# OSPF Packets and Neighbor Discovery

**URL:** https://forum.networklessons.com/t/ospf-packets-and-neighbor-discovery/916
**Category:** Lessons Discussion
**Created:** [December 22, 2016, 1:00am UTC](https://forum.networklessons.com/t/ospf-packets-and-neighbor-discovery/916 "2016-12-22T01:00:06Z")
**Posts on this page:** 1
**Showing post:** 33

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### Author: ![skazeem96](https://cdn-forum.networklessons.com/letter_avatar_proxy/v4/letter/s/a8b319/32.png) [@skazeem96](https://forum.networklessons.com/u/skazeem96)
#### Post date: [October 12, 2016, 2:09am UTC](https://forum.networklessons.com/t/ospf-packets-and-neighbor-discovery/916/33 "2016-10-12T02:09:12Z")

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Below is my scenario with 2 Routers and each router configured with 3 loopback interface and two Fast Ethernet interfaces are up.  
(Router A - loopback networks: 10.1.1.1/8, 11.1.1.1/8, 12.1.1.1/8 and Fast Ethernet 0/1 99.99.99.1/24 and Fast Ethernet 0/0 100.100.100.1/24).

```
 (100.100.100.1) (100.100.100.2)
    Router A Router B

        ----------Down staste----------
	----------init staste----------
	----------two-way staste-------
	----------extart staste--------

```

After master Slave negotiation, Router A considered as Slave and send below DBD packet with a summary of the LSDB which contain LSA-type 1 (Router-LSA)

```
Ethernet II, Src: c4:01:0a:84:00:00 (c4:01:0a:84:00:00), Dst: c4:02:03:f4:00:00 (c4:02:03:f4:00:00)
Internet Protocol Version 4, Src: 100.100.100.1, Dst: 100.100.100.2
Open Shortest Path First
    OSPF Header
    OSPF DB Description
        Interface MTU: 1500
        Options: 0x52 (O, (L) LLS Data block, (E) External Routing)
        DB Description: 0x02 ((M) More)
            .... 0... = (R) OOBResync: Not set
            .... .0.. = (I) Init: Not set
            .... ..1. = (M) More: Set
            .... ...0 = (MS) Master: No
        DD Sequence: 242
    LSA-type 1 (Router-LSA), len 72
        .000 0000 0011 0101 = LS Age (seconds): 53
        0... .... .... .... = Do Not Age Flag: 0
        Options: 0x22 ((DC) Demand Circuits, (E) External Routing)
        LS Type: Router-LSA (1)
        Link State ID: 12.1.1.1
        Advertising Router: 12.1.1.1
        Sequence Number: 0x80000001
        Checksum: 0x2157
        Length: 72
    OSPF LLS Data Block

```

In your explanation,  
“In the exchange state, our routers are sending a DBD with a summary of the LSDB. This way the routers can find out what networks they don’t know about”.

My question:

1. When Router B receives this DBD packet from Router A, how Router B understand that Router A connected networks ( 10.1.1.1/8, 11.1.1.1/8, 12.1.1.1/8,99.99.99.1/24,100.100.100.1/24) ? this network information is not shown in the LSA-type 1 (Router-LSA).Now How Router B will find out that what networks they don’t know about and what networks they know about?

2)Router A is sending Sequence Number: 0x80000001 and now what Sequence Number Router B will have to compare with Sequence Number received from Router A ?

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_[View the full topic](https://forum.networklessons.com/t/ospf-packets-and-neighbor-discovery/916)._