Database Description Packet (DDP) - This contains the summary of all the LSAs, it only contains the headers and not the complete LSA. This allows router to see if its own link state database is updated. This is type 2 OSPF packet. I(nitial)-bit - If set means this is the first DDP sent. M(ore)-bit - … Continue reading Building OSPF adjacency
Tag: ospf
OSPF finite state machine
Reference: Routing TCP/IP 2nd Edition Jeff Doyle, page 353-357. OSPF router goes through several states before adjacency is formed. DOWN - No hello packets was seen from the neighbour router since the last dead interval. If an OSPF router goes to DOWN state from its higher states, its link state retransmission, database summary and link … Continue reading OSPF finite state machine
Understanding OSPF
Open Shortest Path First is using Djikstra's shortest path first algorithm. It is a link-state routing protocol, another link-state routing protocol is Intermediate System to Intermediate System also known as IS-IS. General facts about OSPF 1. Uses its own transport protocol number 89. Protocol number 89 indicates that this packet is destined for OSPF. 2. … Continue reading Understanding OSPF
OSPF over NBMA
R2 hub router interface Serial1/0 no ip address encapsulation frame-relay serial restart-delay 0 interface Serial1/0.102 point-to-point ip address 10.10.10.1 255.255.255.252 frame-relay interface-dlci 102 end interface Serial1/0.103 multipoint ip address 192.168.100.1 255.255.255.248 ip ospf priority 10 frame-relay interface-dlci 103 frame-relay interface-dlci 104 end router ospf 1 router-id 1.1.1.1 log-adjacency-changes passive-interface default no passive-interface Serial1/0.102 no passive-interface … Continue reading OSPF over NBMA
OSPF: A note about OSPF authentication
Interface level command ip ospf authentication message-digest (Enable MD5 authentication) ip ospf message-digest-key 1 md5 cisco ip ospf authentication (Enable clear text authentication) ip ospf authentication-key cisco OSPF authentication type Type 0: Null, no authentication Command: ip ospf authentication null Type 1: Clear text Command: ip ospf authentication Type 2: MD5 hash authentication Command: ip … Continue reading OSPF: A note about OSPF authentication
OSPF: Special area types
Area types planning Area 0 is backbone area. Reason: Backbone area being also the transit area should contain all LSA types. Area 1 and Area 2 are stub OSPF areas. Reason: There's only one ASBR, and only 2651-2 has connection to ASBR 2651-4, hence all areas only need to know how to get to 2651-2 … Continue reading OSPF: Special area types
Connecting Enterprise hub to other enterprise spokes over NBMA (frame-relay)
The original design intention is not to advertise routes using ospf over NBMA network, it is not necessary for this case. However for Buona Vista HQ to be able to reach the two sites, I need entries in the routing table to let other routers know the road map to reach. A way is to … Continue reading Connecting Enterprise hub to other enterprise spokes over NBMA (frame-relay)
OSPF: non-broadcast multiaccess (point-to-multipoint type)
RFC2328 compliant (point-to-multipoint type) 1. RFC2328 compliant mode. 2. No DR and BDR is/are needed. 3. Multicast OSPF hello packet is sent to discover neighbours. 4. Typically for partial mesh or star topology. Debug output of 2651-4 2651-4# *Mar 3 11:26:39.841: OSPF: Interface Serial0/0.301 going Up *Mar 3 11:26:39.841: OSPF: Send hello to 224.0.0.5 … Continue reading OSPF: non-broadcast multiaccess (point-to-multipoint type)
OSPF: non-broadcast multaccess network (broadcast type)
OSPF using broadcast network type over NBMA 1. Uses multicast (224.0.0.5, AllSPFRouter multicast) to automatically discover neighbours, no need to use neighbor command for OSPF router to send hello via unicast. 2. Cisco mode. (non RFC2328-compliant) 3. DR and BDR will be elected. 4. Typically full mesh or partial mesh topology. 5. Hello is sent … Continue reading OSPF: non-broadcast multaccess network (broadcast type)
OSPF: non-broadcast multiaccess network (non-broadcast type)
OSPF network types 1. Point-to-point: Network between a pair of routers. 2. Broadcast multiaccess: If you connect the routers to a switch, the switch is broadcast capable. OSPF over EoMPLS behaves the same as broadcast multiaccess, OSPF routers do not know the existence of EoMPLS at all. DR is required, BDR can be elected if … Continue reading OSPF: non-broadcast multiaccess network (non-broadcast type)
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