12.4.1. Router links
Connected: An Internet Encyclopedia
12.4.1. Router links
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RFC 1583
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12. Link State Advertisements
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12.4. Originating link state advertisements
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12.4.1. Router links
12.4.1. Router links
A router originates a router links advertisement for each
area that it belongs to. Such an advertisement describes
the collected states of the router's links to the area. The
advertisement is flooded throughout the particular area, and
no further.
....................................
. 192.1.2 Area 1 .
. + .
. | .
. | 3+---+1 .
. N1 |--|RT1|-----+ .
. | +---+ .
. | _______N3 .
. + / . 1+---+
. * 192.1.1 *------|RT4|
. + /_______/ . +---+
. | / | .
. | 3+---+1 / | .
. N2 |--|RT2|-----+ 1| .
. | +---+ +---+8 . 6+---+
. | |RT3|----------------|RT6|
. + +---+ . +---+
. 192.1.3 |2 . 18.10.0.6|7
. | . |
. +------------+ .
. 192.1.4 (N4) .
....................................
Figure 15: Area 1 with IP addresses shown
The format of a router links advertisement is shown in
Appendix A (Section A.4.2). The first 20 bytes of the
advertisement consist of the generic link state
advertisement header that was discussed in Section 12.1.
Router links advertisements have LS type = 1. The router
indicates whether it is willing to calculate separate routes
for each IP TOS by setting (or resetting) the T-bit of the
link state advertisement's Options field.
A router also indicates whether it is an area border router,
or an AS boundary router, by setting the appropriate bits
(bit B and bit E, respectively) in its router links
advertisements. This enables paths to those types of routers
to be saved in the routing table, for later processing of
summary link advertisements and AS external link
advertisements. Bit B should be set whenever the router is
actively attached to two or more areas, even if the router
is not currently attached to the OSPF backbone area. Bit E
should never be set in a router links advertisement for a
stub area (stub areas cannot contain AS boundary routers).
In addition, the router sets bit V in its router links
advertisement for Area A if and only if it is the endpoint
of an active virtual link using Area A as its Transit area.
This enables the other routers attached to Area A to
discover whether the area supports any virtual links (i.e.,
is a transit area).
The router links advertisement then describes the router's
working connections (i.e., interfaces or links) to the area.
Each link is typed according to the kind of attached
network. Each link is also labelled with its Link ID. This
Link ID gives a name to the entity that is on the other end
of the link. Table 18 summarizes the values used for the
Type and Link ID fields.
Link type Description Link ID
__________________________________________________
1 Point-to-point Neighbor Router ID
link
2 Link to transit Interface address of
network Designated Router
3 Link to stub IP network number
network
4 Virtual link Neighbor Router ID
Table 18: Link descriptions in the
router links advertisement.
In addition, the Link Data field is specified for each link.
This field gives 32 bits of extra information for the link.
For links to transit networks, numbered links to routers and
virtual links, this field specifies the IP interface address
of the associated router interface (this is needed by the
routing table calculation, see Section 16.1.1). For links
to stub networks, this field specifies the network's IP
address mask. For unnumbered point-to-point networks, the
Link Data field should be set to the unnumbered interface's
MIB-II [RFC 1213] ifIndex value.
Finally, the cost of using the link for output (possibly
specifying a different cost for each Type of Service) is
specified. The output cost of a link is configurable. It
must always be non-zero.
To further describe the process of building the list of link
descriptions, suppose a router wishes to build a router
links advertisement for Area A. The router examines its
collection of interface data structures. For each
interface, the following steps are taken:
- If the attached network does not belong to Area A, no
links are added to the advertisement, and the next
interface should be examined.
- Else, if the state of the interface is Down, no links
are added.
- Else, if the state of the interface is Point-to-Point,
then add links according to the following:
- If the neighboring router is fully adjacent, add a
Type 1 link (point-to-point) if this is an interface
to a point-to-point network, or add a Type 4 link
(virtual link) if this is a virtual link. The Link
ID should be set to the Router ID of the neighboring
router. For virtual links and numbered point-to-
point networks, the Link Data should specify the IP
interface address. For unnumbered point-to-point
networks, the Link Data field should specify the
interface's MIB-II [RFC 1213] ifIndex value.
