The OSPF NSSA Option
draft-ietf-ospf-nssa-option-00
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 1587.
|
|
|---|---|---|---|
| Authors | Rob Coltun , Vince Fuller | ||
| Last updated | 2013-03-02 (Latest revision 1992-12-09) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 1587 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-ospf-nssa-option-00
-------------------------------------------------------------------------------
Internet Draft OSPF NSSA Option October 1992
Expiration Date: April 1993
The OSPF NSSA Option
Rob Coltun
Consultant
(301) 340-9416
rcoltun@ni.umd.edu
Vince Fuller
BARRNet
Stanford University
Pine Hall 115
Stanford, CA, 94305-4122
vaf@Valinor.Stanford.EDU
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its
Areas, and its Working Groups. Note that other groups may also
distribute working documents as Internet Drafts).
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Inter-
net Drafts as reference material or to cite them other than as a
"working draft" or "work in progress."
Please check the I-D abstract listing contained in each Internet
Draft directory to learn the current status of this or any other
Internet Draft.
Coltun & Fuller [Page 1]
Internet Draft OSPF NSSA Option October 1992
Table Of Contents
1.0 Abstract ................................................. 2
2.0 Overview ................................................. 2
2.1 Motivation ............................................... 2
2.2 Proposed Solution ........................................ 3
3.0 Implementation Details ................................... 5
3.1 The N-bit ................................................ 5
3.2 Type-7 LSAs .............................................. 5
3.3 Originating Type-7 LSAs .................................. 6
3.4 Calculating Type-7 AS External Routes .................... 7
3.5 Incremental Updates ...................................... 9
4.0 Originating Type-5 LSAs .................................. 9
4.1 Translating Type-7 LSAs .................................. 9
4.2 Flushing Translated Type-7 LSAs .......................... 10
5.0 Acknowledgements ......................................... 10
6.0 References ............................................... 10
Appendix A: Type-7 LSA Packet Format ......................... 11
Appendix B: The Options Field ................................ 11
Appendix C: Configuration Parameters ......................... 12
1.0 Abstract
This document describes a new optional type of OSPF area, some-
what humorously referred to as a "not-so-stubby" area (or NSSA).
NSSAs are similar to the existing OSPF stub area configuration
option but have the additional capability of importing AS exter-
nal routes in a limited fashion.
2.0 Overview
2.1 Motivation
Wide-area transit networks (such as the NSFNET regionals) often
have connections to moderately-complex "leaf" sites. A leaf site
may have multiple IP network numbers assigned to it.
Typically, one of the leaf site's networks is directly connected
to a router provided and administered by the transit network
while the others are distributed throughout and administered by
the site. From the transit network's perspective, all of the
network numbers associated with the site make up a single "stub"
entity. For example, BARRNet has one site composed of a class-B
network, 130.57.0.0, and a class-C network, 192.31.114.0. From
BARRNet's perspective, this configuration looks something like
this:
Coltun & Fuller [Page 2]
Internet Draft OSPF NSSA Option October 1992
192.31.114
|
(cloud)
-------------- 130.57.4
|
|
------ 131.119.13 ------
|BR18|------------|BR10|
------ ------
|
V
to BARRNet "core" OSPF system
where the "cloud" consists of the subnets of 130.57 and network
192.31.114, all of which are learned by RIP on router BR18.
Topologically, this cloud looks very much like an OSPF stub area.
The advantages of running the cloud as an OSPF stub area are:
1. Type-5 routes (OSPF external link-state advertise-
ments (LSAs)) are not advertised into the cloud
utilizing less resources in the (perhaps limited)
cloud's routers.
2. The transit network is abstracted to the cloud
by advertising a default route into the cloud.
3. The cloud becomes a single manageable entity to the
transit network.
4. The cloud can become part of the transit network's
OSPF routing system.
However, the current definition of the OSPF protocol [OSPF]
imposes topological limitations which restrict simple cloud topo-
logies from becoming OSPF stub areas. In particular, it is ille-
gal for a stub area to import routes external to OSPF; it is not
possible for routers BR18 and BR10 to both be members of the stub
area and to import the routes learned from RIP or other IP rout-
ing protocols as type-5 (OSPF external LSAs) into the OSPF sys-
tem. In order to run OSPF out to BR18, BR18 must be a member of
a non-stub area or the OSPF backbone to import routes other than
its directly-connected network(s). Since it is not acceptable
for BR18 to maintain all of BARRNet's external (type-5) routes,
BARRNet is forced by OSPF's topological limitations to run OSPF
out to BR10 and to run RIP between BR18 and BR10.
