OSPF route redistribution is an important topic on the BSCI exam, and it's a topic full of details and defaults that you need to know for the exam room and the job. To help you pass the BSCI exam, here's a quick review of some of the OSPF route redistribution basics.
To see if a router is an ABR or ASBR, run show ip ospf. This also displays any routes being redistributed into OSPF on this router.
R1#show ip ospf
Routing Process "ospf 1" with ID 1.1.1.1
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
rip, includes subnets in redistribution
When redistributing RIP into OSPF, the “subnets" option is needed to include subnets in redistribution. When redistributing OSPF into RIP, a seed metric must be specified. (OSPF gives redistributed routes a default metric of 20 – this can be changed, but a seed metric does not have to be set.)
R1(config)#router ospf 1
R1(config-router)#redistribute connected
% Only classful networks will be redistributed
R1(config-router)#redistribute connected subnets
R1(config-router)#redistribute rip subnets
R1(config-router)#router rip
R1(config-router)#redistribute connected metric 1
R1(config-router)#redistribute ospf 1 metric 1
By default, routes redistributed into OSPF are marked as E2 routes. The metric for these routes reflects only the cost of the path from the ASBR to the destination network and does not include the cost of the path from the local router to the ASBR. By contrast, E1 routes include the cost of the entire path from the local router to the destination network.
O E2 5.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E2 6.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
O E2 172.12.21.0/30 [110/20] via 172.34.34.3, 00:33:32, Ethernet0
O E2 7.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
15.0.0.0/24 is subnetted, 1 subnets
O E2 15.1.1.0 [110/20] via 172.34.34.3, 00:33:32, Ethernet0
To redistribute routes into OSPF and mark them as E1 upon redistribution, use the metric-type option with the redistribution command.
R1(config)#router ospf 1
R1(config-router)#redistribute rip subnets metric-type ?
1 Set OSPF External Type 1 metrics
2 Set OSPF External Type 2 metrics
R1(config-router)#redistribute rip subnets metric-type 1
Look at the same two routes in R4's routing table, which are now displayed as E1 routes:
O E1 5.1.1.1 [110/94] via 172.34.34.3, 00:04:13, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E1 6.1.1.1 [110/94] via 172.34.34.3, 00:04:14, Ethernet0
BSCI exam success and earning your CCNP certification depends on knowing the details, and there are plenty of details involved in OSPF route redistribution! Keep studying, practice different scenarios in your CCNA / CCNP home lab or rack rental, and you'll master these details and pass the exam!
Showing posts with label external. Show all posts
Showing posts with label external. Show all posts
Thursday, December 25, 2008
Cisco CCNP / BSCI Exam Tutorial: EIGRP Route Summarization
Summarizing routes is a vital skill to learn to pass the BSCI exam and get one step closer to earning your CCNP. The actual binary conversions are only part of the test, though! You've got to know how to correctly apply the summary routes, and that differs from one protocol to the next. In the last few CCNP / BSCI tutorials, we've looked at using the "area range" and "summary-address" commands to perform OSPF route summarization. Today, we'll take a look at summarizing routes in EIGRP.
We'll use the following four loopback addresses in this example:
Loopback 16, 16.16.16.16 /32
Loopback 17, 17.17.17.17 /32
Loopback 18, 18.18.18.18 /32
Loopback 19. 19.19.19.19 /32
On R1, we'll place these four addresses into EIGRP AS 100.
