From: Brian Dennis (bdennis@internetworkexpert.com)
Date: Fri Dec 23 2005 - 22:16:58 GMT-3
When you shut down the serial interface on R4, R4 is not a backbone router anymore since it's only interface in area 0 is now down. R4's path to reach any network is now via the Ethernet. But when the serial interface is up, R4 MUST send the traffic to R2 irrespective of the cost across the Ethernet.
Traffic can not go like this: Area 3 -> Area 2. It must go Area 3 -> Area 0 -> Area 2.
As per the RFC:
3.2. Inter-area routing
When routing a packet between two non-backbone areas the
backbone is used. The path that the packet will travel can be
broken up into three contiguous pieces: an intra-area path from
the source to an area border router, a backbone path between the
source and destination areas, and then another intra-area path
to the destination. The algorithm finds the set of such paths
that have the smallest cost.
Lastly if you are still confused I would highly recommend that you draw this out on paper and then consider how OSPF handles inter-area traffic.
HTH,
Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
________________________________________
From: Zukque [mailto:zukque@gmail.com]
Sent: Friday, December 23, 2005 5:05 PM
To: Brian Dennis
Cc: Cisco certification
Subject: Re: IP OSPF COST for IA's
Brian, Sorry but I do not get it
R4>en
R4#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, 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
Gateway of last resort is not set
2.0.0.0/24 is subnetted, 1 subnets
O E2 2.2.2.0 [110/20] via 183.1.42.2, 05:06:12, Ethernet0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/1065] via 183.1.0.2, 05:06:12, Serial0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
183.1.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 183.1.42.0/24 is directly connected, Ethernet0
O 183.1.0.2/32 [110/1000] via 183.1.0.2, 05:12:17, Serial0
O 183.1.0.3/32 [110/1064] via 183.1.0.2, 05:12:18, Serial0
C 183.1.0.0/24 is directly connected, Serial0
R4#show ip ospf
Routing Process "ospf 1" with ID 183.1.0.4 and Domain ID 0.0.0.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an area border and autonomous system boundary router
Redistributing External Routes from,
connected, includes subnets in redistribution
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 2. Checksum Sum 0x17F1B
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 28 times
Area ranges are
Number of LSA 8. Checksum Sum 0x44A36
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 3
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 18 times
Area ranges are
Number of LSA 13. Checksum Sum 0x5D8B5
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
R4#show ip ospf interface
Serial0 is up, line protocol is up
Internet Address 183.1.0.4/24 , Area 0
Process ID 1, Router ID 183.1.0.4, Network Type POINT_TO_MULTIPOINT, Cost: 1000
Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT,
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
Hello due in 00:00:03
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 2, maximum is 2
Last flood scan time is 4 msec, maximum is 4 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 183.1.0.2
Suppress hello for 0 neighbor(s)
Ethernet0 is up, line protocol is up
Internet Address 183.1.42.4/24, Area 3
Process ID 1, Router ID 183.1.0.4 , Network Type BROADCAST, Cost: 65
Transmit Delay is 1 sec, State DROTHER, Priority 0
Designated Router (ID) 183.1.0.2, Interface address 183.1.42.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Index 1/2, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 5
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 183.1.0.2 (Designated Router)
Suppress hello for 0 neighbor(s)
R4#
R4#
R4#conf ter
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#interface serial 0
R4(config-if)#shutdown
R4(config-if)#^Z
R4#
R4#
3d21h: is_up: 0 state: 6 sub state: 1 line: 0