2.2 Proposed Solution
This document describes a new optional type of OSPF area, some-
what humorously referred to as a "not-so-stubby" area (or NSSA)
which has the capability of importing external routes in a lim-
ited fashion.
Coltun & Fuller [Page 3]
Internet Draft OSPF NSSA Option October 1992
The OSPF specification defines two general classes of area confi-
guration. The first allows type-5 LSAs to be flooded throughout
the area. In this configuration, type-5 LSAs may be originated
by routers internal to the area or flooded into the area by area
border routers. These areas, referred to herein as type-5 capa-
ble areas (or just plain areas in the OSPF spec), are dis-
tinguished by the fact that they can carry transit traffic. The
backbone is always a type-5 capable area. The second type of
area configuration, called stub, allows no type-5 LSAs to be pro-
pagated into/throughout the area and instead depends on default
routing to external destinations.
NSSAs are defined in much the same manner as existing stub areas.
To support NSSAs, a new option bit (the "N" bit) and a new type
of LSA (type-7) area defined. The "N" bit ensures that routers
belonging to an NSSA agree on its configuration. Similar to the
stub area's use of the "E" bit, both NSSA neighbors must agree on
the setting of the "N" bit or the OSPF neighbor adjacency will
not form.
Type-7 LSAs provide for carrying external route information
within an NSSA. Type-7 AS External LSAs have virtually the same
syntax as the Type-5 AS External LSAs with the obvious exception
of the link-state type (see section 3.2 for more details). There
are two major semantic differences between type-5 and type-7
LSAs.
o Type-7 LSAs may be originated by and advertised
throughout an NSSA; as with stub areas, NSSA's do not
receive or originate type-5 LSAs.
o Type-7 LSAs are advertised only within a single NSSA;
they are not flooded into the backbone area or any
other area by border routers, though the information
which they contain can be propagated into the backbone
area (see section 3.6).
In order to allow limited exchange of external information across
an NSSA area border, NSSA border routers will translate selected
type-7 LSAs received from the NSSA into type-5 LSAs. These
type-5 LSAs will be flooded to all type-5 capable areas. In
addition, an NSSA area border router can originate a default
type-7 LSA (IP address of 0.0.0.0) into the NSSA (note that the
default route originated by an NSSA area border router is never
translated into a type-5 LSA). As with stub areas and the sum-
mary default route, default routes are necessary because NSSAs do
not receive full routing information and therefore must have a
default route to route to AS-external destinations. As with stub
areas, NSSAs may be connected to the backbone at more than one
area border router, but may not be used as a transit area.
It should be noted that unlike stub areas, all OSPF summary
routes (type-3 LSAs) must be imported into NSSAs. This is to
ensure that OSPF internal routes are always chosen over OSPF
Coltun & Fuller [Page 4]
Internet Draft OSPF NSSA Option October 1992
external (type-7) routes.
In our example topology the subnets of 130.57 and network
192.31.114, will still be learned by RIP on router BR18 but now
both BR10 and BR18 can be in an NSSA and all of BARRNets external
routes are hidden from BR18; BR10 becomes an NSSA area border
router and BR18 becomes an AS boundary router internal to the
NSSA. BR18 will import the subnets of 130.57 and network
192.31.114 as type-7 LSAs into the NSSA. BR10 then translates
these routes into type-5 LSAs and floods them into BARRNet's
backbone.
3.0 Implementation Details
3.1 The N-bit
The N-bit ensures that all members of a NSSA agree on the area's
configuration. Together, the N-bit and E-bit reflect an
interface's (and consequently the interface's associated area's)
external LSA flooding capability. As explained in section 10.5
of the OSPF specification, if type-5 LSAs are not flooded
into/throughout the area, the E-bit must be clear in the option
field of the received Hello packets. Interfaces associated with
an NSSA will not send or receive type-5 LSAs on that interface
but may send and receive type-7 LSAs. Therefore, if the N-bit is
set in the options field, the E-bit must be cleared.
To support the NSSA option an additional check must be made in
the function that handles receiving Hello packet to verify that
both the N-bit and the E-bit found in the Hello packet's option
field match the value of the options that have been configured in
the receiving interface. A mismatch in the options causes pro-
cessing of the received Hello packet to stop and the packet to be
dropped.
3.2 Type-7 LSAs: NSSA External Link-State Advertisements
External routes are imported into NSSAs as type-7 LSAs by the
NSSA's AS boundary routers. That is, type-7 LSAs are originated
by the NSSA's AS boundary routers that have an interface associ-
ated with the NSSA and are exchanging routing information with
routers belonging to another AS (or importing static routes). As
with type-5 LSAs a separate LSA is originated for each destina-
tion network. The link-state database must therefore be expanded
to contain a type-7 LSA.
