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A.4.5 AS external link advertisements Connected: An Internet Encyclopedia
A.4.5 AS external link advertisements

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A.4.5 AS external link advertisements

A.4.5 AS external link advertisements

AS external link advertisements are the 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) which is external to the AS. For details concerning the construction of AS external link advertisements, see Section 12.4.3.

AS external link advertisements usually describe a particular external destination. For these advertisements the Link State ID field specifies an IP network number (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix F for details). AS external link 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 State ID is always set to DefaultDestination ( and the Network Mask is set to

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |            LS age             |     Options   |      5        |
       |                        Link State ID                          |
       |                     Advertising Router                        |
       |                     LS sequence number                        |
       |         LS checksum           |             length            |
       |                         Network Mask                          |
       |E|    TOS      |                  metric                       |
       |                      Forwarding address                       |
       |                      External Route Tag                       |
       |                              ...                              |

Separate costs may be advertised for each IP Type of Service. The encoding of TOS in OSPF link state advertisements is described in Section 12.3. 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.

Network Mask

The IP address mask for the advertised destination. For example, when advertising a class A network the mask 0xff000000 would be used.

For each specified Type of Service, the following fields are defined. The number of TOS routes included can be calculated from the link state advertisement header's length field. Values for TOS 0 must be specified; they are listed first. Other values must be listed in order of increasing TOS encoding. For example, the cost for TOS 16 must always follow the cost for TOS 8 when both are specified.

bit E

The type of external metric. If bit E is set, the metric specified is a Type 2 external metric. This means the metric is considered larger than any link state path. If bit E is zero, the specified metric is a Type 1 external metric. This means that is is comparable directly (without translation) 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, data traffic will be forwarded instead to the advertisement's originator (i.e., the responsible AS boundary router).


The Type of Service that the following cost concerns. The encoding of TOS in OSPF link state advertisements is described in Section 12.3.


The cost of this route. Interpretation depends on the external type indication (bit E above).

External Route Tag

A 32-bit field attached to each external route. This is not used by the OSPF protocol itself. It may be used to communicate information between AS boundary routers; the precise nature of such information is outside the scope of this specification.

Next: B. Architectural Constants

Connected: An Internet Encyclopedia
A.4.5 AS external link advertisements


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