On demand routing (ODR)

1. Must turn on CDP for all routers; hence this routing protocol is Cisco proprietary. 2. Only works in hub and spoke topology. 3. Has a default administrative distance of 160. 4. Extremely easy to configure. 5. Makes default route easier to configure for every spoke to hub communication. The hub router actually advertises default … Continue reading On demand routing (ODR) →

Type 10 LSA – Opaque LSA

This LSA is an extension of OSPF. The extension is for doing OSPF traffic engineering. MAX-LSA - Protects against adjacent router's excessive LSA in the OSPF domain. Excessive LSA will cost router's high cpu resources. This is to limit the LSA that is not generated by the router itself. CAUTION: Router adjacency will be affected … Continue reading Type 10 LSA – Opaque LSA →

LSA types

1. Router LSA - This LSA is produced by every router. The LSA contains information of the router's connected interfaces, states, neighbour router type, any known ospf routers connected to the router's link and outgoing cost of each link. This LSA floods within its own area (intra-area flooding), hence this LSA describes intra-area route. 2. … Continue reading LSA types →

Building OSPF adjacency

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 →

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 →

Routing table – a router’s decision

How router chooses a route. A router that has multiple exit to a destined network will decide which route will be in routing table by these: 1st: Choose the route that has the longest prefix match. 2nd: Choose the route that has the lowest administrative distance. Administrative distance is a value a router uses to trust … Continue reading Routing table – a router’s decision →