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Home > Archive > CCNP > September 2001 > ospf link state requests and updates
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ospf link state requests and updates
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| muckfish 2001-09-05, 7:48 pm |
| hello everyone,
a fundamental question on ospf....
during the final stages of establishing an adjacency(loading stage ... i think), link state updates are sent in reply to link state requests. r these updates flooded to other neighbours of the recepient or are the link state updates meant only for that particular router?
Cheerz | |
| dmaftei 2001-09-06, 7:17 am |
| LS requests and LS updates sent in response to requests are unicast, so they're "meant only for that particular router". | |
| muckfish 2001-09-06, 9:20 am |
| this is where i'm confused. so when does flooding occur? therefore during the loading stage LSA flooding does not occur?
i read somewhere that link state updates and acknowledgements are part of the synchronization process. and these are flooded out. i just want to confirm that i've read it correctly
i think maybe the method that the message is being relayed (unicasts or multicasts) is configurable (i.e. broadcast, NBMA, P2P, P2MP) | |
| dmaftei 2001-09-06, 9:52 am |
| You're talking about different situations. The first one (the one with the requests) is the synchronization, or forming an adjacency between routers. Once the two routers determine that they sould form the adjacency, they describe their LS databases, figure out what LSAs they need and request those LSAs. The answers to these requests are unicast.
The flooding happens when a router detects a change. That change must be propagated throughout the are (or throughout the OSPF domain in the case of type 5 LSAs), which is done by sending updates. DRs and BDRs will send the update multicast to AllSPFRouters (224.0.0.5), while DROthers will send the update multicast to AllDRouters, on their adjacencies. There's a lot of details involved in the flooding process (determining the LSA's age, handling self originated LSAs, where to resend a freshly received, etc.), but I guess you're not too worried about those now. | |
| muckfish 2001-09-07, 1:17 am |
| establishing adjacencies is the process that occurs between two router. if we were to take this to the macro level where there are say five routers
A-----B-----C-----D-----E
So when all the routers are configured with OSPF (assume all routers have just started up), router A will establish an adjacency with B, B with C, C with D etc. etc.
When synchronization is taking place (depending on which is the master/slave), lets say router C informs its link to D in an LS update packet to router B
how will router A know about router D and E links when forming an adjacency with router B
how will router B know about router E's links
if the LS update intended for router B is flooded to router A, then A would be informed of links to D.
anyway, this is the bit that is bugging me big time. i know the example is very simplified, if anyone can help i'd be more than grateful ..... plzzzzz | |
| dmaftei 2001-09-07, 1:39 pm |
| quote:
Originally posted by muckfish
...how will router A know about router D and E links when forming an adjacency with router B
1. Router B starts synchronizing with router A before B knows of D and E. In this case, after A and B are synchronized, A will not know about D and E. Later B learns about D and E, and it will send updates to A, contaning D's and E's router LSAs. This is flooding.
2. Router B starts synchronizing with router A after B knows of D and E. In this case, B already has D's and E's router LSAs in its database, while A, obviously, doesn't. In the process of database description, A will notice that it's missing D's and E's LSAs, and it will request these LSAs from B. B will honor the request and will send the LSAs to A. This is not flooding; it's syncronization. | |
| muckfish 2001-09-07, 2:33 pm |
| thank you ... that helped. was just wondering,
what would happen if routers A and C was forming an adjacency at the same time with router B. so routers A and B will sync and routers B and C will sync first. new info that B gained from C will be flooded out to A and similarly to C? | |
| dmaftei 2001-09-07, 2:53 pm |
| quote:
Originally posted by muckfish
what would happen if routers A and C was forming an adjacency at the same time with router B.
Frankly, I don't know. I'll need to study the spec for this one, and I wouldn't be surprised if this is left up to the implementations. You could look it up in RFC 2328, and let us know what you found. | |
| muckfish 2001-09-10, 1:53 am |
| ok i got this from rfc 2328 ......
" The adjacency is used by the flooding procedure as soon as the Database Exchange Process begins. This simplifies database
synchronization, and guarantees that it finishes in a predictable period of time."
does this mean that LS Updates that contain multiple LSAs received since the exchange state are flooded even during the final stages of adjacency? ..... hope someone can tell me if i'm reading this wrong ..... thx |
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