Re: OSPF ADV Router is not-reachable

I was more speaking specifically about intra-area LSAs, but I certainly get
your point.

Keller Giacomarro
keller.g_at_gmail.com

On Fri, Dec 14, 2012 at 8:01 PM, Marko Milivojevic <markom_at_ipexpert.com>wrote:

> This is not always true, especially when dealing with external LSAs
> that carry FA.
>
> On Fri, Dec 14, 2012 at 5:31 PM, Keller Giacomarro <keller.g_at_gmail.com>
> wrote:
> > 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 When the link between them goes down the routes are withdrawn from the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 routing table, but the LSAs are not withdrawn from the database.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
> >> >
> >> >
> >> > Blogs and organic groups at http://www.ccie.net
> >> >
> >> >
> _______________________________________________________________________
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> >> >
> >> >
> >> >
> >> >
> >> >
> >> >
> >> >
> >> >
> >>
> >>
> >> --
> >> @ccie99999
> >>
> >>
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> >>
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> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> --
> >>
> >> Regards
> >> Hemanth Raj
> >> CCIE#28593 (R&S)
> >>
> >>
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> >
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Received on Fri Dec 14 2012 - 20:08:28 ART