OSPF study notes

Packet type:

  • Hello: discover and maintain neighbornood
  • DBD: LSA header information, like a dictionary of LSAs
  • LSR: request LSA
  • LSU: update LSA (may contain several LSA)
  • LS ACK: confirmation

LSA type:

Type advertizer scope content
1 all routers running ospf local area directly connected interfaces
2 DR local area mask of a MA network
3 ABR AS summary of LSA 1 & 2
4 ABR in which area an ASBR exist AS where the ASBR is
5 ASBR AS external routes
6 Group Membership LSA
7 ASBR of NSSA in the NSSA external routes
8 all routers in IPv6 (LSA 1 for ipv6)
9 DR in IPv6 (LSA 2 for ipv6)
10 MPLS TE
11 MPLS TE

Route types:

  • O: LSA 1, 2
  • O IA, LSA 3
  • O E1, LSA 5 cost increase
  • O E2, LSA 5 cost does NOT increase
  • O N1, LSA 7 cost increase
  • O N2, LSA 7 cost does NOT increase

Network types:

  • BMA: hello timer 10s, DR will be selected
  • P2P: hello timer 10s, DR will be selected
  • P2MP
  • P2MP NB
  • NBMA: hello timer 30s
  • Loopback

Area types:

Type characteristics
stub no LSA 4, 5. default route from ABR (LSA 3), no external routes allowed
totally stub no LSA 3, 4, 5. default route from ABR (LSA 3), no external routes allowed
NSSA no LSA 4, 5. no default route from ABR, external routes allowed
totally NSSA (Cisco Only) no LSA 3, 4, 5. default route from ABR (LSA 3), external routes allowed

neighbor relationship:

Type characteristics
down only in frame-relay
attempt only in frame-relay
init send hello packet
2-way elect DR & BDR
ex-start send DBD packets, select master/slave (whose sequence number will be used)
exchange exchange LSA information (LSR, LSU)
loading respond ack, start SPF caculation
full put best routes into routing table, neighbor connection finished

DR & BDR election:

  • priority, default is 1, 0 means the router doesn’t attempt election
  • Router ID, stuck in down if router ids are the same