Re: OSPF ADV Router is not-reachable

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 When the link between them goes down the routes are withdrawn from the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 routing table, but the LSAs are not withdrawn from the database.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
>> >
>> >
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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 - 18:01:35 ART