Re: OSPF ADV Router is not-reachable

Keep in mind that reachability has to do with OSPF SPF reachability, not IP
reachability. "Adv Router is not reachable" basically means that in the
SPF graph, there are no paths to the node.

Note that the same kind of thing happens when the DR for an Ethernet
segments goes down. Normally, the T2 LSA is originated by the DR and lists
all the attached routers. The routers, in turn, list the T2 LSA as
attached. If the DR goes down without withdrawing the T2 LSA, a new DR is
elected, a new T2 LSA is generated, and all the routers attach to the new
T2 LSA instead of the old one. However, the OLD T2 LSA is still in the
LSDB, but shows as not reachable.

This whole topic really confused me for a bit too, until I realized that
LSAs have everything to do with the SPF graph and how the nodes connect,
and less to do with actual IP subnets on the network.

Keller Giacomarro
keller.g_at_gmail.com

On Fri, Dec 14, 2012 at 4:33 PM, Brian McGahan <bmcgahan_at_ine.com> wrote:

> 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 When the link between them goes down the routes are withdrawn from the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 routing table, but the LSAs are not withdrawn from the database.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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> >
> >
> >
> >
> >
> >
>
>
> --
> @ccie99999
>
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> Regards
> Hemanth Raj
> CCIE#28593 (R&S)
>
>
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Received on Fri Dec 14 2012 - 19:31:03 ART