RE: OSPF ADV Router is not-reachable

I don't know if Juniper implements it that way but that's how Cisco does it. In IOS you don't need to enable anything, it works like this by default.

For example take a very simple topology where R1 and R2 are OSPF adjacent. When the link between them goes down the routes are withdrawn from the routing table, but the LSAs are not withdrawn from the database. Instead the LSA is marked as " Adv Router is not-reachable"

R1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/DR 00:00:03 10.0.0.2 FastEthernet0/0

R1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 18 0x80000003 0x002BD1 2
2.2.2.2 2.2.2.2 19 0x80000003 0x001BD4 2

                Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
10.0.0.2 2.2.2.2 19 0x80000001 0x0043D6

R1#show ip ospf database router 2.2.2.2

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

  LS age: 25
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 2.2.2.2
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000003
  Checksum: 0x1BD4
  Length: 48
  Number of Links: 2

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 2.2.2.2
     (Link Data) Network Mask: 255.255.255.255
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.0.0.2
     (Link Data) Router Interface address: 10.0.0.2
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

R1#sh ip route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 10.0.0.2, 00:00:03, FastEthernet0/0

R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int f0/0
R1(config-if)#shut
R1(config-if)#end
R1#
*Dec 14 22:29:50.381: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Dec 14 22:29:50.801: %SYS-5-CONFIG_I: Configured from console by console
*Dec 14 22:29:52.377: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Dec 14 22:29:53.377: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down

R1#show ip ospf database router 2.2.2.2

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

  Adv Router is not-reachable in topology Base with MTID 0 <---------
  LS age: 37
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 2.2.2.2
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000003
  Checksum: 0x1BD4
  Length: 48
  Number of Links: 2

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 2.2.2.2
     (Link Data) Network Mask: 255.255.255.255
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.0.0.2
     (Link Data) Router Interface address: 10.0.0.2
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

R1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 14 0x80000004 0x00CA58 1
2.2.2.2 2.2.2.2 45 0x80000003 0x001BD4 2

                Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
10.0.0.2 2.2.2.2 45 0x80000001 0x0043D6

R1#show ip route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP
       + - replicated route, % - next hop override

Gateway of last resort is not set

R1#

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com

Internetwork Expert, Inc.
http://www.INE.com

From: HEMANTH RAJ [mailto:hemanthrj_at_gmail.com]
Sent: Thursday, December 13, 2012 6:50 PM
To: ccie99999
Cc: Brian McGahan; Cisco certification
Subject: Re: OSPF ADV Router is not-reachable

Hi Brian

I dont see this OSPF Optimization in Cisco Gear as wel as Juniper box where the Adjacency goes down adn all teh databases gets flushed immediately. Is there any special command we need to enable inorder to achieve this.

On Thu, Dec 13, 2012 at 10:15 PM, ccie99999 <ccie99999_at_gmail.com> wrote:
Thanks Brian,

it's more than clear now!

On Thu, Dec 13, 2012 at 4:38 PM, Brian McGahan <bmcgahan_at_ine.com> wrote:

> That's normal. This is a convergence optimization of OSPF in which it
> retains old LSAs until the MAXAGE is reached. The idea is that if the
> neighbor temporarily flaps, once they come back and they send a database
> descriptor you will realize that you already have their LSAs in the
> database from before, and the only thing you need to do is update the
> neighbor as reachable and reset the age & checksum. Otherwise OSPF would
> have to go through full flooding every time there is a neighbor flap. If
> you wait an hour (MAXAGE) you will see the LSA eventually disappear.
> Normally only the originator of the LSA can prematurely flush the LSA out
> of the database by reflooding with MAXAGE, and if that router is cut off
> from the entire OSPF network it cannot flood, hence your case.
>
>
> HTH,
>
> Brian McGahan, CCIE #8593 (R&S/SP/Security)
> bmcgahan_at_INE.com
>
> Internetwork Expert, Inc.
> http://www.INE.com
>
> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> ccie99999
> Sent: Thursday, December 13, 2012 5:56 AM
> To: Cisco certification
> Subject: OSPF ADV Router is not-reachable
>
> Hello friends,
>
> I'm working on OSPF in these days and I don't undertand this thing:
>
> I've a stable topology and I've one router (R3) that is my exit point for
> my nssa area..
> R3 indeed is configured as nssa no-summary.
> And in the routers in this area (it's area2) get him as only summary LSAs.
>
> Summary Net Link States (Area 2)
>
> Link ID ADV Router Age Seq# Checksum
> 0.0.0.0 150.1.3.3 1091 0x80000001 0x00BED9
>
> If now I decide to kill R3 (shut all interfaces) and even do a clear of
> ospf process, I still see the summary in the ospf database.
>
> If then I check the LSA1 on in it I see ADV Router is not-reachable.
>
> Rack1SW1#show ip ospf database router 150.1.3.3
>
> Adv Router is not-reachable
> LS age: 2292
>
> I don't understand the reason it's still there. Should't be flushed? and
> why by design it's not?
>
> thank you
>
>
> --
> @ccie99999
>
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