R1(config-if)#router eigrp 100
R1(config-router)#network 16.16.16.16 0.0.0.0
R1(config-router)#network 17.17.17.17 0.0.0.0
R1(config-router)#network 18.18.18.18 0.0.0.0
R1(config-router)#network 19.19.19.19 0.0.0.0
R3 is an EIGRP neighbor of R1, and that router's EIGRP routing table now looks like this:
R3#show ip route eigrp
17.0.0.0/32 is subnetted, 1 subnets
D 17.17.17.17 [90/2297856] via 172.12.123.1, 00:00:29, Serial0
16.0.0.0/32 is subnetted, 1 subnets
D 16.16.16.16 [90/2297856] via 172.12.123.1, 00:00:36, Serial0
19.0.0.0/32 is subnetted, 1 subnets
D 19.19.19.19 [90/2297856] via 172.12.123.1, 00:00:08, Serial0
18.0.0.0/32 is subnetted, 1 subnets
D 18.18.18.18 [90/2297856] via 172.12.123.1, 00:00:22, Serial0
To perform manual route summarization, write out the network addresses in binary and then determine the point at which the addresses no longer have a bit in common. For these four addresses, it will be enough to write out the first octet in binary:
16 00010000
17 00010001
18 00010010
19 00010011
Working from left to right, the common bits are the first six bits - 000100xx. In decimal, this value is 16. The summary mask must be determined as well, and that value is derived from putting a "1" in the mask for each common bit. With the first six bits all set to one - 11111100 - the resulting mask is 252.0.0.0. The full summary address is 16.0.0.0 252.0.0.0.
In EIGRP, the summary address is actually configured on an interface, not under the routing process.
R1(config)#interface serial0
R1(config-if)#ip summary-address eigrp 100 16.0.0.0 252.0.0.0
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is down: summary configured
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is down: summary configured
02:40:16: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is up : new adjacency
02:40:17: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is up: new adjacency
There's an immediate side effect here that most books leave out. Your EIGRP adjacencies are going to come down after you configure this summary, but they should come back up quickly. The key word there is "should". If you configure EIGRP summary addresses on a production network, you may want to do this during non-peak hours. The timestamps on the above commands indicate that the adjacencies were down for about 27 seconds over the NBMA network. That's about 30 minutes in end-user time. ;)
Check R3's EIGRP routing table.
R3#show ip route eigrp
D 16.0.0.0/6 [90/2297856] via 172.12.123.1, 00:01:46, Serial0
The four summarized routes are no longer in the routing table, and they have been replaced by the summary route shown at the bottom of the routing table. Notice the mask is /5, which is prefix notation for 248.0.0.0.
Knowing how and why to summarize routes is a valuable skill, regardless of the protocol in use. But before you take the BSCI exam on your way to the CCNP, make sure you know how to perform summarization with all of the core protocols!
We'll use the following four loopback addresses in this example:
Loopback 16, 16.16.16.16 /32
Loopback 17, 17.17.17.17 /32
Loopback 18, 18.18.18.18 /32
Loopback 19. 19.19.19.19 /32
On R1, we'll place these four addresses into EIGRP AS 100.
R1(config-if)#router eigrp 100
R1(config-router)#network 16.16.16.16 0.0.0.0
R1(config-router)#network 17.17.17.17 0.0.0.0
R1(config-router)#network 18.18.18.18 0.0.0.0
R1(config-router)#network 19.19.19.19 0.0.0.0
R3 is an EIGRP neighbor of R1, and that router's EIGRP routing table now looks like this:
R3#show ip route eigrp
17.0.0.0/32 is subnetted, 1 subnets
D 17.17.17.17 [90/2297856] via 172.12.123.1, 00:00:29, Serial0
16.0.0.0/32 is subnetted, 1 subnets
D 16.16.16.16 [90/2297856] via 172.12.123.1, 00:00:36, Serial0
19.0.0.0/32 is subnetted, 1 subnets
D 19.19.19.19 [90/2297856] via 172.12.123.1, 00:00:08, Serial0
18.0.0.0/32 is subnetted, 1 subnets
D 18.18.18.18 [90/2297856] via 172.12.123.1, 00:00:22, Serial0
To perform manual route summarization, write out the network addresses in binary and then determine the point at which the addresses no longer have a bit in common. For these four addresses, it will be enough to write out the first octet in binary:
16 00010000
17 00010001
18 00010010
19 00010011
Working from left to right, the common bits are the first six bits - 000100xx. In decimal, this value is 16. The summary mask must be determined as well, and that value is derived from putting a "1" in the mask for each common bit. With the first six bits all set to one - 11111100 - the resulting mask is 252.0.0.0. The full summary address is 16.0.0.0 252.0.0.0.
