16. Calculation of the Routing Table
Summary-LSAs are originated by the area border routers. Each
summary-LSA in Area A is considered in turn. Remember that the
destination described by a summary-LSA is either a network (Type
3 summary-LSAs) or an AS boundary router (Type 4 summary-LSAs).
For each summary-LSA:
(1) If the cost specified by the LSA is LSInfinity, or if the
LSA's LS age is equal to MaxAge, then examine the the next
LSA.
(2) If the LSA was originated by the calculating router itself,
examine the next LSA.
(3) If it is a Type 3 summary-LSA, and the collection of
destinations described by the summary-LSA equals one of the
router's configured area address ranges (see Section 3.5),
and the particular area address range is active, then the
summary-LSA should be ignored. "Active" means that there
are one or more reachable (by intra-area paths) networks
contained in the area range.
(4) Else, call the destination described by the LSA N (for Type
3 summary-LSAs, N's address is obtained by masking the LSA's
Link State ID with the network/subnet mask contained in the
body of the LSA), and the area border originating the LSA
BR. Look up the routing table entry for BR having Area A as
its associated area. If no such entry exists for router BR
(i.e., BR is unreachable in Area A), do nothing with this
LSA and consider the next in the list. Else, this LSA
describes an inter-area path to destination N, whose cost is
the distance to BR plus the cost specified in the LSA. Call
the cost of this inter-area path IAC.
(5) Next, look up the routing table entry for the destination N.
(If N is an AS boundary router, look up the "router" routing
table entry associated with Area A). If no entry exists for
N or if the entry's path type is "type 1 external" or "type
2 external", then install the inter-area path to N, with
associated area Area A, cost IAC, next hop equal to the list
of next hops to router BR, and Advertising router equal to
BR.
(6) Else, if the paths present in the table are intra-area
paths, do nothing with the LSA (intra-area paths are always
preferred).
(7) Else, the paths present in the routing table are also
inter-area paths. Install the new path through BR if it is
cheaper, overriding the paths in the routing table.
Otherwise, if the new path is the same cost, add it to the
list of paths that appear in the routing table entry.
16.3. Examining transit areas' summary-LSAs
This step is only performed by area border routers attached to
one or more non-backbone areas that are capable of carrying
transit traffic (i.e., "transit areas", or those areas whose
TransitCapability parameter has been set to TRUE in Step 2 of
the Dijkstra algorithm (see Section 16.1).
The purpose of the calculation below is to examine the transit
areas to see whether they provide any better (shorter) paths
than the paths previously calculated in Sections 16.1 and 16.2.
Any paths found that are better than or equal to previously
discovered paths are installed in the routing table.
The calculation also determines the actual next hop(s) for those
destinations whose next hop was calculated as a virtual link in
Sections 16.1 and 16.2. After completion of the calculation
below, any paths calculated in Sections 16.1 and 16.2 that still
have unresolved virtual next hops should be discarded.
The calculation proceeds as follows. All the transit areas'
summary-LSAs are examined in turn. Each such summary-LSA
describes a route through a transit area Area A to a Network N
(N's address is obtained by masking the LSA's Link State ID with
the network/subnet mask contained in the body of the LSA) or in
the case of a Type 4 summary-LSA, to an AS boundary router N.
Suppose also that the summary-LSA was originated by an area
border router BR.
(1) If the cost advertised by the summary-LSA is LSInfinity, or
if the LSA's LS age is equal to MaxAge, then examine the
next LSA.
(2) If the summary-LSA was originated by the calculating router
itself, examine the next LSA.
(3) Look up the routing table entry for N. (If N is an AS
boundary router, look up the "router" routing table entry
associated with the backbone area). If it does not exist, or
if the route type is other than intra-area or inter-area, or
if the area associated with the routing table entry is not
the backbone area, then examine the next LSA. In other
words, this calculation only updates backbone intra-area
routes found in Section 16.1 and inter-area routes found in
Section 16.2.
(4) Look up the routing table entry for the advertising router
BR associated with the Area A. If it is unreachable, examine
the next LSA. Otherwise, the cost to destination N is the
sum of the cost in BR's Area A routing table entry and the
cost advertised in the LSA. Call this cost IAC.
(5) If this cost is less than the cost occurring in N's routing
table entry, overwrite N's list of next hops with those used
for BR, and set N's routing table cost to IAC. Else, if IAC
is the same as N's current cost, add BR's list of next hops
to N's list of next hops. In any case, the area associated
with N's routing table entry must remain the backbone area,
and the path type (either intra-area or inter-area) must
also remain the same.
