Re: OSPF cost behaviour

The reason behind this is order of OSPF preference of routes
1) Intra-area routes
2) Inter-area routes
3) E1 routes
4) E2 routes

The Type-5 LSA is originated by the ASBR R3 (Router-ID:172.30.0.1). And the
forward address would be 0.0.0.0, indicating to forward traffic for that
prefix to the ASBR itself. If you do a show ip ospf border-router on R2,
you'll see it has an intra area route to R3 (Router-ID:172.30.0.1) over
E0/1. When you shutdown the E0/1 interface R2 loses it's intra area route to
the ASBR, it will in turn have an inter area route to the ASBR via R1.

So as long as there is the intra-area route to the ASBR on R2, i.e since
ospf process on R2 finds both an intra area route and inter area route to
the ASBR, it will ignore the cost and simply choose the intra area route to
the ASBR.

Lejoe

On Wed, Jul 15, 2009 at 10:47 AM, <fernandosetegarcia_at_uol.com.br> wrote:

> Dear GS,
>
> I'm trying to figure out why manipulating interface OSPF cost does't
> change the path to get to a network on a scenario like this:
>
> 172.30.0.0/20
> R3 <http://172.30.0.0/20%0AR3>
> /B B B B B B \
> R1 --- R2
>
> R1-R3 - Area 10
> R2-R3 - Area 10
> R1-R2 - Area 0
>
> Increasing OSPF cost on path R2->R3 should't R2 prefer R1 to get to
> 172.30.0.0/20 ?
>
> R2#show ip route 172.30.0.0
> Routing entry for 172.30.0.0/20, 1 known subnets
>
> O E1B B B 172.30.0.0 [110/30] via 172.29.4.33, 00:00:01, Ethernet0/1
>
> After changing Cost to 1000, R2 still prefers same path, even with a
> higher cost:
>
> R2#conf t
> Enter configuration commands, one per line.B End with CNTL/Z.
> R2(config)#int et 0/1
> R2(config-if)#ip ospf cost 1000
> R2(config-if)#^Z
> R2#show ip route 172.30.0.0
> Routing entry for 172.30.0.0/20, 1 known subnets
>
> O E1B B B 172.30.0.0 [110/1020] via 172.29.4.33 , 00:00:10,
> Ethernet0/1B B <<<<----- still prefers same path
> R2#
>
> B
>
> I noticed route through R1 is not on LSDB (maybe it is the same ?):
>
> R2#show ip ospf database | b Type-5
> Type-5 AS External Link States
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Tag
> 172.30.0.0B B B B B 172.30.0.1B B B B B 342B B B B B B B B 0x80000002
> 0x00DFB8 0
> R2#
>
> B
>
> But if R2 loose path to R3, it works:
>
> R2#conf t
> R2(config)#int et 0/1
> R2(config-if)#shutdown
> R2(config-if)#^Z
> R2#show ip route 172.30.0.0
> Routing entry for 172.30.0.0/20, 1 known subnets
>
> O E1B B B 172.30.0.0 [110/40] via 192.168.1.62, 00:00:04, Ethernet0/0B
> <<<-----
>
> R2#show ip ospf database | b Type-5
> Type-5 AS External Link States
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Tag
> 172.30.0.0B B B B B 172.30.0.1B B B B B 599B B B B B B B B 0x80000002
> 0x00DFB8 0
> R2#
>
> Since this is my first post, hope it's not too long and not a dumb issue
> :)
>
> Kindest Regards,
>
> ---- checks ----
>
> R2#show ip ospf database
>
> OSPF Router with ID (192.168.1.61) (Process ID 1)
>
> Router Link States (Area 0)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Link count
> 192.168.1.61B B B 192.168.1.61B B B 1323B B B B B B B 0x80000007
> 0x007A09 1
> 192.168.1.62B B B 192.168.1.62B B B 1870B B B B B B B 0x80000003
> 0x008004 1
>
> Net Link States (Area 0)
>
> Link IDB B B B B B B B ADV Rout erB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 192.168.1.62B B B 192.168.1.62B B B 1871B B B B B B B 0x80000001
> 0x002A7D
>
> Summary Net Link States (Area 0)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 172.29.4.32B B B B 192.168.1.61B B B 153B B B B B B B B 0x80000006
> 0x00477E
> 172.29.4.32B B B B 192.168.1.62B B B 1304B B B B B B B 0x8000000B
> 0x00712F
> 172.29.4.48B B B B 192.168.1.61B B B 153B B B B B B B B 0x80000004
> 0x00B402
> 172.29.4.48B B B B 192.168.1.62B B B 1823B B B B B B B 0x80000003
> 0x00D6C2
>
> Summary ASB Link States (Area 0)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 172.30.0.1B B B B B 192.168.1.61B B B 153B B B B B B B B 0x80000004
> 0x00EEEA
> 172.30.0.1B B B B B 192.168.1.62B B B 1608B B B B B B B 0x80000001
> 0x001F9E
>
> Router Link States (Area 10)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Link count
> 172.30.0.1B B B B B 172.30.0.1B B B B B 1317B B B B B B B 0x80000008
> 0x00A8F8 2