In EIGRP, the summary address is actually configured on an interface, not under the routing process.
R1(config)#interface serial0
R1(config-if)#ip summary-address eigrp 100 16.0.0.0 252.0.0.0
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is down: summary configured
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is down: summary configured
02:40:16: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is up : new adjacency
02:40:17: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is up: new adjacency
There's an immediate side effect here that most books leave out. Your EIGRP adjacencies are going to come down after you configure this summary, but they should come back up quickly. The key word there is "should". If you configure EIGRP summary addresses on a production network, you may want to do this during non-peak hours. The timestamps on the above commands indicate that the adjacencies were down for about 27 seconds over the NBMA network. That's about 30 minutes in end-user time. ;)
Check R3's EIGRP routing table.
R3#show ip route eigrp
D 16.0.0.0/6 [90/2297856] via 172.12.123.1, 00:01:46, Serial0
The four summarized routes are no longer in the routing table, and they have been replaced by the summary route shown at the bottom of the routing table. Notice the mask is /5, which is prefix notation for 248.0.0.0.
Knowing how and why to summarize routes is a valuable skill, regardless of the protocol in use. But before you take the BSCI exam on your way to the CCNP, make sure you know how to perform summarization with all of the core protocols!
Cisco CCNP / BSCI Certification Exam: Five OSPF Details You Must Know
Preparing for your BSCI exam on your way to the Cisco CCNP certification, you can quickly get overwhelmed by the details! Here are five commonly overlooked points you should keep in mind when it comes to your OSPF studies.
The virtual link command includes the area number of the transit area, and if authentication is being used on Area 0, the virtual link command must include the authentication statement. Since the virtual link is a logical extension of Area 0, it stands to reason that it has to be configured with the authentication type and password configured on Area 0.
OSPF requires no seed metric when routes are being redistributed into an OSPF domain. The default cost for such routes is 20, but you do need to use the "subnets" option if you want to redistribute subnets into OSPF.
There are two kinds of external OSPF routes. The default, E2, reflects the cost of the path from the ASBR to the external destination. The other option, E1, has a cost reflecting the entire path from the local router to the external destination.
When configuring stub areas, each router in the area must agree that the area is stub. For a total stub area, only the ABR needs to be configured with the "no-summary" option, but all routers in the area still must agree that the area is stub.
Routers in a stub area will have a default route to use to reach external destinations; routers in total stub areas will have a default route to use in order to reach both external and inter-area networks.
The BSCI exam and CCNP certification require a great deal of dedication and hard work. Keep studying and paying attention to the details, and you will get there!
The virtual link command includes the area number of the transit area, and if authentication is being used on Area 0, the virtual link command must include the authentication statement. Since the virtual link is a logical extension of Area 0, it stands to reason that it has to be configured with the authentication type and password configured on Area 0.
OSPF requires no seed metric when routes are being redistributed into an OSPF domain. The default cost for such routes is 20, but you do need to use the "subnets" option if you want to redistribute subnets into OSPF.
There are two kinds of external OSPF routes. The default, E2, reflects the cost of the path from the ASBR to the external destination. The other option, E1, has a cost reflecting the entire path from the local router to the external destination.
When configuring stub areas, each router in the area must agree that the area is stub. For a total stub area, only the ABR needs to be configured with the "no-summary" option, but all routers in the area still must agree that the area is stub.
Routers in a stub area will have a default route to use to reach external destinations; routers in total stub areas will have a default route to use in order to reach both external and inter-area networks.
The BSCI exam and CCNP certification require a great deal of dedication and hard work. Keep studying and paying attention to the details, and you will get there!
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