3d21h: %OSPF-5-ADJCHG: Process 1, Nbr 183.1.0.2 on Serial0 from FULL to DOWN, Neighbor Down: Interface down or detached
3d21h: RT: interface Serial0 removed from routing table
3d21h: RT: del 183.1.0.0/24 via 0.0.0.0, connected metric [0/0]
3d21h: RT: delete subnet route to 183.1.0.0/24
3d21h: RT: delete route to 3.3.3.3 via 183.1.0.2, Serial0
3d21h: RT: no routes to 3.3.3.3 , flushing
3d21h: RT: delete network route to 3.0.0.0
3d21h: RT: delete route to 183.1.0.2 via 183.1.0.2, Serial0
3d21h: RT: no routes to 183.1.0.2, flushing
3d21h: RT: delete route to 183.1.0.3 via 183.1.0.2, Serial0
3d21h: RT: no routes to 183.1.0.3, flushing
R4#
3d21h: %SYS-5-CONFIG_I: Configured from console by console
R4#
R4#
R4#
3d21h: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
R4#
R4#
R4#
3d21h: is_up: 0 state: 6 sub state: 1 line: 0
3d21h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down
R4#
3d21h: is_up: 0 state: 6 sub state: 1 line: 0
R4#
R4#
R4#
3d21h: RT: add 3.3.3.3/32 via 183.1.42.2 , ospf metric [110/130]
3d21h: RT: network 183.1.0.0 is now variably masked
3d21h: RT: add 183.1.0.2/32 via 183.1.42.2, ospf metric [110/65]
3d21h: RT: add 183.1.0.3/32 via 183.1.42.2, ospf metric [110/129]
R4#
R4#
R4#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, 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
Gateway of last resort is not set
2.0.0.0/24 is subnetted, 1 subnets
O E2 2.2.2.0 [110/20] via 183.1.42.2, 00:00:04, Ethernet0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/130] via 183.1.42.2, 00:00:05, Ethernet0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
183.1.0.0/16 is variably subnetted, 3 subnets, 2 masks
C 183.1.42.0/24 is directly connected, Ethernet0
O IA 183.1.0.2/32 [110/65] via 183.1.42.2, 00:00:05, Ethernet0
O IA 183.1.0.3/32 [110/129] via 183.1.42.2, 00:00:06, Ethernet0
R4#
R4#show ip int brief
Interface IP-Address OK? Method Status Protocol
BRI0 unassigned YES unset administratively down down
BRI0:1 unassigned YES unset administratively down down
BRI0:2 unassigned YES unset administratively down down
Ethernet0 183.1.42.4 YES manual up up
Loopback0 4.4.4.4 YES manual up up
Serial0 183.1.0.4 YES manual administratively down down
Serial1 unassigned YES unset administratively down down
On 12/23/05, Brian Dennis <bdennis@internetworkexpert.com> wrote:
Remember that all inter-area traffic must transit area 0 (an ABR). It doesn't matter what the cost is to reach R3's loopback which is in area 2 across the Ethernet segment that is area 3.
To reach a route in area 2 the traffic must use the area 0 path over the path across area 3. If you want to "transit" area 3 to reach area 2 look into using a virtual-link across the Ethernet or a tunnel.
HTH,
Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
________________________________________
From: Zukque [mailto:zukque@gmail.com ]
Sent: Friday, December 23, 2005 3:06 PM
To: Brian Dennis
Cc: Cisco certification
Subject: Re: IP OSPF COST for IA's
Thanks for your help, this is a very simple topology..sorry not good drawing also :)
R2 is the Hub
R3 and R4 are the spokes
R4 is also conected to R2 via Ethernet
Lo0 3.3.3.3/32
!
R3 -S0-------------------------------
---------------------------------------S0-R2 -Lo0 2.2.2.2/32
R4-S0--------------------------------- |
Eth-----------------------------------Eth
R3 Lo0 belongs to area 2
R2, R3, R4 S0s are in area 0
R4 Eth to R2 Ethe si in area 3
R2 is redistributing this Lo0 in OSPF using the default type
------------------------------------------------
R2
interface Ethernet0
ip address 183.1.42.2 255.255.255.0
!
interface Serial0
ip address 183.1.0.2 255.255.255.0
encapsulation frame-relay
ip ospf network point-to-multipoint
no frame-relay inverse-arp
frame-relay map ip 183.1.0.3 203 broadcast
frame-relay map ip 183.1.0.4 204 broadcast
!
router ospf 1
log-adjacency-changes
redistribute connected subnets route-map Lo0
network 183.1.0.0 0.0.0.255 area 0
network 183.1.42.2 0.0.0.0 area 3
!