Type 7-LSAs are identical to type-5 LSAs except for the following
(see section 12.3.4 "AS external links" in the OSPF specifica-
tion).
1. The type field in the LSA header is 7.
2. Type-7 LSAs are only flooded within the NSSA.
The flooding of type-7 LSAs follow the same rules
Coltun & Fuller [Page 5]
Internet Draft OSPF NSSA Option October 1992
as the flooding of type 1-4 LSAs.
3. Type-7 LSAs are kept within the NSSA's LSDB (are area
specific) whereas because type-5 LSAs are flooded to
all type-5 capable areas, type-5 LSAs have global scope
in the router's LSDB.
4. At the area border router, selected type-7 LSAs are
translated into type 5-LSAs and flooded into the
backbone.
5. Type 7 LSAs have a propagate (P) bit which is
used to flag the area border router to translate the
type-7 LSA into a type-5 LSA. Examples of how the P-bit
is used for loop avoidance are in the following sections.
6. Those type-7 LSAs that are to be translated into type-5
LSAs must have their forwarding address set;
all type-5 LSAs that have been translated from type-7 LSAs
must contain a forwarding address.
The forwarding address contained in type-5 LSAs will
result in more efficient routing to the AS external
networks when there are multiple NSSA area
border routers. Having the forwarding address in the
type-7 LSAs will ease the translation of type-7 into
type-5 LSAs as the NSSA area border router will
not be required to compute the forwarding address.
If the network between the NSSA AS boundary router and the
adjacent AS is advertised into OSPF as an internal OSPF
route, the forwarding address should be the next hop
address as is currently done in type-5 LSAs, but unlike
type-5 LSAs if the intervening network is not advertised
into OSPF as an internal OSPF route, the forwarding
address should be any one of the router's active OSPF
interface addresses.
Type-5 and type-7 metrics and path types are directly comparable.
3.3 Originating Type-7 LSAs
NSSA AS boundary routers may originate type-7 LSAs. All NSSA
area border routers must also be AS boundary routers since they
all must have the capability of translating a type-7 LSAs into a
type-5 LSAs (see section 3.6 routes for the translation algo-
rithm). NSSA area border routers must set the E-bit (external
bit) as well as the B-bit (border bit) in its router (type-1)
LSA.
When an NSSA internal AS boundary router originates a type-7 LSA
that it wants to be translated into a type-5 LSA by the NSSA area
border router (and subsequently flooded into the backbone), it
must set the P-bit in the LS header's option field and add a
valid forwarding address in the type-7 LSA.
Coltun & Fuller [Page 6]
Internet Draft OSPF NSSA Option October 1992
If a router is attached to another AS and is also an NSSA area
border router, it may originate a both a type-5 and a type-7 LSA
for the same network. The type-5 LSA will be flooded to the
backbone (and all attached type-5 capable areas) and the type-7
will be flooded into the NSSA. If this is the case, the P-bit
must be reset in the type-7 NSSA so the type-7 LSA isn't again
translated into a type-5 LSA by another NSSA area border router.
A type-7 default route (network 0.0.0.0) may be originated into
the NSSA by an NSSA area border router or by an NSSA AS boundary
router which is internal to the NSSA. The type-7 default route
originated by the NSSA area border router must have the P-bit
reset so that the default route originated by the NSSA area
border router will not find its way out of the NSSA into the rest
of the AS system via another NSSA area border router. The type-7
default originated by an NSSA AS boundary router which is inter-
nal to the NSSA may have the P-bit set. Type-7 routes which are
originated by the NSSA area border router will not get added to
other NSSA area border router's routing table.
A default route must not be injected into the NSSA as a summary
(type-3) LSA as in the stub area case. The reason for this is
that the preferred summary default route would be chosen over all
more specific type-7 route. Because summary routes are preferred
to external routes and to ensure that summary routes are chosen
over external within the NSSA, all summary routes (unlike stub
areas in which this is optional) must be imported into an NSSA.
3.4 Calculating Type-7 AS External Routes
This section is very similar to section 16.4 (Calculating AS
external routes) in the OSPF specification. An NSSA area border
router should examine both type-5 LSAs and type-7 LSAs if either
type-5 or type-7 routes need to be updated. Type-7 LSAs should
be examined after type-5 LSAs. An NSSA internal router should
examine type-7 LSAs when type-7 routes need to be recalculated.
In relation to the steps to calculate the routing table as
presented in the OSPF specification (chapter 16, "Calculation of
the Routing Table"), type-7 LSAs should be examined after step 5
where the routes to external destinations are calculated.
