1. 왜 BDR이 필요하고, Multi-Access 망에서는 왜 Subnet이 반드시 같아야 Neighbor가 되는가?
  2. DR/BDR을 선출하는 기준은 무엇이고, 언제 DR/BDR이 변경되는가?
  3. DR/BDR이 존재하는 Multi-Access 구조에서 Update는 어떠한 과정으로 전달되는가?

Routing Protocol과 Multicast Address

  PC나 서버, 네트워크 장비는 IGMP Join한 Multicast address에 대해서만 Multicast Packet을 처리한다. 그러면 OSPF가 사용하는 Multicast address인 ‘224.0.0.5’도 Interface에서 IGMP Join을 하여야 처리할 수 있을 것이다. 그럼, 장비에 OSPF, EIGPR, RIP을 따로 올린 후에 Interface 정보를 확인해 보자.

OSPF Routing Protocol Enable

R1(config)#router ospf 1
R1(config-router)#network 0.0.0.0 255.255.255.255 area 0

R1#show ip int fa0/0
FastEthernet0/0 is up, line protocol is up
  Internet address is 10.10.100.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.5
  Outgoing access list is not set
  Inbound  access list is not set
  --- (생략) ---

 

EIGRP Routing Protocol Enable

R1(config)#router eigrp 1 
R1(config-router)#network 0.0.0.0 
R1#show ip int fa0/0 
 FastEthernet0/0 is up, line protocol is up
  Internet address is 10.10.100.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.10
  Outgoing access list is not set
  Inbound access list is not set
  --- (생략) ---

 

RIP Routing Protocol Enable

R1(config)#router rip 
R1(config-router)#network 0.0.0.0 
R1#show ip int fa0/0 
 FastEthernet0/0 is up, line protocol is up
  Internet address is 10.10.100.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.9
  Outgoing access list is not set
  Inbound access list is not set
  --- (생략) ---

  이와같이 각각의 Routing Protocol을 Interface에 enable 시키게 되면, 자신이 사용하는 Multicast address에 자동으로 IGMP Join이 되는 것을 확인할 수 있다.


224.0.0.5 vs. 224.0.0.6

DR에서 Interface 정보 확인

R1#show ip ospf int fa0/0
FastEthernet0/0 is up, line protocol is up 
  Internet Address 10.10.100.1/24, Area 0, Attached via Interface Enable
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Enabled by interface config, including secondary ip addresses
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 1.1.1.1, Interface address 10.10.100.1
  Backup Designated router (ID) 2.2.2.2, Interface address 10.10.100.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
  --- (생략) ---

R1#show ip int fa0/0
FastEthernet0/0 is up, line protocol is up
  Internet address is 10.10.100.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.5 224.0.0.6
  Outgoing access list is not set
  Inbound  access list is not set
  --- (생략) ---

 

BDR에서 Interface 정보 확인

R2#show ip ospf int fa0/0
*Nov  4 17:06:35.370: %SYS-5-CONFIG_I: Configured from console by console
R14#show ip ospf int fa0/0
FastEthernet0/0 is up, line protocol is up 
  Internet Address 10.10.100.2/24, Area 0, Attached via Interface Enable
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Enabled by interface config, including secondary ip addresses
  Transmit Delay is 1 sec, State BDR, Priority 50
  Designated Router (ID) 1.1.1.1, Interface address 10.10.100.1
  Backup Designated router (ID) 2.2.2.2, Interface address 10.10.100.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
  --- (생략) ---

R2#show ip int fa0/0
FastEthernet0/0 is up, line protocol is up
  Internet address is 10.10.100.2/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.5 224.0.0.6
  Outgoing access list is not set
  Inbound  access list is not set
  --- (생략) ---

 

DROTHER에서 Interface 정보 확인

R3#show ip ospf int fa0/0
FastEthernet0/0 is up, line protocol is up 
  Internet Address 10.10.100.3/24, Area 0, Attached via Interface Enable
  Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Enabled by interface config, including secondary ip addresses
  Transmit Delay is 1 sec, State DROTHER, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 10.10.100.1
  Backup Designated router (ID) 2.2.2.2, Interface address 10.10.100.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
  --- (생략) ---

R1#show ip int fa0/0
FastEthernet0/0 is up, line protocol is up
  Internet address is 10.10.100.3/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.5
  Outgoing access list is not set
  Inbound  access list is not set
  --- (생략) ---

 

Multicast 224.0.0.5 vs. 224.0.0.6

  Multi-access 망에서 DR과 BDR만 ‘224.0.0.6’에 추가로 IGMP Join이 되어 있는 것을 학인할 수 있다. Multicast ‘224.0.0.6’은 DR과 BDR에게 Update Packet을 전달할 때 사용한다. 즉, 다른 Neighbor에게는 전달하지 않고 DR/BDR에게만 Update Packet을 전달하기 위해 추가로 사용하는 주소이다.

  그리고, ‘224.0.0.6’에 대해서는 LSAck Packet을 전달하지 않는다. DR/BDR이 해당 Packet을 받으면 DR이 다시 타 Neighbor에게 ‘224.0.0.5’로 Update Packet을 전달하는데 이 Packet이 LSAck 역할도 같이 수행하게 된다.

  또한, ‘224.0.0.5’로 Update Packet을 받은 장비들은 모두 LSAck Packet을 전달하는데, 최초로 ‘224.0.0.6’으로 Update Packet을 전달한 장비는 자신이 보낸 정보가 다시 돌아온 것일 뿐이기 때문에 LSAck Packet을 전달하지 않고 DR도 LSAck를 기다리지 않는다.