앞에서 LSA Type 5가 E1과 E2로 구분되며, E1은 ‘External metric’ + ‘Forward metic’이 Routing table에 세팅된다고 설명을 하였다. 그런가 하면, E2는 ‘External metric’ 값이 다른 경우 ‘Forward metic’이 가장 작은 경로가 bestpath가 된다고 설명하였다.

  그럼, ‘Forward metric’은 무엇인가? ‘Forward metric’은 ‘Forward address’ 까지의 metric 값이다. 만일, ‘Forward address’가 설정되어 있지 않으면 ASBR까지의 metric 값이 ‘Forward metric’ 값이 된다. 이제는 ‘Forward address’가 무엇인지만 확인하면 될 것이다.

  ‘Forward address’는 LSA Type 5 내에 들어 있는 External Network으로 가기 위해 어느 OSPF internal address를 찾아가면 되는지를 나타내는 것이다. 다음 Topology를 가지고 확인해 보자.

  현재 ASBR 2대가 EIGRP Network 정보를 OSPF로 redistribute 하였다. 이 경우 EIGRP Network에 대하여 ASBR까지 최소 metric 값을 구하여 ‘Forward metric’을 계산하면 된다. 그러면 ‘Forward metic’은 다음과 같이 될 것이다.

Node ASBR R1 metric ASBR R2 metric Forwarding metric
R3 1 2 1 (R1)
R4 2 1 1 (R2)
R5 2 3 2 (R1)
R6 3 2 2 (R2)

  그래서, 각 Router에서 Routing table을 보면 다음과 같다.

R3#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 1
  Last update from 10.10.13.1 on FastEthernet0/0, 00:00:28 ago
  Routing Descriptor Blocks:
  * 10.10.13.1, from 1.1.1.1, 00:00:28 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1
R4#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 1
  Last update from 10.10.24.2 on FastEthernet0/0, 00:07:44 ago
  Routing Descriptor Blocks:
  * 10.10.24.2, from 2.2.2.2, 00:07:44 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1
R5#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.10.35.3 on FastEthernet0/0, 00:00:23 ago
  Routing Descriptor Blocks:
  * 10.10.35.3, from 1.1.1.1, 00:00:23 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1
R6#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.10.46.4 on FastEthernet0/0, 00:06:07 ago
  Routing Descriptor Blocks:
  * 10.10.46.4, from 2.2.2.2, 00:06:07 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1

  그런데, R1은 R7과 10Mbps(Cost=10)로 연결되어 있고, R2는 R7과 100Mbps(Cost=1)로 연결되어 있다. 그렇다면 실제로 EIGRP Network으로 가는 경로는 R2가 bestpath가 되어야 할 것이다. 여기서 R1과 R2에서 ‘7.7.7.7/32’ Network에 대해 Routing table을 확인해 보자.

R1#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "eigrp 1", distance 90, metric 409600, type internal
  Redistributing via ospf 1, eigrp 1
  Advertised by ospf 1 subnets
  Last update from 100.100.100.100 on FastEthernet0/0, 00:17:59 ago
  Routing Descriptor Blocks:
  * 100.100.100.100, from 100.100.100.100, 00:17:59 ago, via FastEthernet0/0
      Route metric is 409600, traffic share count is 1
      Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1
R2#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "eigrp 1", distance 90, metric 409600, type internal
  Redistributing via ospf 1, eigrp 1
  Advertised by ospf 1 subnets
  Last update from 200.200.200.200 on FastEthernet0/0, 00:09:12 ago
  Routing Descriptor Blocks:
  * 200.200.200.200, from 200.200.200.200, 00:09:12 ago, via FastEthernet0/0
      Route metric is 409600, traffic share count is 1
      Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

  ‘7.7.7.7/32’ Network에 대하여 R1은 next-hop을 ‘100.100.100.100’으로 알고 있고, R2는 next-hop을 ‘200.200.200.200’으로 알고 있다. 여기서 next-hop은 R1과 R7 사이 Network과 R2와 R7 사이의 Network에 포함되어 있는 address일 것이다.

  그럼, R1과 R2에서 R7과 연결된 구간을 OSPF로 광고해주면 해당 Network ‘100.100.100.0/24’와 ‘200.200.200.0/24’는 OSPF internal network이 될 것이다. 이렇게 OSPF external network의 next-hop이 존재하는 Network을 OSPF internal network으로 설정하게 되면 next-hop 정보가 ‘Forward address’로 설정된다.

OSPF external network의 next-hop network을 OSPF로 광고하기 전

show ip ospf database external 7.7.7.7

            OSPF Router with ID (5.5.5.5) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 1726
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 7.7.7.7 (External Network Number )
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000001
  Checksum: 0x86F9
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0 
        Metric: 20 
        Forward Address: 0.0.0.0
        External Route Tag: 0

  LS age: 1673
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 7.7.7.7 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0x6814
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0 
        Metric: 20 
        Forward Address: 0.0.0.0
        External Route Tag: 0

OSPF external network의 next-hop network을 OSPF로 광고한 후

show ip ospf database external 7.7.7.7

            OSPF Router with ID (5.5.5.5) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 13
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 7.7.7.7 (External Network Number )
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000002
  Checksum: 0x20CD
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0 
        Metric: 20 
        Forward Address: 100.100.100.100
        External Route Tag: 0

  LS age: 6
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 7.7.7.7 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000002
  Checksum: 0x9DBA
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0 
        Metric: 20 
        Forward Address: 200.200.200.200
        External Route Tag: 0

  이제 Forward address가 설정되었으니 Forward metric이 변경되고, R7과 연결된 구간의 cost 값이 적용되었으니 bestpath도 변경될 것이다.

Node 100.100.100.100 200.200.200.200 Forwarding metric
R3 11 3 3 (200.200.200.200)
R4 12 2 2 (200.200.200.200)
R5 12 4 4 (200.200.200.200)
R6 13 3 3 (200.200.200.200)

  그래서 각 Router에서 Routing table을 보면 다음과 같다.

R3#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 3
  Last update from 10.10.34.4 on FastEthernet1/0, 00:17:23 ago
  Routing Descriptor Blocks:
  * 10.10.34.4, from 2.2.2.2, 00:17:23 ago, via FastEthernet1/0
      Route metric is 20, traffic share count is 1
R4#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.10.24.2 on FastEthernet0/0, 00:44:17 ago
  Routing Descriptor Blocks:
  * 10.10.24.2, from 2.2.2.2, 00:44:17 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1
R5#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 4
  Last update from 10.10.35.3 on FastEthernet0/0, 00:16:48 ago
  Routing Descriptor Blocks:
    10.10.56.6, from 2.2.2.2, 00:16:48 ago, via FastEthernet0/1
      Route metric is 20, traffic share count is 1
  * 10.10.35.3, from 2.2.2.2, 00:16:48 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1
R6#show ip route 7.7.7.7
Routing entry for 7.7.7.7/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 3
  Last update from 10.10.46.4 on FastEthernet0/0, 00:16:48 ago
  Routing Descriptor Blocks:
  * 10.10.46.4, from 2.2.2.2, 00:16:48 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1