OECG – Chapter 10


Last post for this chapter. It should be noted that there’s a lot of detail in the “Foundation Summary” section of this chapter. Pages of OSPF-related commands and what they do. I’m not writing that all up here, or I’ll never get through this chapter. Or this book. Or take the test. :-) Moving on to the definitions for OSPF, of which there appears to be a ton.

LSDB – link state database. The topological database of all known routers, areas and links in the OSPF system.

Dijkstra – the shortest path first algorithm, used to compute the fastest way to get to a particular network.

link-state routing protocol – exchanges information about links, as opposed to exchanging routes.

LSA – link-state advertisement.

LSU – link-state update.

DD – database description. It’s what gets sent in an LSU.

Hello – used for several things in an OSPF network, including discovery, keep-alive, initial neighbor adjacency setup, and DR/BDR election.

LSAck – sent as an acknowledgement of an LSU.

RID – router ID. Each router ID in an OSPF system must be unique. Can be set manually, or will be chosen by the highest up/up loopback, or if no loopback, by the highest up/up IP interface.

neighbor state – an OSPF neighbor can have several states. init, 2-way, exstart, exchange, loading, full.

neighbor – A discovered OSPF router on the same segment.

adjacent – an OSPF router that is “full”, i.e. the 2 routers are exchanges link-state information. Not all neighbors are considered adjacent in OSPF-speak.

fully-adjacent – the OSPF database flooding process has completed.

2-way – routers that have sent initial hellos to one another – the multicast address for “all SPF routers” – the multicast address for “all DR router”

area – a logical boundary group of OSPF routers.

network type – OSPF networks can be broadcast, point to point, point to multipoint, etc. with bearing on whether hellos will be used, whether DRs will be elected, and whether static neighbors must be assigned.

external route – a route redistributed into OSPF from outside the routing protocol

E1 route – external route where internal and external metrics are considered.

E2 route – external route where only external metrics are considered.

Hello timer – the interval between hellos, 10 seconds by default on broadcast, 30 seconds on T1 or slower.

Dead interval – defaults to 4x the hello time, the amount of time hellos are missed before the neighbor is considered dead.

sequence number – placed on an LSA, it is used by routers to determine if their LSDB is current. Routers with old sequence numbers will request an update for that LSA.

DR – designated router. The router responsible for flooding LSAs to other routers on the segment. On segments where there are DRs, OSPF routers will only become fully adjacent to DRs and BDRs.

BDR – backup designated router. The router that takes over the DR role when the DR falls off the network.

DROther – routers that are not DRs or BDRs.

priority – the higher the OSPF priority number, the more likely the router will be elected the DR.

LSA flooding – sending LSAs to the multicast address of (all SPF speakers on the segment).

DR election – the process whereby a designated router is elected.

SPF calculation – the process of determining the most efficient way to forward traffic for a specific network.

partial SPF calculation – what routers in one area do when routers in another area experience an LSA change.

full SPF calculation – what routers in one area do when there’s a change to an LSA in their area.

LSRefresh – 30 minutes by default, how long between LSA flooding a particular LSA.

hello time/interval – is this different from the hello timer above? I will check…

MaxAge – the timer tracking LSA age, default 2x LSRefresh. If no one’s heard about this LSA by MaxAge, it’s dea.

ABR – area border router. The router sitting in between area 0 and 1 or more other areas.

ASBR – autonomous system boundary router. A router that redistributes routes from another routing protocol into the OSPF system. He could be sitting in any area.

Internal router – not an ABR or ASBR. All interfaces connect to a single OSPF area.

transit network – the network 2 OSPF routers found each other on, implying that it can be used to forward packets for other networks.

stub network – a network with only one OSPF router connected.

LSA type – the format for a particular LSA is defined by its type.

stub area – an area where type 5 LSAs are not injected.

NSSA – not-so-stubby area. An area where type 5 LSAs are not injected in, but type 7 LSAs are allowed out.

totally stubby area – an area where type 5 LSA’s are not injected in, and neither are type 3 LSAs. You get a default route, and that’s about it. And you’re going to like it, mister!

totally NSSA – totally not-so-stubby area. An area where type 5 LSA’s are not injected in, and neither are type 3’s. OTOH, type 7’s are allowed out.

virtual link – a logical connection that allows for areas with no physical connection to area 0 to be connected to it. Can also be used to connect partitioned areas.

stub router – a router that will only forward inbound packets for connected networks.

transit router – a router that will forward to another router if needed to get the packet to its destination.

SPF algorithm – used by OSPF to compute routes based on the LSDB.

All OSPF DR router –

All OSPF routers –

About the author

Ethan Banks

Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.

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Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.

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