-----------------------------------------------
R3:
!
interface Loopback0
ip address 3.3.3.3 255.255.255.255
interface Serial0
ip address 183.1.0.3 255.255.255.0
encapsulation frame-relay
ip ospf network point-to-multipoint
frame-relay map ip 183.1.0.2 302 broadcast
frame-relay map ip 183.1.0.4 302
no frame-relay inverse-arp
router ospf 1
log-adjacency-changes
network 3.3.3.3 0.0.0.0 area 23
network 183.1.0.0 0.0.0.255 area 0
------------------------------------------
R4:
interface Loopback0
ip address 4.4.4.4 255.255.255.255
ip ospf network point-to-point
!
interface Ethernet0
ip address 183.1.42.4 255.255.255.0
ip ospf priority 0
interface Serial0
ip address 183.1.0.4 255.255.255.0
encapsulation frame-relay
ip ospf network point-to-multipoint
ip ospf cost 1000
ip ospf priority 0
frame-relay map ip 183.1.0.2 402 broadcast
frame-relay map ip 183.1.0.3 402
no frame-relay inverse-arp
On 12/23/05, Brian Dennis < bdennis@internetworkexpert.com> wrote:
Can you post your configurations and a little information about the topology?
HTH,
Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
________________________________________
From: Zukque [mailto: zukque@gmail.com]
Sent: Friday, December 23, 2005 1:37 PM
To: Brian Dennis
Cc: Cisco certification
Subject: Re: IP OSPF COST for IA's
Brain, but why to get to route 3.3.3.3 the router is choosing S0, E0 has a better cost
On 12/23/05, Brian Dennis < bdennis@internetworkexpert.com > wrote:
You have four OSPF routes in your routing table:
2.0.0.0/24 is subnetted, 1 subnets
O E22.2.2.0 [110/20] via 183.1.0.2, 00:24:45, Serial0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/129] via 183.1.0.2, 00:24:45, Serial0
O 183.1.0.2/32 [110/64] via 183.1.0.2, 00:31:22, Serial0
O 183.1.0.3/32 [110/128] via 183.1.0.2, 00:31:24, Serial0
You adjusted the OSPF cost on the interface to 1000:
2.0.0.0/24 is subnetted, 1 subnets
O E2 2.2.2.0 [110/20] via 183.1.42.2 , 00:01:24, Ethernet0
3.0.0.0/32 is subnetted, 1 subnets
O IA3.3.3.3 [110/1065] via 183.1.0.2, 00:01:24, Serial0
4.0.0.0/32 is subnetted, 1 subnets
O 183.1.0.2/32 [110/1000] via 183.1.0.2, 00:01:24, Serial0
O 183.1.0.3/32 [110/1064] via 183.1.0.2, 00:01:25, Serial0
All of the intra and inter area routes cost have changed.The only
route that did not change is the E2 route which as you know should not
change.
As per RFC 2328:
Cost
The link state cost of the path to the destination.For all
paths except type 2 external paths this describes the entire
path's cost.For Type 2 external paths, this field describes
the cost of the portion of the path internal to the AS.This
cost is calculated as the sum of the costs of the path's
constituent links.
HTH,
Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com ] On Behalf Of
Zukque
Sent: Friday, December 23, 2005 11:50 AM
To: Cisco certification
Subject: IP OSPF COST for IA's
Dears
Why IA OSPF route's are not affected with the ip ospf cost command?
Is there a preference for Intra / Inter / External Route? Please can you
show me where I can find this in 2328
Thanks!
