As interesting as it may be, the details of OSPF can be a bit daunting. A big part of that is that there are types of this and types of that all over the place. Two important types that you\u2019ll use a lot and should know pretty well are area types and LSA types. I\u2019ve come up with a pretty good way to learn both and to correctly associate the two.<\/p>\n\n\n\n
To save you some time if you are already familiar with OSPF and just want to get the skinny, here it is (we\u2019ll recap at the end too):<\/p>\n\n\n\n
OSPF Special Area Types<\/strong><\/p>\n\n\n\n OSPF has a few extra types of areas aside from your standard, run-of-the-mill area. All of the special area types revolve around the term \u201cstub\u201d. A stub area is an area that only has 1 way in and out (as far as the OSPF process is concerned). Their purpose typically has to do with minimizing the utilization of memory, CPU, and bandwidth by limiting\/filtering certain types of link-state announcements the areas send and receive. The flexibility required to make this possible is actually where the link-state announcement (LSA) types come into play.<\/p>\n\n\n\n Aside from a regular OSPF area, Cisco routers support the following area types:<\/p>\n\n\n\n OSPF LSA Types<\/strong><\/p>\n\n\n\n I\u2019ll keep this part as short as possible, but keep in mind that one could go on for pages discussing LSA type specifics. There are 11 types of LSAs in OSPFv2. For our discussion, I\u2019m going to just tell you what a hand full (plus 1) of the LSA types are and what they are for, then we\u2019ll get down to associating them to the various area types.<\/p>\n\n\n\n Type 1 LSAs are your standard intra-area link-state announcement. There is nothing special about them, they just do their job, punch the clock, and head home. These are the bread and butter of intra-area routing.<\/p>\n\n\n\n Type 2 LSAs only, only, only come from a DR (designated router). Remember those? They are the elected (by priority or by router-id) official of the broadcast network. Type 2 LSAs come from the DR to tell the rest of the world who is attached to the broadcast link.<\/p>\n\n\n\n Type 1 and 2 never cross area borders, and by virtue that the protocol is link-state, you can\u2019t keep them from moving about the area.<\/p>\n\n\n\n Type 3 LSAs are summaries of routes that are sent inter-area.<\/p>\n\n\n\n Type 4 LSAs are also summary routes, but they specifically deal with ASBR (autonmous system boundry routers). In fact, the only thing that they do is provide information about the ASBR.<\/p>\n\n\n\n Type 5 LSAs are used to deliver external information. This is the information that is learned from outside of OSPF and was introduced by redistribution at some point.<\/p>\n\n\n\n Type 7 LSAs are the same as type 5 LSAs. Redundant? Kinda, but hang in there, there is a reason for it. As stated above:<\/p>\n\n\n\n Now that you know what the LSAs are, what the area types are, and these rules, let\u2019s try to figure out what kind of LSAs are allowed to cross the ABR for the following special area types:<\/p>\n\n\n\n All of these are valid area types on Cisco routers.<\/p>\n\n\n\n So with the 4 rules in mind, starting with example #1. We won\u2019t have type 1 or 2 for sure \u2013 they never cross an ABR anyway. \u201cstub\u201d means that we won\u2019t have type 5. As it looks, we\u2019ll be permitting type 3 and 4 (summaries), blocking 5 (info from outside of OSPF), and letting everything else through.<\/p>\n\n\n\n Example 2 is the same as above, only it has our key words of \u201cnot-so\u201d. That means that even though we\u2019re not letting type 5 through, we\u2019re allowing type 7. When the type 7 hit the ABR, they get translated into type 5 to go out to the rest of the network. This may seem stupid on the surface, but digging deeper reveals the point. The key is that we\u2019re saving our area from processing all of the type 5 LSAs from all of the other areas. Let\u2019s say we have enough power (processor\/memory, etc) to process our own area\u2019s information, but we\u2019re not able to handle dealing with everyone else\u2019s.<\/p>\n\n\n\n Example #3 is the most efficient of any of these. Looking at the rules we see that stub filters type 5, and \u201ctotally\u201d filters 3 and 4. Since we know that ABRs don\u2019t transit type 1 and 2, we\u2019re pretty much done. So how does it work? Well, the point of a stub, if you recall, is that there is only 1 exit point for the network. So\u2026. OSPF just sends a default route and that is it. Intra-area routes within the stub area are still going to make their way into the LSDB and the routing table, but the only way out to the rest of the world is via the default route.<\/p>\n\n\n\n Example #4 is like #3, but type 7 LSAs get through for the same reason as example #2. Did you get all that? =) We filter everything, but we have an ASBR somewhere within our totally stubby area. As such, we need routes that are being introduced to us from that avenue to be sent out to the rest of the network. We send type 7 LSAs to the ABR, it changes them to type 5 on the way out, and everyone is happy.<\/p>\n","protected":false},"excerpt":{"rendered":" As interesting as it may be, the details of OSPF can be a bit daunting. A big part of that is that there are types of this and types of that all over the place. Two important types that you\u2019ll use a lot and should know pretty well are area types and LSA types. I\u2019ve […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[],"class_list":["post-9","post","type-post","status-publish","format-standard","hentry","category-network"],"_links":{"self":[{"href":"https:\/\/n0c.us\/index.php?rest_route=\/wp\/v2\/posts\/9","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/n0c.us\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/n0c.us\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/n0c.us\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/n0c.us\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=9"}],"version-history":[{"count":1,"href":"https:\/\/n0c.us\/index.php?rest_route=\/wp\/v2\/posts\/9\/revisions"}],"predecessor-version":[{"id":10,"href":"https:\/\/n0c.us\/index.php?rest_route=\/wp\/v2\/posts\/9\/revisions\/10"}],"wp:attachment":[{"href":"https:\/\/n0c.us\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/n0c.us\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/n0c.us\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n
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