Understand Open Shortest Path First (ospf) Design Guide

LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 203.0.113.16
LS Seq Number: 80000002
Checksum: 0x96CF
Length: 36
Network Mask: 0.0.0.0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 10
Forward Address: 0.0.0.0
External Route Tag: 10
Routing Bit Set on this LSA
LS age: 226
Options: (No TOS-capability)
LS Type: AS External Link
Link State ID: 203.0.113.1288 (External Network Number)
Advertising Router: 203.0.113.16
LS Seq Number: 80000002
Checksum: 0x93C4
Length: 36
Network Mask: 255.255.255.192
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 10
Forward Address: 0.0.0.0
External Route Tag: 0
Note the forward address. Whenever this address is 0.0.0.0 it indicates that the external routes are
reachable via the advertising router, in this case 203. 250.16.130.
This is why the identity of the ASBR is injected by ABRs into other areas which use ASBR
summary links.
This forward address is not always 0.0.0.0. In some cases, it could be the IP address of another
router on the same segment. This diagram illustrates this situation:
In this situation RTB, runs BGP with RTA, and OSPF with the rest of the domain. RTA does not

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run OSPF. RTB redistributes BGP routes into OSPF.
In accord with OSPF, RTB is an ASBR advertising external routes. The forwarding address in this
case is set to 172.16.0.11 and not to the advertising router (0.0.0.0) RT B.
There is no need to make the extra hop. Routers inside the OSPF domain must reach the
forwarding address via OSPF in order for the external routes to be put in the IP routing table.
If the forwarding address is reached via some other protocol or no t accessible, the external
entries would be in the database but not in the IP routing table.
Another situation would arise if both RTB and RTC are ASBRs (RTC runs BGP with RTA). In this
situation, in order to eliminate the duplication of the effort, one of the two routers do not advertise
(flushes) the external routes. The router with the higher RID prevails.