OSPF Prefix Suppression

OSPF prefix-suppression is a useful feature in order to reduce the number of Link State Advertisement (LSA) that are flooded within an area. In an OSPF area which has multiple transit links between hosts and actual communication is between the hosts. There is no need to advertise the transit link LSAs to all the routers. You can only advertise the LSAs related to end hosts. By default, OSPF advertises all the LSAs that include the transit link LSAs.

OSPF prefix-suppression feature helps to overcome this behavior and reduces the number of Type 1(router) and Type 2(network) LSAs advertised.

This feature can be enabled globally on a router or on per interfaces basis.

OSPF prefix-suppression helps in faster Shortest Path First (SPF) calculation due to less number of prefixes in the database (DB). OSPF Type 3, Type 4, Type 5, or Type 7 LSAs are not suppressed.

OSPFv2 and OSPFv3 headers

#DoYouKnow #OSFP #Routing

All OSPFv2 packets have a common 24-byte header, and OSPFv3 packets have a common 16-byte header, that contains all information necessary to determine whether OSPF should accept the packet. The header consists of the following fields:

• Version number—The current OSPF version number. This can be either 2 or 3.
• Type—Type of OSPF packet.
• Packet length—Length of the packet, in bytes, including the header.
• Router ID—IP address of the router from which the packet originated.
• Area ID—Identifier of the area in which the packet is traveling. Each OSPF packet is associated with a single area. Packets traveling over a virtual link are labeled with the backbone area ID, 0.0.0.0. .
• Checksum—Fletcher checksum.
• Authentication—(OSPFv2 only) Authentication scheme and authentication information.
• Instance ID—(OSPFv3 only) Identifier used when there are multiple OSPFv3 realms configured on a link.

Multicast OSPF LSA (Type 6) on Cisco router

Cisco routers do not support LSA Type 6 Multicast OSPF (MOSPF), and they generate syslog messages if they receive such packets. If the router is receiving many MOSPF packets, you might want to configure the router to ignore the packets and thus prevent a large number of syslog messages.


Commands:
#Router ospf 0.0.0.1
#ignore lsa mospf

Does ospf having backup path?

 OSPF uses the SPF algorithm. The information contained in a router’s OSPF link state database is the “MAP” that is used to calculate the best path to a remote network. However, unlike EIGRP, OSPF does not keep backup paths to routes, rather, when a route to a network goes down, the SPF algorithm is run again to determine a backup or alternate path.
Keep in mind no backup link.. if there are any dual active paths to any destination with same metric then load balancing will work (default up to 4 Path).

What Do We Mean by Link-States?

What Do We Mean by Link-States?

OSPF is a link-state protocol. We could think of a link as being an interface on the router. The state of the link is a description of that interface and of its relationship to its neighboring routers. A description of the interface would include, for example, the IP address of the interface, the mask, the type of network it is connected to, the routers connected to that network and so on. The collection of all these link-states would form a link-state database.

OSPF Tips - Summarization of Network

#CiscoTips #OSPF #DoYouKnow

An internal summary route is generated if at least one subnet within the area falls in the summary address range and the summarized route metric is equal to the lowest cost of all the subnets within the summary address range. Interarea summarization can only be done for the intra-area routes of connected areas, and the ABR creates a route to Null0 to avoid loops in the absence of more specific routes.

OSPF network planning tips - Summarization of network

#CiscoTips #DoYouKnow #OSPF #Design Summarization design is a process of network planning: One step fails, Network fail::: If the OSPF design includes many ABRs or ASBRs, suboptimal routing is possible. This is one of the drawbacks of summarization. Route summarization requires a good addressing plan—an assignment of subnets and addresses that are based on the OSPF area structure and lends itself to aggregation at the OSPF area borders.

Cisco Real Time Configurations‏ - OSPF on L3 Switches

Dear Friends,

Today is my first series on Cisco Real Time Configuration.

Now you can Download PKT and Test on your system.

Today Topic -  OSPF Configuration on L3 Switch (Routing).

I hope you will like it.

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Download:- Download Packet Tracer Real Time Configuration .. OSPF Routing on L3 Switch.

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