It is important to note that the above calculation never makes
unreachable destinations reachable, but instead just potentially
finds better paths to already reachable destinations. The
calculation installs any better cost found into the routing
table entry, from which it may be readvertised in summary-LSAs
to other areas.
As an example of the calculation, consider the Autonomous System
pictured in Figure 17. There is a single non-backbone area
(Area 1) that physically divides the backbone into two separate
pieces. To maintain connectivity of the backbone, a virtual link
has been configured between routers RT1 and RT4. On the right
side of the figure, Network N1 belongs to the backbone. The
dotted lines indicate that there is a much shorter intra-area
........................
. Area 1 (transit) . +
. . |
. +---+1 1+---+100 |
. |RT2|----------|RT4|=========|
. 1/+---+********* +---+ |
. /******* . |
. 1/*Virtual . |
1+---+/* Link . Net|work
=======|RT1|* . | N1
+---+\ . |
. \ . |
. \ . |
. 1\+---+1 1+---+20 |
. |RT3|----------|RT5|=========|
. +---+ +---+ |
. . |
........................ +
Figure 17: Routing through transit areas
backbone path between router RT5 and Network N1 (cost 20) than
there is between Router RT4 and Network N1 (cost 100). Both
Router RT4 and Router RT5 will inject summary-LSAs for Network
N1 into Area 1.
After the shortest-path tree has been calculated for the
backbone in Section 16.1, Router RT1 (left end of the virtual
link) will have calculated a path through Router RT4 for all
data traffic destined for Network N1. However, since Router RT5
is so much closer to Network N1, all routers internal to Area 1
(e.g., Routers RT2 and RT3) will forward their Network N1
traffic towards Router RT5, instead of RT4. And indeed, after
examining Area 1's summary-LSAs by the above calculation, Router
RT1 will also forward Network N1 traffic towards RT5. Note that
in this example the virtual link enables transit data traffic to
be forwarded through Area 1, but the actual path the transit
data traffic takes does not follow the virtual link. In other
words, virtual links allow transit traffic to be forwarded
through an area, but do not dictate the precise path that the
traffic will take.
16.4. Calculating AS external routes
AS external routes are calculated by examining AS-external-LSAs.
Each of the AS-external-LSAs is considered in turn. Most AS-
external-LSAs describe routes to specific IP destinations. An
AS-external-LSA can also describe a default route for the
Autonomous System (Destination ID = DefaultDestination,
network/subnet mask = 0x00000000). For each AS-external-LSA:
(1) If the cost specified by the LSA is LSInfinity, or if the
LSA's LS age is equal to MaxAge, then examine the next LSA.
(2) If the LSA was originated by the calculating router itself,
examine the next LSA.
(3) Call the destination described by the LSA N. N's address is
obtained by masking the LSA's Link State ID with the
network/subnet mask contained in the body of the LSA. Look
up the routing table entries (potentially one per attached
area) for the AS boundary router (ASBR) that originated the
LSA. If no entries exist for router ASBR (i.e., ASBR is
unreachable), do nothing with this LSA and consider the next
in the list.
Else, this LSA describes an AS external path to destination
N. Examine the forwarding address specified in the AS-
external-LSA. This indicates the IP address to which
packets for the destination should be forwarded.
If the forwarding address is set to 0.0.0.0, packets should
be sent to the ASBR itself. Among the multiple routing table
entries for the ASBR, select the preferred entry as follows.
If RFC1583Compatibility is set to "disabled", prune the set
of routing table entries for the ASBR as described in
Section 16.4.1. In any case, among the remaining routing
table entries, select the routing table entry with the least
cost; when there are multiple least cost routing table
entries the entry whose associated area has the largest OSPF
Area ID (when considered as an unsigned 32-bit integer) is
chosen.
If the forwarding address is non-zero, look up the
forwarding address in the routing table.[24] The matching
routing table entry must specify an intra-area or inter-area
path; if no such path exists, do nothing with the LSA and
consider the next in the list.
(4) Let X be the cost specified by the preferred routing table
entry for the ASBR/forwarding address, and Y the cost
specified in the LSA. X is in terms of the link state
metric, and Y is a type 1 or 2 external metric.
(5) Look up the routing table entry for the destination N. If
no entry exists for N, install the AS external path to N,
with next hop equal to the list of next hops to the
forwarding address, and 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.
(6) Compare the AS external path described by the LSA with the
existing paths in N's routing table entry, as follows. If
the new path is preferred, it replaces the present paths in
N's routing table entry. If the new path is of equal
preference, it is added to N's routing table entry's list of
paths.
(a) Intra-area and inter-area paths are always preferred
over AS external paths.