> 192.168.1.61B B B 192.168.1.61B B B 159B B B B B B B B 0x8000000B
> 0x00DC2E 1
> 192.168.1.62B B B 192.168.1.62B B B 1833B B B B B B B 0x80000002
> 0x00B31B 1
>
> Net Link States (Area 10)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 172.29.4.33B B B B 172.30.0.1B B B B B 1313B B B B B B B 0x80000003
> 0x00E830
> 172.29.4.54B B B B 192.168.1.62B B B 1833B B B B B B B 0x80000001
> 0x0035F2
>
> Summary Net Link States (Area 10)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 192.168.1.48B B B 192.168.1.61B B B 1317B B B B B B B 0x80000005
> 0x006A91
> 192.168.1.48B B B 192.168.1.62B B B 1862B B B B B B B 0x80000003
> 0x006894
> Type-5 AS External Link States
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Tag
> 172.30.0.0B B B B B 172.30.0.1B B B B B 1590B B B B B B B 0x80000001
> 0x00E1B7 0
> R2#
>
> ---------------
>
> R1#show ip ospf database
>
> OSPF Router with ID (192.168.1.62) (Process ID 1)
>
> Router Link States (Area 0)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Link count
> 192.168.1.61B B B 192.168.1.61B B B 1393B B B B B B B 0x80000007
> 0x007A09 1
> 192.168.1.62B B B 192.168.1.62B B B 16B B B B B B B B B 0x80000004
> 0x007E05 1
>
> Net Link States (Area 0)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 192.168.1.62B B B 192.168.1.62B B B 16B B B B B B B B B 0x80000002
> 0x00287E
>
> Summary Net Link States (Area 0)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 172.29.4.32B B B B 192.168.1.61B B B 224B B B B B B B B 0x80000006
> 0x00477E
> 172.29.4.32B B B B 192.168.1.62B B B 1372B B B B B B B 0x8000000B
> 0x00712F
> 172.29.4.48B B B B 192.168.1.61B B B 224B B B B B B B B 0x80000004
> 0x00B402
> 172.29.4.48B B B B 192.168.1.62B B B 16B B B B B B B B B 0x80000004
> 0x00D4C3
>
> Summary ASB Link States (Area 0)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 172.30.0.1B B B B B 192.168.1.61B B B 224B B B B B B B B 0x80000004
> 0x00EEEA
> 172.30.0.1B B B B B 192.168.1.62B B B 1677B B B B B B B 0x80000001
> 0x001F9E
>
> Router Link States (Area 10)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Link count
> 172.30.0.1B B B B B 172.30.0.1B B B B B 1388B B B B B B B 0x80000008
> 0x00A8F8 2
> 192.168.1.61B B B 192.168.1.61B B B 231B B B B B B B B 0x8000000B
> 0x00DC2E 1
> 192.168.1.62B B B 192.168.1.62B B B 17B B B B B B B B B 0x80000003
> 0x00B11C 1
>
> Net Link States (Area 10)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 172.29.4.33B B B B 172.30.0.1B B B B B 1383B B B B B B B 0x80000003
> 0x00E830
> 172.29.4.54B B B B 192.168.1.62B B B 17B B B B B B B B B 0x80000002
> 0x0033F3
>
> Summary Net Link States (Area 10)
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum
> 192.168.1.48B B B 192.168.1.61B B B 1389B B B B B B B 0x80000005
> 0x006A91
> 192.168.1.48B B B 192.168.1.62B B B 17B B B B B B B B B 0x80000004
> 0x006695
>
> Type-5 AS External Link States
>
> Link IDB B B B B B B B ADV RouterB B B B B AgeB B B B B B B B
> Seq#B B B B B B Checksum Tag
> 172.30.0.0B B B B B 172.30.0.1B B B B B 1660B B B B B B B 0x80000001
> 0x00E1B7 0
> R1#
>
> ------ confs -----
>
> R1#
> router ospf 1
> log-adjacency-changes
> network 172.29.4.54 0.0.0.0 area 10
> network 192.168.1.62 0.0.0.0 area 0
>
> interface Ethernet0/1
> ip address 172.29.4.54 255.255.255.240
> full-duplex
> end
>
> R2#
> router ospf 1
> log-adjacency-changes
> network 172.29.4.38 0.0.0.0 area 10
> network 192.168.1.61 0.0.0.0 area 0
>
> interface Ethernet0/1
> ip address 172.29.4.38 255.255.255.240
> ip ospf cost 1000
> full-duplex
> end
>
> R3#
> interface FastEthernet1/1
> no switchport
> ip address 172.29.4.49 255.255.255.240
> end
>
> interface FastEthernet1/2
> no switchport
> ip address 172.29.4.33 255.255.255.240
> duplex full
> end
>
> !
> interface Loopback0
> ip address 172.30.0.1 255.255.240.0
> end
>
> router o spf 1
> log-adjacency-changes
> redistribute connected metric-type 1 subnets route-map CONN->OSPF
> network 172.29.4.33 0.0.0.0 area 10
> network 172.29.4.49 0.0.0.0 area 10
>
> route-map CONN->OSPF permit 10
> match interface Loopback0
>
> B
>
> B
>
> Fernando Sete Garcia
>
>
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Received on Wed Jul 15 2009 - 14:26:51 ART