- If this is a numbered point-to-point network (i.e,
not a virtual link and not an unnumbered point-to-
point network) and the neighboring router's IP
address is known, add a Type 3 link (stub network)
whose Link ID is the neighbor's IP address, whose
Link Data is the mask 0xffffffff indicating a host
route, and whose cost is the interface's configured
output cost.
- Else if the state of the interface is Loopback, add a
Type 3 link (stub network) as long as this is not an
interface to an unnumbered serial line. The Link ID
should be set to the IP interface address, the Link Data
set to the mask 0xffffffff (indicating a host route),
and the cost set to 0.
- Else if the state of the interface is Waiting, add a
Type 3 link (stub network) whose Link ID is the IP
network number of the attached network and whose Link
Data is the attached network's address mask.
- Else, there has been a Designated Router selected for
the attached network. If the router is fully adjacent
to the Designated Router, or if the router itself is
Designated Router and is fully adjacent to at least one
other router, add a single Type 2 link (transit network)
whose Link ID is the IP interface address of the
attached network's Designated Router (which may be the
router itself) and whose Link Data is the router's own
IP interface address. Otherwise, add a link as if the
interface state were Waiting (see above).
Unless otherwise specified, the cost of each link generated
by the above procedure is equal to the output cost of the
associated interface. Note that in the case of serial
lines, multiple links may be generated by a single
interface.
After consideration of all the router interfaces, host links
are added to the advertisement by examining the list of
attached hosts. A host route is represented as a Type 3
link (stub network) whose Link ID is the host's IP address
and whose Link Data is the mask of all ones (0xffffffff).
As an example, consider the router links advertisements
generated by Router RT3, as pictured in Figure 6. The area
containing Router RT3 (Area 1) has been redrawn, with actual
network addresses, in Figure 15. Assume that the last byte
of all of RT3's interface addresses is 3, giving it the
interface addresses 192.1.1.3 and 192.1.4.3, and that the
other routers have similar addressing schemes. In addition,
assume that all links are functional, and that Router IDs
are assigned as the smallest IP interface address.
RT3 originates two router links advertisements, one for Area
1 and one for the backbone. Assume that Router RT4 has been
selected as the Designated router for network 192.1.1.0.
RT3's router links advertisement for Area 1 is then shown
below. It indicates that RT3 has two connections to Area 1,
the first a link to the transit network 192.1.1.0 and the
second a link to the stub network 192.1.4.0. Note that the
transit network is identified by the IP interface of its
Designated Router (i.e., the Link ID = 192.1.1.4 which is
the Designated Router RT4's IP interface to 192.1.1.0).
Note also that RT3 has indicated that it is capable of
calculating separate routes based on IP TOS, through setting
the T-bit in the Options field. It has also indicated that
it is an area border router.
; RT3's router links advertisement for Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 2
Link ID = 192.1.1.4 ;IP address of Desig. Rtr.
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# other metrics = 0
TOS 0 metric = 1
Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network
# other metrics = 0
TOS 0 metric = 2
Next RT3's router links advertisement for the backbone is
shown. It indicates that RT3 has a single attachment to the
backbone. This attachment is via an unnumbered point-to-
point link to Router RT6. RT3 has again indicated that it
is TOS-capable, and that it is an area border router.
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's router ID
Advertising Router = 192.1.1.3 ;RT3's router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# other metrics = 0
TOS 0 metric = 8
Even though Router RT3 has indicated that it is TOS-capable
in the above examples, only a single metric (the TOS 0
metric) has been specified for each interface. Different
metrics can be specified for each TOS. The encoding of TOS
in OSPF link state advertisements is described in Section
12.3.
As an example, suppose the point-to-point link between
Routers RT3 and RT6 in Figure 15 is a satellite link. The
AS administrator may want to encourage the use of the line
for high bandwidth traffic. This would be done by setting
the metric artificially low for the appropriate TOS value.
Router RT3 would then originate the following router links
advertisement for the backbone (TOS 8 = maximize
throughput):
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# other metrics = 1
TOS 0 metric = 8
TOS = 8 ;maximize throughput
metric = 1 ;traffic preferred
Next: 12.4.2. Network links
Connected: An Internet Encyclopedia
12.4.1. Router links
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