of the next hop to a subnet of
...
...
There are several workarounds to this problem. Obviously, one would
be to change to a routing mechanism that does advertise subnets. An
alternative would be to establish an IP ...
... to R2. This would work as long as there is no problem in advertising
the 10.0.0.0 subnets into the R2 routing system. There would be a
problem, for example, if the 10.0.0.0 address were in the private
...
... RIP on the hosts, so that they hear routing updates. They assume any
host that issues routing ...
... routing updates. They assume any
host that issues routing updates must be a router, so traffic for
...
... support arbitrary prefixes, the router(s) issuing the routing updates
may have additional capabilities that let them correctly forward such
traffic ...
... such an approach, some sites have VLSM-aware hosts but others do not.
As long as the routing structure supports VLSM, workarounds can be
applied where needed.
...
... address space. This might be done to clean up an
address space into contiguous blocks prior to introducing a routing
system that aggregates addresses, such as OSPF.
...
...
Especially on UNIX and othe that do routing, there may be static name
definitions. Such definitions are probably harder to keep maintained
than entries in the DNS ...
... router's interaction
with neighbors below the full routing level (e.g., default gateways,
ARP ...
... router ID may not require it be a
"real" IP address that is advertised through the routing domain, but
is simply a 32-bit ...
... accessed from the Internet unless explicitly used for exterior
routing. External traceroutes will also fail reverse DNS lookup.
...
... unicast packets (the most common case) or
for multicast packets (including routing updates). Filtering rules
may contain source and/or destination IP addresses ...
... networks
is the bastion host, in which reside various non-routing isolation
and authentication functions, beyond the scope of this document.
...
... Interior Routing ...
...
This section deals with routing inside an enterprise, which generally
follows, ignoring default routes, the rules:
...
... Most enterprises do not directly participate in global Internet
routing mechanisms, the details of which are of concern to their
service providers. The next section deals with those more complex
...
... restart routers or
explicitly clear a routing table entry to force the changed static
route to take effect.
...
...
The Routing Information Protocol (RIP) has long been with us, as one
of the first interior routing protocols ...
... Routing Information Protocol (RIP) has long been with us, as one
of the first interior routing protocols. It still does that job in
small networks, and also has been used for assorted functions that
...
... small networks, and also has been used for assorted functions that
are not strictly part of interior routing. In this discussion, we
will first deal with pure interior routing ...
... routing. In this discussion, we
will first deal with pure interior routing applications.
In a renumbering effort that involves classless addressing ...
... not be able to cope with the new addressing scheme. Officially, this
protocol is Historic and should be avoided in new routing plans.
Where legacy support requirements dictate it be retained, it is
...
... to moving to RIPv2, OSPF, or other classless routing protocols as the
primary means of interior routing. Doing so, however, may not remove ...
... OSPF, or other classless routing protocols as the
primary means of interior routing. Doing so, however, may not remove
the need to run RIP ...
... subnet. In these applications, RIP need
not be the only routing protocol in the domain; RIP may be present
...
... subnets. Destination information from more capable
routing protocols may be translated into RIP updates. While it is
generally reasonable to minimize or remove ...
...
RIP is also used as a quasi-exterior routing mechanism between some
customers and their ISPs ...
... OSPF has several sensitivities to renumbering beyond those of simpler
routing protocols. If router IDs are assigned to be part of the
registered address space ...
... Exterior Routing ...
...
Exterior routes may be defined statically. If dynamic routing is
involved, such routes are learned primarily from BGP. RIP ...
... routers of other AS
and in routing registries. This will require careful administrative
coordination.
...
... coordination.
If for no other reason than documentation, consider use of a routing
policy notation [RIPE-181++] [RPSL] to describe exterior routing
policies ...
... routing
policy notation [RIPE-181++] [RPSL] to describe exterior routing
policies
...
...
Organizations who participate in exterior routing usually will have
routing information not only in their routers ...
... Organizations who participate in exterior routing usually will have
routing information not only in their routers, but in databases
...
... route objects with old prefixes in
routing registries need to be deleted and route objects with new
...
... Other autonomous systems, including nonadjacent ones, can contain
direct or indirect (e.g., aggregated) references to the above routing
information. Tools exist that can do preliminary checking of
connectivity to given external destinations ...
... addresses not consistent with the
active routing environment. Such encapsulating functions usually
have a tunneling model, where an end-to-end ...
... case, the legacy IP addresses can be tunneled over the renumbered
routing environment.
Also note that IP ...
... During a renumbering operation, it may be appropriate to introduce
authentication mechanisms for routing updates.
...
...
What is your plan of retreat if major problems develop?
Make a distinction between problems in the routing system
and unforeseen problems in hosts affected by renumbering.
...
... Coordinate changes with affected external organizations (e.g.,
ISPs, business partners, routing registries)
Step 6--Document what isn't already documented. Make notes to help
...
... routers will have to be restarted. The
sequence in which changes are applied must be carefully thought out,
to avoid loss of connectivity, routing loops, etc., while the
renumbering is in process.
...
...
Routing processes tend towards instability when they suddenly need to
handle very large numbers of updates, as might occur if a "flag day"
cutover is not carefully planned. A general guideline is to make
...
... handle very large numbers of updates, as might occur if a "flag day"
cutover is not carefully planned. A general guideline is to make
changes in only one part of a routing hierarchy at a time.
Routing system ...
... routing hierarchy at a time.
Routing system design should be hierarchical in all but the smallest
domains. While OSPF ...
... IS-IS have explicit area-based hierarchical
models, hierarchical principles can be used with most implementations
of modern routing protocols. Hierarchy can be imposed on a protocol
such as RIPv2 or EIGRP by judicious use of route ...
...
Respecting a hierarchical model during renumbering means such things
as renumbering a "stub" part of the routing domain and letting that
part stabilize before changing other parts. Alternatively, it may be
...
... Alaettinoglu, C., Bates, T., Gerich, E., Terpstra, M., and C. Villamizer, "Routing Policy Specification Language", Work in Progress. ...