Type-7 routes are calculated by examining type-7 LSAs. Each of
LSAs are considered in turn. Most type-7 LSAs describe routes to
specific IP destinations. A type-7 LSA can also describe a
default route for the NSSA (destination = DefaultDestination).
For each type-7 LSA:
1. If the metric specified by the LSA is LSInfinity, the
age of the LSA equals MaxAge or the advertising router
field is equal to this router's router ID, examine the
next advertisement.
2. Call the destination described by the LSA N. Look up the
Coltun & Fuller [Page 7]
Internet Draft OSPF NSSA Option October 1992
routing table entry for the AS boundary router (ASBR) that
originated the LSA. If no entry exists for the ASBR
(i.e., ASBR is unreachable), do nothing with this LSA and
consider the next in the list.
If the destination is the default route (destination =
DefaultDestination) and if the originator of the LSA and
the calculating router are both NSSA are border routers
do nothing with this LSA and consider the next in the list.
Else, this LSA describes an AS external path to destination
N. If the forwarding address (as specified in the forwarding
address field of the LSA) is 0.0.0.0, the packets routed
to the external destination N will be routed to the
originating ASBR. If the forwarding address is not 0.0.0.0,
look up the forwarding address in the routing table. Packets
routed to the external destination N will be routed within
the NSSA to the this forwarding address. An intra-area path
must therefore exist to the forwarding address. If no such
path exists, do nothing with the LSA and consider the next
in the list.
Call the routing table distance to the forwarding address
(or the distance to the originating ASBR if the forwarding
address is 0.0.0.0) X, and the cost specified in the type-7
LSA Y. X is in terms of the link-state metric, and Y is a
Type-1 or external metric.
3. Now, look up the routing table entry for the destination
N. If no entries exist for N, install the AS external path
to N, with the next hop equal to the list of next hops to
the forwarding address/ASBR, and the advertising router equal
to ASBR. If the external metric type is 1, then the
path-type is set to Type-1 external and the cost is equal
to X + Y. If the external metric type is 2, the path-type
is set to Type-2 external, the link-state component of the
route's cost is X, and the Type-2 cost is Y.
4. Else, if the paths present in the table are not Type-1 or
Type-2 external paths, do nothing (AS external paths have
the lowest priority).
5. Otherwise, compare the cost of this new AS external path
to the ones present in the table. Note that type-5 and
type-7 routes are directly comparable. Type-1 external
paths are always shorter than Type-2 external paths.
Type-1 external paths are compared by looking at the sum
of the distance to the forwarding address/ASBR and the
advertised Type-1 paths (X+Y). Type-2 external paths are
compared by looking at the advertised Type-2 metrics,
and then if necessary, the distance to the forwarding
address/ASBR.
When a type-5 LSA and a type-7 LSA are found to have the
same type and an equal distance, the following priorities
Coltun & Fuller [Page 8]
Internet Draft OSPF NSSA Option October 1992
apply (listed from highest to lowest) for breaking the tie.
a. Any type 5 LSA.
b. A type-7 LSA with the P-bit set and the forwarding
address non-zero.
c. Any other type-7 LSA.
If the new path is shorter, it replaces the present paths
in the routing table entry. If the new path is the same
cost, it is added to the routing table entry's list of
paths.
3.5 Incremental Updates
Incremental updates for type-7 LSAs should be treated the same as
incremental updates for type-5 LSAs (see section 16.6 of the OSPF
specification). That is, if a new instance of a type-7 LSA is
received it is not necessary to recalculate the entire routing
table. If there is already an OSPF internal route to the destina-
tion represented by the type-7 LSA, no recalculation is neces-
sary. Otherwise, the procedure in the proceeding section will
have to be performed but only for the external routes (type-5 and
type-7) whose networks describe the same networks as the newly
received LSA.
4.0 Originating Type-5 LSAs
4.1 Translating Type-7 LSAs Into Type-5 LSAs
This step is performed as part of the NSSA's Dijkstra calculation
after type-5 and type-7 routes have been calculated. If the cal-
culating router is not an area border router this translation
algorithm should be skipped. All reachable area border routers
in the NSSA should now be examined noting the one with the
highest router ID. If this router has the highest router ID, it
will be the one translating type-7 LSAs into type-5 LSAs for the
NSSA, otherwise the translation algorithm should not be per-
formed.
All type-7 routes that have been added to the routing table
should be examined. If the type-7 LSA being examined has the P-
bit set and a non-zero forwarding address a new instance of a
type-5 LSA should be originated and flooded to all attached
type-5 capable areas if one of the following is true.
