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  1. #1
    Join Date
    Dec 2008
    Posts
    88

    Default Forming Adjacencies

    Hey,

    Trying to differentiate between Neighbors and Adjacency.
    I understand that Neighbors will be two interfaces on a common network (such as a serial link). My question is more regarding Adjacency. Do two routers have to be directly connected to form an adjacency? Or, can routers form adjacencies with routers that aren't directly connected (but still in the same Area)?

    Thanks!

  2. #2
    Join Date
    Mar 2008
    Location
    Winston Salem, NC
    Posts
    432

    Default

    Ospf neighbors simply mean that they can exchange hello messages ,and that they are connected on the same subnetwork point of attatchment (SNPA), they are eighter in a point-to-point design, or that they are directly attacted to the same ethernet segment.

    OSPF adjacencies means that the neighbors have fully gone through all phases of the ospf process, and their databases are fully syncronized. on broadcast, and specific NMBA networks, routers will only form adjacencies with DR/BDR.

    hope this helps
    Cisco---------------------- CompTIA
    -CCNA ----------------------Security +
    -CCNP (BSCI,BCMSN,ISC W)

  3. #3
    Join Date
    Dec 2008
    Posts
    88

    Default

    Quote Originally Posted by lildeezul View Post

    OSPF adjacencies means that the neighbors have fully gone through all phases of the ospf process, and their databases are fully syncronized. on broadcast, and specific NMBA networks, routers will only form adjacencies with DR/BDR.

    hope this helps
    Page 447 says: All routers on the shared network will establish adjacencies with the DR and BDR.

    What if the DR is not directly connected to one of the routers?
    Last edited by typesh; 01-13-2009 at 08:11 PM.

  4. #4
    Join Date
    Mar 2008
    Location
    Winston Salem, NC
    Posts
    432

    Default

    The DR/BDR is for the local ethernet segment.... in reality the routers will connect to a switch, which will connect all other routers. therefore there are not directly connected, but they are on the same SNPA or ethernet segment. other routers will establish an adjacency with the DR/BDR of their segment
    Cisco---------------------- CompTIA
    -CCNA ----------------------Security +
    -CCNP (BSCI,BCMSN,ISC W)

  5. #5
    Join Date
    Jun 2009
    Posts
    31

    Smile Switch in the center doesn't look practical!

    hey lildeezul,

    Is switch in the center is practical? I mean in real-time situations 2(or more) routers would be logically connected(via broadband or ISDN).

    Switch in center with all routers connected to it through fast Ethernet is just impractical and wont happen in reality!(Its just my guess) please correct me if I am wrong!!, Any comment is appreciated!


    So What I mean from this is, DR/BDR elections in reality is not possible or they won't make anything better because most probably one router will be elected out of two( or three max.), generally!!

    What the norm is! hosts are connected to switch, Switch is connected to router and each router is connected to each other via ISDN (assuming one router for each branch)

    Regards!

  6. #6
    Join Date
    Nov 2008
    Location
    Birmingham, UK
    Posts
    1,437

    Default

    What happens when you have four or more routers all needing direct connectivity to each other? You can either install an interface for each router-router link (as in a full mesh), or you can install a single FE/GE link from each router to a switch and connect them all that way.

    If you are an ISP you may have many, many routers. Instead of daisy chaining them together, you'd connect them to a switch. Imagine the hop count if your packet had to go through 20 routers when all the routers were in the same physical location. What a waste of bandwidth!!
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  7. #7
    Join Date
    Mar 2008
    Posts
    2,888

    Default

    Quote Originally Posted by Fuzz View Post
    If you are an ISP you may have many, many routers. Instead of daisy chaining them together, you'd connect them to a switch. Imagine the hop count if your packet had to go through 20 routers when all the routers were in the same physical location. What a waste of bandwidth!!
    Welcome to BT wholesales network.....LOL !
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  8. #8
    Join Date
    Jun 2009
    Posts
    31

    Talking Contented!

    Yh! I was looking for an example actually! and you gave one! thanks buddy!

    But still even in medium-to-large companies, you won't find more then 10 or so routers(and again, assuming one router for each branch)! but as you said, Its(Switch-as-a-center) worth if we need more than one router in a branch, but again, Isn't this is a bit uncommon? I mean they can opt for something like VLANs!(segmenti ng their branch-network for each department!).

    And one more question is raised! We don't necessary need BD/BDR election-option in our network right?(from this I mean, for small or medium enterprise), so the use of DR/BDR elections comes into play on something exceptionally odd(as you mentioned, for ISPs), and its not the norm, right?

    Buying routers is expensive and not many companies would buy more than one for there entire branch! so OSPF is basically for very large or a bit odd networks(connec ting more than one router for a branch), do you agree?

    It would be nice to have your opinion!

    Regards,



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