(b) Type 1 external paths are always preferred over type 2
external paths. When all paths are type 2 external
paths, the paths with the smallest advertised type 2
metric are always preferred.
(c) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
and RFC1583Compatibility is set to "disabled", select
the preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section
16.4.1.
(d) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
select the preferred path based on a least cost
comparison. Type 1 external paths are compared by
looking at the sum of the distance to the forwarding
address and the advertised type 1 metric (X+Y). Type 2
external paths advertising equal type 2 metrics are
compared by looking at the distance to the forwarding
addresses.
16.4.1. External path preferences
When multiple intra-AS paths are available to
ASBRs/forwarding addresses, the following rules indicate
which paths are preferred. These rules apply when the same
ASBR is reachable through multiple areas, or when trying to
decide which of several AS-external-LSAs should be
preferred. In the former case the paths all terminate at the
same ASBR, while in the latter the paths terminate at
separate ASBRs/forwarding addresses. In either case, each
path is represented by a separate routing table entry as
defined in Section 11.
This section only applies when RFC1583Compatibility is set
to "disabled".
The path preference rules, stated from highest to lowest
preference, are as follows. Note that as a result of these
rules, there may still be multiple paths of the highest
preference. In this case, the path to use must be determined
based on cost, as described in Section 16.4.
o Intra-area paths using non-backbone areas are always the
most preferred.
o The other paths, intra-area backbone paths and inter-
area paths, are of equal preference.
16.5. Incremental updates -- summary-LSAs
When a new summary-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination
described by the summary-LSA N (N's address is obtained by
masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA), and let Area A be the area to
which the LSA belongs. There are then two separate cases:
Case 1: Area A is the backbone and/or the router is not an area
border router.
In this case, the following calculations must be performed.
First, if there is presently an inter-area route to the
destination N, N's routing table entry is invalidated,
saving the entry's values for later comparisons. Then the
calculation in Section 16.2 is run again for the single
destination N. In this calculation, all of Area A's
summary-LSAs that describe a route to N are examined. In
addition, if the router is an area border router attached to
one or more transit areas, the calculation in Section 16.3
must be run again for the single destination. If the
results of these calculations have changed the cost/path to
an AS boundary router (as would be the case for a Type 4
summary-LSA) or to any forwarding addresses, all AS-
external-LSAs will have to be reexamined by rerunning the
calculation in Section 16.4. Otherwise, if N is now newly
unreachable, the calculation in Section 16.4 must be rerun
for the single destination N, in case an alternate external
route to N exists.
Case 2: Area A is a transit area and the router is an area
border router.
In this case, the following calculations must be performed.
First, if N's routing table entry presently contains one or
more inter-area paths that utilize the transit area Area A,
these paths should be removed. If this removes all paths
from the routing table entry, the entry should be
invalidated. The entry's old values should be saved for
later comparisons. Next the calculation in Section 16.3 must
be run again for the single destination N. If the results of
this calculation have caused the cost to N to increase, the
complete routing table calculation must be rerun starting
with the Dijkstra algorithm specified in Section 16.1.
Otherwise, if the cost/path to an AS boundary router (as
would be the case for a Type 4 summary-LSA) or to any
forwarding addresses has changed, all AS-external-LSAs will
have to be reexamined by rerunning the calculation in
Section 16.4. Otherwise, if N is now newly unreachable, the
calculation in Section 16.4 must be rerun for the single
destination N, in case an alternate external route to N
exists.
16.6. Incremental updates -- AS-external-LSAs
When a new AS-external-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination
described by the AS-external-LSA N. N's address is obtained by
masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA. If there is already an intra-
area or inter-area route to the destination, no recalculation is
necessary (internal routes take precedence).
Otherwise, the procedure in Section 16.4 will have to be
performed, but only for those AS-external-LSAs whose destination
is N. Before this procedure is performed, the present routing
table entry for N should be invalidated.
16.7. Events generated as a result of routing table changes
Changes to routing table entries sometimes cause the OSPF area
border routers to take additional actions. These routers need
to act on the following routing table changes:
o The cost or path type of a routing table entry has changed.
If the destination described by this entry is a Network or
AS boundary router, and this is not simply a change of AS
external routes, new summary-LSAs may have to be generated
(potentially one for each attached area, including the
backbone). See Section 12.4.3 for more information. If a
previously advertised entry has been deleted, or is no
longer advertisable to a particular area, the LSA must be
flushed from the routing domain by setting its LS age to
MaxAge and reflooding (see Section 14.1).
o A routing table entry associated with a configured virtual
link has changed. The destination of such a routing table
entry is an area border router. The change indicates a
modification to the virtual link's cost or viability.