1. There currently is no type-5 LSA originated from this
router corresponding to the type-7 LSA.
2. The path type or the metric in the corresponding
type-5 LSA is different from the type-7 LSA.
3. The forwarding address in the corresponding
type-5 LSA is different from the type-7 LSA.
Coltun & Fuller [Page 9]
Internet Draft OSPF NSSA Option October 1992
The newly originated type-5 LSAs will describe the same network
and have the same network mask, metrics, forwarding address, and
external route tag as the type-7 LSA. The advertising router
field will be the router ID of this area border router.
4.2 Flushing Translated Type-7 LSAs
If an NSSA area border router has translated a type-7 LSA to a
type-5 LSA that should no longer be translated, the type-5 LSA
should be flushed (set to MaxAge and flooded). The translated
type-5 LSA should be flushed whenever the routing table entry
that caused the translation changes so that either the routing
table entry is unreachable or the entry's associated LSA is not a
type-7 with the P-bit set and a non-zero forwarding address.
5.0 Acknowledgements
This document was produced by the OSPF Working Group, chaired by
John Moy.
In addition, the comments of the following individuals are also
acknowledged:
Phani Jajjarvarpu cisco
Dino Farinacci cisco
Jeff Honig Cornell University
John Moy Proteon, Inc.
Doug Williams IBM
6.0 References
[OSPF] Moy, J. OSPF Version 2. RFC 1247. July 1991.
[MOSPF] Moy, J. Multicast Extensions to OSPF.
Internet Draft. September 1992.
Coltun & Fuller [Page 10]
Internet Draft OSPF NSSA Option October 1992
Appendix A: Type-7 Packet Format
0 32
-----------------------------------
| | OPTS | 7 |
| ------------------
| Link-State Header |
| |
-----------------------------------
| Network Mask |
----------------------------------- ______
|E| Tos | metric | .
----------------------------------- . repeated for each TOS
| Forwarding Address | .
----------------------------------- .
| External Route Tag | ______
-----------------------------------
The definitions of the link-state ID, network mask, metrics and
external route tag are the same as the definitions for the type-5
LSAs (see A.4.5 in the OSPF specification) except for:
The Forwarding Address
If the network between the NSSA AS boundary router and the adja-
cent AS is advertised into OSPF as an internal OSPF route, the
forwarding address should be the next hop address but if the
intervening network is not advertised into OSPF as an internal
OSPF route, the forwarding address should be any one of the
router's active OSPF interface addresses.
Appendix B: The Options Field
The OSPF options field is present in OSPF Hello packets, Database
Description packets and all link-state advertisements. See appen-
dix A.2 in the OSPF specification for a description of option
field.
------------------------------------
| * | * | * | * | N/P | MC | E | T |
------------------------------------
The Type-7 LSA options field
T-bit: The T-bit describes the router's TOS capability.
E-bit: Type-5 AS external link advertisements are not
flooded into/through OSPF stub and NSSA areas.
The E-bit ensures that all members of a stub area
agree on that area configuration. The E-bit is
meaningful only in OSPF Hello packets. When the
E-bit is reset in the Hello packet sent out a
Coltun & Fuller [Page 11]
Internet Draft OSPF NSSA Option October 1992
particular interface, it means that the router
will neither send nor receive type-5 AS external
link state advertisements on that interface (in other
words, the interface connects to a stub area). Two
routers will not become neighbors unless they agree
on the state of the E-bit.
MC-bit: The MC-bit describes the multicast capability of the
various pieces of the OSPF routing domain [MOSPF].
N-bit: The N-bit describes the the router's NSSA capability.
The N-bit is used only in Hello packets and ensures
that all members of an NSSA agree on that area's
configuration. When the N-bit is reset in the Hello
packet sent out a particular interface, it means
that the router will neither send nor receive
type-7 LSAs on that interface. Two routers will not
form an adjacency unless they agree on the state
of the N-bit. If the N-bit is set in the options
field, the E-bit must be reset.
P-bit: The P-bit is used only in the type-7 LSA header.
It flags the NSSA area border router to translate the
type-7 LSA into a type-5 LSA.
Appendix C: Configuration Parameters
Appendix C.2 in the OSPF specification lists the area parameters.
Area ID, list of address ranges and authentication type remain
unchanged. For NSSAs the external capabilities of the area must
be set to accept type-7 external routes. Additionally there must
be a way of configuring the NSSA area border router to send a
default route into the NSSA using a specific metric (type-1 or
type-2 and the actual cost).
Coltun & Fuller [Page 12]