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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.
Netw
ork Links Advertisements
Netw
ork 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:
Figure 6- 108.Network Link Advertisements
Field Description
Network Mask
The IP address mask for the network.
Attached Router The Router Ids of each of the r
outers attached to the netw
ork.
Only those routers that are fully adjacent to
the Designated Router (DR) are listed.The DR includes itself in this list.
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 boundary router’s OSPF Router ID.Other
that the diff
er
ence in the Link State ID field,
the format of Type 3 and 4 link state advertisements is identical.
115
Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Octets
Network Link Advertisements
Link-State ID
Advertising Router
Link-State Checksum Length
Link-State Sequence Number
Link State Age
Options
2
01 23
4
Network Mask
Attached Router