OSPF. OSPF processs can be enabled on 2 levels

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1 OSPF UDP port 89 Metic cost Link state protocol Flood the link state information in the entire topology Builds the topology table Stores in LSDB Runs SPF(Djsktra algorithm) for best path to reach destination OSPF processs can be enabled on 2 levels 1. Interface level 2. Under ospf process Neighbor command is used for unicast hellos Area 0 is required to establish full reachbility between areas OSPF neighborship parameter Hello timer,dead timer Authentication MTU Same Network Type Neighborship stages in OSPF Down Attempt : Only applicable for a NBMA segment Init : When a hello is received then the neighbor is placed in init state (Neighbors)2way : List of seen routers includes the neighbor along with the hello packet. It is stable state when the router don t wish to become adjacent Ex start : Master/Slave relationship is determined Exchange : Database description packets are exchanged i.e LSA s Loading : when all the LSA s are downloaded from its neighbor (Adjacent)Full : it s a stable state for routers which want to be fully adjacent LSA : (Link state advertisement) IN IOS 15 and above There is no need that packet should pass through area 0,however all the areas should be directly connected to area 0

2 1. Router ID 2. Net summary 3. ABR summary 4. ASBR summary 5. External AS The LSA types defined in OSPF are as follows: Type 1 - Router LSA - the router announces its presence and lists the links to other routers or networks in the same area, together with the metrics to them. Type 1 LSAs are flooded across their own area only. The link-state ID of the type 1 LSA is the originating router ID. Type 2 - Network LSA - the designated router (DR) on a broadcast segment (e.g. Ethernet) lists which routers are joined together by the segment. Type 2 LSAs are flooded across their own area only. The link-state ID of the type 2 LSA is the IP interface address of the DR. Type 3 - Summary LSA - an Area Border Router (ABR) takes information it has learned on one of its attached areas and summarizes it before sending it out on other areas it is connected to. This summarization helps provide scalability by removing detailed topology information for other areas, because their routing information is summarized into just an address prefix and metric. The summarization process can also be configured to remove a lot of detailed address prefixes and replace them with a single summary prefix, helping scalability. The link-state ID is the destination network number for type 3 LSAs. Type 4 - ASBR-Summary LSA - this is needed because Type 5 External LSAs are flooded to all areas and the detailed next-hop information may not be available in those other areas because it may be using a different routing protocol. This is solved by an Area Border Router flooding the information for the router (i.e. the Autonomous System Boundary Router) where the type 5 originated. The link-state ID is the router ID of the described ASBR for type 4 LSAs. Type 5 - External LSA - these LSAs contain information imported into OSPF from other routing processes. They are flooded to all areas unchanged (except stub and NSSA areas). For "External Metric Type 1" LSAs routing decisions are made using the Type 1 metric cost sent, as the total cost to get to the external destination and includes the cost to the ASBR; while for "External Type 2" LSAs the metric sent is the cost from the ASBR to the External destination network and must be added to the OSPF cost to the ASBR advertising the Type 5. The link-state ID of the type 5 LSA is the external network number. DR/BDR

3 DR/BDR election takes place only for the network type broadcast and non broadcast Different network types OSPF Cost Broadcast : default on LAN segment DR/BDR Non Broadcast : default on DMVPN. Manual neighborship is to given DR/BDR P2P : ATM, GRE tunnel, uses multicast hellos, NO DR/BDR P2MP : uses multicast hellos, multiple neighborships are allowed but there is no DR/BDR P2MP non broadcast : uses unicast hellos, Doesn t support DR/BDR, Neigbhors are manually configured Loopback : the advertised loopback in an area are seen as /32 instead of their original subnet Ospf uses bandwidth to calculate the best path to reach destination Bandwidth = Reference bandwidth/interface bandwidth By default ref bandwidth is 100 so, the GIG links cannot be distinguished Changing the bandwidth to manipulate the cost is not preferred You can manipulate cost under router process, interface base and per neighbor base OSPF Authentication Clear Text : Ip ospf authentication key Ip ospf authentication MD5 ip ospf authentication message-digest ip ospf message-digest-key 1 md5 KEYONE MD5 supports key-rotation i.e we can use multiple keys and our adjacency will also be maintained even if the one key is removed SHA Crypto-encryption mode SHA key-chain OSPF Summarization External summ. Summary address on the redrisbution point Internal summ. Under the router process AREA range Only on ABR and ASBR Also used to filter routes. OSPF looks for the longest match to reach the destination. If the route to the destination is not specific/shorter match the traffic is not send. To alter this feature we use command no discard route Filtering OSPF routes

4 Filter list (Only for inter area routes) Intra area routes are filtered by summarization Distribute list : Limitation don t support outbound filtering. It is applied inbound Route map Route is removed from routing table RIB but it is present in Database.and Ping will not happen It filters routes only from the routing table. they are still present in ospf database Adminstritive distance Summarization Stub/Special areas Database filtering : database filter all out its similar to the command passive interface Applied on interface as ip ospf Under router process in hub and spoke topology using Neighbor databse filter all out Ospf advance features Virtual link It is used to make 2 noncontigious area communicate We give command under the router process Area (in the interface is ) virtual link ip (of the ABR in A0) Same config is done on the ABR Virtual link cost is 1 OSPF area capability transit It is enabled by default, allowing the OSPF Area Border Router to install better cost routes to the backbone area through the transit area instead of the virtual links. If you want to retain a traffic pattern through the virtual link path, you can disable capability transit by entering the no capability transit command Defuault route Passive interface Special areas : stub, totally stub, NSSA, totally NSSA In ospfv3 a router can become stub by advertising the max metric value for its originated lsa s Max-Metric Router LSA

5 Stub Area : Blocks the external routes type 5 and replace them with a default route Area stub on every router in the stub area Default route is generated by default Totally stubby Filter intra area routes and external routes (Type 3 and 5) Area stub no summary on ABR and area stub on other routers Default route is generated by default NSSA/totally NSSA (ASBR is allowed) N2: External routes which in the NSSA/totally NSSA Area Because NSSA doesn t support E bit therefore there are no E1 and E2 routes. ABR convert these routes into E1 or E2 to flood them in entire OSPF domain. ABR with highest router id convert the route types Reverse Lookup in NSSA/totally NSSA Forward address: Only If the forward address is we can do reverse lookup i.e can ping the source NSSA No external routes are allowes(blocks type 5) Default route is originated by command Default info originate The default route is injected as a type 7 LSA Totally NSSA Totally stubby + NSSA Inter area and external routes are blocked Only intra area routes,nssa external, inter area default route is allowed If the ASBR is in Totally NSSA the forward address is of the ASBR and to change that we use the following command : area 2 NSSA no summary translate type 7 no summary OIA>O>E1>E2>NSSA external

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