If the entry indicates that the area border router is newly
reachable, the corresponding virtual link is now
operational. An InterfaceUp event should be generated for
the virtual link, which will cause a virtual adjacency to
begin to form (see Section 10.3). At this time the virtual
link's IP interface address and the virtual neighbor's
Neighbor IP address are also calculated.
If the entry indicates that the area border router is no
longer reachable, the virtual link and its associated
adjacency should be destroyed. This means an InterfaceDown
event should be generated for the associated virtual link.
If the cost of the entry has changed, and there is a fully
established virtual adjacency, a new router-LSA for the
backbone must be originated. This in turn may cause further
routing table changes.
16.8. Equal-cost multipath
The OSPF protocol maintains multiple equal-cost routes to all
destinations. This can be seen in the steps used above to
calculate the routing table, and in the definition of the
routing table structure.
Each one of the multiple routes will be of the same type
(intra-area, inter-area, type 1 external or type 2 external),
cost, and will have the same associated area. However, each
route may specify a separate next hop and Advertising router.
There is no requirement that a router running OSPF keep track of
all possible equal-cost routes to a destination. An
implementation may choose to keep only a fixed number of routes
to any given destination. This does not affect any of the
algorithms presented in this specification.
Footnotes
[1]The graph's vertices represent either routers, transit networks,
or stub networks. Since routers may belong to multiple areas, it is
not possible to color the graph's vertices.
[2]It is possible for all of a router's interfaces to be unnumbered
point-to-point links. In this case, an IP address must be assigned
to the router. This address will then be advertised in the router's
router-LSA as a host route.
[3]Note that in these cases both interfaces, the non-virtual and the
virtual, would have the same IP address.
[4]Note that no host route is generated for, and no IP packets can
be addressed to, interfaces to unnumbered point-to-point networks.
This is regardless of such an interface's state.
[5]It is instructive to see what happens when the Designated Router
for the network crashes. Call the Designated Router for the network
RT1, and the Backup Designated Router RT2. If Router RT1 crashes
(or maybe its interface to the network dies), the other routers on
the network will detect RT1's absence within RouterDeadInterval
seconds. All routers may not detect this at precisely the same
time; the routers that detect RT1's absence before RT2 does will,
for a time, select RT2 to be both Designated Router and Backup
Designated Router. When RT2 detects that RT1 is gone it will move
itself to Designated Router. At this time, the remaining router
having highest Router Priority will be selected as Backup Designated
Router.
[6]On point-to-point networks, the lower level protocols indicate
whether the neighbor is up and running. Likewise, existence of the
neighbor on virtual links is indicated by the routing table
calculation. However, in both these cases, the Hello Protocol is
still used. This ensures that communication between the neighbors
is bidirectional, and that each of the neighbors has a functioning
routing protocol layer.
[7]When the identity of the Designated Router is changing, it may be
quite common for a neighbor in this state to send the router a
Database Description packet; this means that there is some momentary
disagreement on the Designated Router's identity.
[8]Note that it is possible for a router to resynchronize any of its
fully established adjacencies by setting the adjacency's state back
to ExStart. This will cause the other end of the adjacency to
process a SeqNumberMismatch event, and therefore to also go back to
ExStart state.
[9]The address space of IP networks and the address space of OSPF
Router IDs may overlap. That is, a network may have an IP address
which is identical (when considered as a 32-bit number) to some
router's Router ID.
[10]"Discard" entries are necessary to ensure that route
summarization at area boundaries will not cause packet looping.
[11]It is assumed that, for two different address ranges matching
the destination, one range is more specific than the other. Non-
contiguous subnet masks can be configured to violate this
assumption. Such subnet mask configurations cannot be handled by the
OSPF protocol.
[12]MaxAgeDiff is an architectural constant. It indicates the
maximum dispersion of ages, in seconds, that can occur for a single
LSA instance as it is flooded throughout the routing domain. If two
LSAs differ by more than this, they are assumed to be different
instances of the same LSA. This can occur when a router restarts
and loses track of the LSA's previous LS sequence number. See
Section 13.4 for more details.
[13]When two LSAs have different LS checksums, they are assumed to
be separate instances. This can occur when a router restarts, and
loses track of the LSA's previous LS sequence number. In the case
where the two LSAs have the same LS sequence number, it is not
possible to determine which LSA is actually newer. However, if the
wrong LSA is accepted as newer, the originating router will simply
originate another instance. See Section 13.4 for further details.
[14]There is one instance where a lookup must be done based on
partial information. This is during the routing table calculation,
when a network-LSA must be found based solely on its Link State ID.