That is: (take a look at 3.3.3.3)
R#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address
Interface
183.1.0.2 1 FULL/-00:01:51 183.1.0.2
Serial0
183.1.0.2 1 FULL/BDR00:00:33 183.1.42.2
Ethernet0
R#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, 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
Gateway of last resort is not set
2.0.0.0/24 is subnetted, 1 subnets
O E22.2.2.0 [110/20] via 183.1.0.2 , 00:24:45, Serial0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/129] via 183.1.0.2, 00:24:45, Serial0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
183.1.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 183.1.42.0/24 is directly connected, Ethernet0
O 183.1.0.2/32 [110/64] via 183.1.0.2, 00:31:22, Serial0
O 183.1.0.3/32 [110/128] via 183.1.0.2, 00:31:24, Serial0
C 183.1.0.0/24 is directly connected, Serial0
R#
R(config)#interface s0
R(config-if)#ip ospf cost 1000
R(config-if)#^Z
R#clear ip route *
3d16h: RT: add 183.1.42.0/24 via 0.0.0.0, connected metric [0/0]
3d16h: RT: add 4.4.4.4/32 via 0.0.0.0, connected metric [0/0]
3d16h: RT: add 183.1.0.0/24 via 0.0.0.0, connected metric [0/0]
3d16h: RT: network 183.1.0.0 is now variably masked
3d16h: RT: add 183.1.0.2/32 via 183.1.0.2, ospf metric [110/1000]
3d16h: RT: add 183.1.0.3/32 via 183.1.0.2, ospf metric [110/1064]
3d16h: RT: add 3.3.3.3/32 via 183.1.0.2, ospf metric [110/1065]
3d16h: RT: add 2.2.2.0/24 via 183.1.42.2, ospf metric [110/20]
R#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, 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
Gateway of last resort is not set
2.0.0.0/24 is subnetted, 1 subnets
O E22.2.2.0 [110/20] via 183.1.42.2 , 00:01:24, Ethernet0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/1065] via 183.1.0.2, 00:01:24, Serial0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
183.1.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 183.1.42.0/24 is directly connected, Ethernet0
O 183.1.0.2/32 [110/1000] via 183.1.0.2, 00:01:24, Serial0
O 183.1.0.3/32 [110/1064] via 183.1.0.2, 00:01:25, Serial0
C 183.1.0.0/24 is directly connected, Serial0
R#show ip ospf database
OSPF Router with ID ( 183.1.0.4) (Process ID 1)
Router Link States (Area 0)
Link ID ADV RouterAge Seq# Checksum Link
count
183.1.0.2 183.1.0.2 427 0x80000010 0x4A1C 3
183.1.0.3 183.1.0.3 14440x80000007 0xDD56 2
183.1.0.4 183.1.0.4 232 0x80000013 0xEE87 2
Summary Net Link States (Area 0)
Link ID ADV RouterAge Seq# Checksum
3.3.3.3 183.1.0.3 17010x80000002 0x77FB
183.1.42.0183.1.0.2 427 0x8000000B 0xBD69
183.1.42.0183.1.0.4 14800x80000009 0x4014
Summary ASB Link States (Area 0)
Link ID ADV RouterAge Seq# Checksum
183.1.0.2 183.1.0.4 19440x80000004 0xF788
183.1.0.4 183.1.0.2 179 0x80000004 0x65ED
Router Link States (Area 3)
Link ID ADV RouterAge Seq# Checksum Link
count
183.1.0.2 183.1.0.2 510x8000000A 0x66CA 2
183.1.0.4 183.1.0.4 530x8000000B 0xB8C9 2
Net Link States (Area 3)
Link ID ADV RouterAge Seq# Checksum
183.1.42.2 183.1.0.2 510x80000001 0x1D06
183.1.42.4 183.1.0.4 959 0x80000005 0xEC2E
Summary Net Link States (Area 3)
Link ID ADV RouterAge Seq# Checksum
3.3.3.3 183.1.0.2 16910x80000002 0xFF34
3.3.3.3 183.1.0.4 228 0x80000004 0x2321
183.1.0.2 183.1.0.2 11940x80000004 0x853F
183.1.0.2 183.1.0.4 228 0x80000004 0xAD29
183.1.0.3 183.1.0.2 11940x80000004 0xFD85
183.1.0.3 183.1.0.4 228 0x80000004 0x2570
183.1.0.4 183.1.0.2 429 0x80000002 0xF78C
183.1.0.4 183.1.0.4 14820x80000002 0x6959
Summary ASB Link States (Area 3)
Link ID ADV RouterAge Seq# Checksum
183.1.0.2 183.1.0.4 228 0x80000004 0x9F36
183.1.0.4 183.1.0.2 429 0x80000002 0xE999
Type-5 AS External Link States
Link ID ADV RouterAge Seq# Checksum Tag
2.2.2.0 183.1.0.2 429 0x80000004 0x10CC 0
4.4.4.4 183.1.0.4 214 0x80000004 0x933D 0
Thanks !!!!
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