When you're preparing to pass the BSCI exam on the way to the coveted Cisco CCNP certification, you can be quickly overwhelmed by the sheer amount of BGP and OSPF knowledge you must demonstrate a mastery of. One set of details that some BSCI and CCNP candidates underestimate are the differences between the OSPF router types. The OSPF router types seem straightforward enough, but what you must keep in mind is that a router can fill more than one of these roles!
An OSPF Internal router has one rule - it must have all its interfaces in a single area. It does not mean that area has to be Area 0.
An OSPF Backbone router is a router with at least a single area in the OSPF backbone area, Area 0. A router can be both an Internal and Backbone router if all its interfaces are in Area 0.
An Area Border Router has at least one interface in Area 0 and another interface in a non-backbone area. ABRs are also one of two router types that can perform OSPF route summarization. (To advertise a summary route from one OSPF area to another, use the area range command on the ABR.)
Finally, an ASBR is an OSPF router that is performing route redistribution by injecting routes from another source into the OSPF domain. This is the other OSPF router type that can perform route summarization; to summarize routes being redistributed into OSPF, use the summary-address command on the ASBR.
There are several commands you can use to determine the router types in a given OSPF area. The command "show ip ospf" will display quite a bit of information regarding the local router, and this includes whether that router is acting as an ABR or ASBR. To see the routes to the ABRs and ASBRs from the local router, run "show ip ospf border-routers".
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The aim of both is to allow hosts to quickly discover a standby router when the primary router fails. IRDP is commonly used by Windows DHCP clients and several Unix variations, but you do see it in Cisco routers as well. IRDP is defined in RFC 1256.
IRDP routers will multicast Hello messages that host devices hear. If a host hears from more than one IRDP router, it will choose one as its primary and will start using the other router if the primary it's chosen goes down.
HSRP is a Cisco-proprietary protocol that is designed for quick cutover to a secondary router if the primary fails, but the host devices don't "see" either the primary or secondary router. The hosts use a virtual router as their default gateway. This virtual router has its own IP and MAC address! All the while, the router chosen as the primary is actually the one doing the routing. If the primary router goes down, the secondary router quickly takes over with no major interruption to network services.
The HSRP routers communicate by multicasting updates to 224.0.0.2, and its through these hellos that the HSRP routers decide which router is primary and which is secondary. HSRP is defined in RFC 2281.
The configuration of each of these will be covered in a future tutorial. In the meantime, I urge you to read the RFCs mentioned in this article, and visit www.cisco.com/univercd to read about the configurations and options available for both of these vital protocols.
Chris Bryant has sinced written about articles on various topics from CISCO CCNA, Personal Desktop and Cisco CCNP. Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNP and. Chris Bryant's top article generates over 27100 views. Bookmark Chris Bryant to your Favourites.
