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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
103
Link-State Age
Options
Link-State Type
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
Routers Links Advertisements
Reserved
Reserved
Number of Links
Link ID
Link Data
Type
No. Of TOS
TOS 0 Metric
TOS
Metric
...
TOS
Metric
...
Link ID
Link Data
0
0
V
B
E
In router links advertisements, the Link State ID field is set to the router’s OSPF Router ID.
The T – bit is set in the advertisement’s Option field if and only if the router is able to
calculate a separate set of routes for each IP Type of Service (TOS).
Router links
advertisements are flooded throughout a single area only.
Field
Description
V – bit
When set, the router is an endpoint of an active virtual link that is
using the described area as a Transit area (V is for Virtual link
endpoint).
E – bit
When set, the router is an Autonomous System (AS) boundary
router (E is for External).
B – bit
When set, the router is an area border router (B is for Border).
Number of Links
The number of router links described by this advertisement.
This
must be the total collection of router links to the area.
Routers Links Advertisement
The following fields are used to describe each router link.
Each router link is typed.
The
Type field indicates the kind of link being described.
It may be a link to a transit network, to
another router or to a stub network.
The values of all the other fields describing a router link
depend on the link’s Type.
For example, each link has an associated 32-bit data field.
For
links to stub networks this field specifies the network’s IP address mask.
For other link types
the Link Data specifies the router’s associated IP interface address.
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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
104
Field
Description
Type
A quick classification of the router link.
One of the following:
Type
Description
Point-to-point connection to another router.
Connection to a transit network.
Connection to a stub network.
Virtual link.
Link ID
Identifies the object that this router link connects to.
Value
depends on the link’s Type.
When connecting to an object that
also originates a link state advertisement (i.e. another router or a
transit network) the Link ID is equal to the neighboring
advertisement’s Link State ID.
This provides the key for looking
up an advertisement in the link state database.
Type
Link ID
Neighboring router’s Router ID.
IP address of Designated Router.
IP network/subnet number.
Neighboring router’s Router ID
Link Data
Contents again depend on the link’s Type field.
For connections
to stub networks, it specifies the network’s IP address mask.
For
unnumbered point-to-point connection, it specifies the interface’s
MIB-II ifIndex value.
For other link types it specifies the router’s
associated IP interface address.
This latter piece of information is
needed during the routing table build process, when calculating
the IP address of the next hop.
No. of TOS
The number of different Type of Service (TOS) metrics given for
this link, not counting the required metric for TOS 0.
If no
additional TOS metrics are given, this field should be set to 0.
TOS 0 Metric
The cost of using this router link for TOS 0.
Routers Links Advertisements
For each link, separate metrics may be specified for each Type of Service (TOS).
The metric
for TOS 0 must always be included, and was discussed above.
Metrics for non-zero TOS are
described below.
Note that the cost for non-zero TOS values that are not specified defaults to
the TOS 0 cost.
Metrics must be listed in order of increasing TOS encoding.
For example,
the metric for TOS 16 must always follow the metric for TOS 8 when both are specified.
Field
Description
TOS
IP Type of Service that this metric refers to.
Metric
The cost of using this outbound router link, for traffic of the
specified TOS.
Routers Links Advertisement
−
Continued
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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
105
Network Links Advertisements
Network links advertisements are Type 2 link state advertisements.
A network links
advertisement is originated for each transit network in the area.
A transit network is a multi-
access network that has more than one attached router.
The network links advertisement is
originated by the network’s Designated router.
The advertisement describes all routers
attached to the network, including the Designated Router itself.
The advertisement’s Link
State ID field lists the IP interface address of the Designated Router.
The distance form the network to all attached routers is zero, for all TOS.
This is why the
TOS and metric fields need not be specified in the network links advertisement.
The format of the Network Links Advertisement is shown below:
Link-State Age
Options
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
Network Link Advertisements
2
Network Mask
Attached Router
Field
Description
Network Mask
The IP address mask for the network.
Attached Router
The Router Ids of each of the routers attached to the network.
Only
those routers that are fully adjacent to the Designated Router (DR)
are listed.
The DR includes itself in this list.
Network Link Advertisement
Summary Link Advertisements
Summary link advertisements are Type 3 and 4 link state advertisements.
These
advertisements are originated by Area Border routers.
A separate summary link
advertisement is made for each destination known to the router that belongs to the
Autonomous System (AS), yet is outside the area.
Type 3 link state advertisements are used when the destination is an IP network.
In this case
the advertisement’s Link State ID field is an IP network number.
When the destination is an
AS boundary router, a Type 4 advertisement is used, and the Link State ID field is the AS
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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
106
boundary router’s OSPF Router ID. Other that the difference in the Link State ID field, the
format of Type 3 and 4 link state advertisements is identical.
Link-State Age
Options
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
2
Network Mask
Summary Link Advertisements
TOS
Metric
For stub area, Type 3 summary link advertisements can also be used to describe a default
route on a per-area basis.
Default summary routes are used in stub area instead of flooding a
complete set of external routes.
When describing a default summary route, the
advertisement’s Link State ID is always set to the Default Destination
−
0.0.0.0, and the
Network Mask is set to 0.0.0.0.
Separate costs may be advertised for each IP Type of Service.
Note that the cost for TOS 0
must be included, and is always listed first.
If the T-bit is reset in the advertisement’s Option
field, only a route for TOS 0 is described by the advertisement.
Otherwise, routes for the
other TOS values are also described.
If a cost for a certain TOS is not included, its cost
defaults to that specified for TOS 0.
Field
Description
Network Mask
For Type 3 link state advertisements, this indicates the destination
network’s IP address mask.
For example, when advertising the
location of a class A network the value 0xff000000
TOS
The Type of Service that the following cost is relevant to.
Metric
The cost of this route.
Expressed in the same units as the
interface costs in the router links advertisements.
Summary Link Advertisement
Autonomous Systems External Link Advertisements
Autonomous Systems (AS) link advertisements are Type 5 link state advertisements.
These
advertisements are originated by AS boundary routers.
A separate advertisement is made for
each destination known to the router, that is external to the AS.
AS external link advertisements usually describe a particular external destination.
For these
advertisements the Link State ID field specifies an IP network number.
AS external link
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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
107
advertisements are also used to describe a default route.
Default routes are used when no
specific route exists to the destination.
When describing a default route, the Link Stat ID is
always set the Default Destination address (0.0.0.0) and the Network Mask is set to 0.0.0.0.
The format of the AS External Link Advertisement is shown below:
Link-State Age
Options
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
Network Mask
TOS
Metric
AS External Link Advertisements
5
Forwarding Address
External Route Tag
E
Field
Description
Network Mask
The IP address mask for the advertised destination.
E – bit
The type of external metric.
If the E – bit is set, the metric specified
is a Type 2 external metric.
This means the metric is considered
larger than any link state path.
If the E – bit is zero, the specified
metric is a Type 1 external metric. This means that is comparable
directly to the link state metric.
Forwarding Address
Data traffic for the advertised destination will be forwarded to this
address.
If the Forwarding Address is set to 0.0.0.0, data traffic will
be forwarded instead to the advertisement’s originator.
TOS
The Type of Service that the following cost is relevant to.
Metric
The cost of this route.
The interpretation of this metric depends on
the external type indication (the E – bit above).
External Route Tag
A 32-bit field attached to each external route.
This is not used by
the OSPF protocol itself.
AS External System Advertisement
General OSPF Settings
The
OSPF General Setting
menu allows OSPF to be enabled or disabled on the switch
−
without changing the switch’s OSPF configuration.

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