The lookup in this case is still well defined, since no two
network-LSAs can have the same Link State ID.
[15]This is the way RFC 1583 specified point-to-point
representation. It has three advantages: a) it does not require
allocating a subnet to the point-to-point link, b) it tends to bias
the routing so that packets destined for the point-to-point
interface will actually be received over the interface (which is
useful for diagnostic purposes) and c) it allows network
bootstrapping of a neighbor, without requiring that the bootstrap
program contain an OSPF implementation.
[16]This is the more traditional point-to-point representation used
by protocols such as RIP.
[17]This clause covers the case: Inter-area routes are not
summarized to the backbone. This is because inter-area routes are
always associated with the backbone area.
[18]This clause is only invoked when a non-backbone Area A supports
transit data traffic (i.e., has TransitCapability set to TRUE). For
example, in the area configuration of Figure 6, Area 2 can support
transit traffic due to the configured virtual link between Routers
RT10 and RT11. As a result, Router RT11 need only originate a single
summary-LSA into Area 2 (having the collapsed destination N9-
N11,H1), since all of Router RT11's other eligible routes have next
hops belonging to Area 2 itself (and as such only need be advertised
by other area border routers; in this case, Routers RT10 and RT7).
[19]By keeping more information in the routing table, it is possible
for an implementation to recalculate the shortest path tree for only
a single area. In fact, there are incremental algorithms that allow
an implementation to recalculate only a portion of a single area's
shortest path tree [Ref1]. However, these algorithms are beyond the
scope of this specification.
[20]This is how the Link state request list is emptied, which
eventually causes the neighbor state to transition to Full. See
Section 10.9 for more details.
[21]It should be a relatively rare occurrence for an LSA's LS age to
reach MaxAge in this fashion. Usually, the LSA will be replaced by
a more recent instance before it ages out.
[22]Strictly speaking, because of equal-cost multipath, the
algorithm does not create a tree. We continue to use the "tree"
terminology because that is what occurs most often in the existing
literature.
[23]Note that the presence of any link back to V is sufficient; it
need not be the matching half of the link under consideration from V
to W. This is enough to ensure that, before data traffic flows
between a pair of neighboring routers, their link state databases
will be synchronized.
[24]When the forwarding address is non-zero, it should point to a
router belonging to another Autonomous System. See Section 12.4.4
for more details.
References
[Ref1] McQuillan, J., I. Richer and E. Rosen, "ARPANET Routing
Algorithm Improvements", BBN Technical Report 3803, April
1978.
[Ref2] Digital Equipment Corporation, "Information processing
systems -- Data communications -- Intermediate System to
Intermediate System Intra-Domain Routing Protocol", October
1987.
[Ref3] McQuillan, J., et.al., "The New Routing Algorithm for the
ARPANET", IEEE Transactions on Communications, May 1980.
[Ref4] Perlman, R., "Fault-Tolerant Broadcast of Routing
Information", Computer Networks, December 1983.
[Ref5] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[Ref6] McKenzie, A., "ISO Transport Protocol specification ISO DP
8073", RFC 905, April 1984.
[Ref7] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, May 1988.
[Ref8] McCloghrie, K., and M. Rose, "Management Information Base
for network management of TCP/IP-based internets: MIB-II",
STD 17, RFC 1213, March 1991.
[Ref9] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[Ref10] Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC1519, September 1993.
[Ref11] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.
[Ref12] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC 1349, July 1992.
[Ref13] Leiner, B., et.al., "The DARPA Internet Protocol Suite", DDN
Protocol Handbook, April 1985.
[Ref14] Bradley, T., and C. Brown, "Inverse Address Resolution
Protocol", RFC 1293, January 1992.
[Ref15] deSouza, O., and M. Rodrigues, "Guidelines for Running OSPF
Over Frame Relay Networks", RFC 1586, March 1994.
[Ref16] Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", ACM Computer Communications Review, Volume 19,
Number 2, pp. 32-38, April 1989.
[Ref17] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[Ref18] Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
1994.
[Ref19] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC 1587,
March 1994.
[Ref20] Ferguson, D., "The OSPF External Attributes LSA", work in
progress.
[Ref21] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
1793, April 1995.
[Ref22] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[Ref23] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
4)", RFC 1771, March 1995.
[Ref24] Hinden, R., "Internet Routing Protocol Standardization
Criteria", BBN, October 1991.
[Ref25] Moy, J., "OSPF Version 2", RFC 2178, July 1997.
[Ref26] Rosen, E., "Vulnerabilities of Network Control Protocols: An
Example", Computer Communication Review, July 1981.