To pass the BSCI exam and earn your CCNP certification, you've got to master the (many) details of OSPF. You might have thought there were quite a few OSPF details in your CCNA studies, but you'll now build on that foundation on the way to earning your CCNP.
One such detail is the role of the Autonomous System Border Router (ASBR) in OSPF. The name itself raises some eyebrows, since you learned in your CCNA studies that OSPF doesn't use autonomous systems! Just as an OSPF Area Border Router borders multiple OSPF areas, the ASBR borders the entire OSPF domain and another source of routes. This can be another dynamic routing protocol, or directly connected networks that are not being advertised into OSPF by the network command.
Let's say we have a router running both OSPF and RIP version 2. By default, the RIP process will not contain any OSPF-discovered routes, and vice versa. The two separate routing processes are just that - separate. If we want the other OSPF routers to know about the RIP routes, route redistribution must be configured. When the RIP routes are redistributed into OSPF, that router is then an ASBR.
In the below example, RIP subnets have been redistributed into OSPF. A seed metric is not necessary when redistributing routes into OSPF. The command "show ip ospf" confirms that this router is now an ASBR.
R1(config)#router ospf 1
R1(config-router)#redistribute rip subnets
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 autonomous system boundary router
The ASBR can also perform route summarization on the routes being injected into OSPF with the summary-address command. (To configure OSPF inter-area summarization, use the area range command.) By mastering route summarization and route redistribution, you're well on your way to passing the BSCI exam and earning your CCNP certification!
Showing posts with label asbr. Show all posts
Showing posts with label asbr. Show all posts
Thursday, December 25, 2008
Cisco CCNP / BSCI Exam Tutorial: OSPF Router Types
When you're preparing to pass the BSCI exam on the way to the coveted Cisco CCNP certification, you can be quickly overwhelmed by the sheer amount of BGP and OSPF knowledge you must demonstrate a mastery of. One set of details that some BSCI and CCNP candidates underestimate are the differences between the OSPF router types.
An OSPF Internal router has one rule - it must have all its interfaces in a single area. It does not mean that area has to be Area 0.
An OSPF Backbone router is a router with at least a single area in the OSPF backbone area, Area 0. A router can be both an Internal and Backbone router if all its interfaces are in Area 0.
An Area Border Router has at least one interface in Area 0 and another interface in a non-backbone area. ABRs are also one of two router types that can perform OSPF route summarization. (To advertise a summary route from one OSPF area to another, use the area range command on the ABR.)
Finally, an ASBR is an OSPF router that is performing route redistribution by injecting routes from another source into the OSPF domain. This is the other OSPF router type that can perform route summarization; to summarize routes being redistributed into OSPF, use the summary-address command on the ASBR.
There are several commands you can use to determine the router types in a given OSPF area. The command "show ip ospf" will display quite a bit of information regarding the local router, and this includes whether that router is acting as an ABR or ASBR. To see the routes to the ABRs and ASBRs from the local router, run "show ip ospf border-routers".
An OSPF Internal router has one rule - it must have all its interfaces in a single area. It does not mean that area has to be Area 0.
An OSPF Backbone router is a router with at least a single area in the OSPF backbone area, Area 0. A router can be both an Internal and Backbone router if all its interfaces are in Area 0.
An Area Border Router has at least one interface in Area 0 and another interface in a non-backbone area. ABRs are also one of two router types that can perform OSPF route summarization. (To advertise a summary route from one OSPF area to another, use the area range command on the ABR.)
Finally, an ASBR is an OSPF router that is performing route redistribution by injecting routes from another source into the OSPF domain. This is the other OSPF router type that can perform route summarization; to summarize routes being redistributed into OSPF, use the summary-address command on the ASBR.
There are several commands you can use to determine the router types in a given OSPF area. The command "show ip ospf" will display quite a bit of information regarding the local router, and this includes whether that router is acting as an ABR or ASBR. To see the routes to the ABRs and ASBRs from the local router, run "show ip ospf border-routers".
Tuesday, December 23, 2008
Cisco CCNA / CCNP Certification: OSPF E2 vs. E1 Routes
OSPF is a major topic on both the CCNA and CCNP exams, and it's also the topic that requires the most attention to detail. Where dynamic routing protocols such as RIP and IGRP have only one router type, a look at a Cisco routing table shows several different OSPF route types.
R1#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
In this tutorial, we'll take a look at the difference between two of these route types, E1 and E2.
Route redistribution is the process of taking routes learned via one routing protocol and injecting those routes into another routing domain. (Static and connected routes can also be redistributed.) When a router running OSPF takes routes learned by another routing protocol and makes them available to the other OSPF-enabled routers it's communicating with, that router becomes an Autonomous System Border Router (ASBR).
Let's work with an example where R1 is running both OSPF and RIP. R4 is in the same OSPF domain as R1, and we want R4 to learn the routes that R1 is learning via RIP. This means we have to perform route redistribution on the ASBR. The routes that are being redistributed from RIP into OSPF will appear as E2 routes on R4:
R4#show ip route ospf
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
E2 is the default route type for routes learned via redistribution. The key with E2 routes is that the cost of these routes reflects only the cost of the path from the ASBR to the final destination; the cost of the path from R4 to R1 is not reflected in this cost. (Remember that OSPF's metric for a path is referred to as "cost".)
In this example, we want the cost of the routes to reflect the entire path, not just the path between the ASBR and the destination network. To do so, the routes must be redistributed into OSPF as E1 routes on the ASBR, as shown here.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#redistribute rip subnets metric-type 1
Now on R4, the routes appear as E1 routes and have a larger metric, since the entire path cost is now reflected in the routing table.
O E1 5.1.1.1 [110/94] via 172.34.34.3, 00:33:21, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E1 6.1.1.1 [110/100] via 172.34.34.3, 00:33:21, Ethernet0
172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
O E1 172.12.21.0/30 [110/94] via 172.34.34.3, 00:33:32, Ethernet0
O E1 7.1.1.1 [110/94] via 172.34.34.3, 00:33:21, Ethernet0
15.0.0.0/24 is subnetted, 1 subnets
O E1 15.1.1.0 [110/94] via 172.34.34.3, 00:33:32, Ethernet0
Knowing the difference between E1 and E2 routes is vital for CCNP exam success, as well as fully understanding a production router's routing table. Good luck in your studies!
R1#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
In this tutorial, we'll take a look at the difference between two of these route types, E1 and E2.
Route redistribution is the process of taking routes learned via one routing protocol and injecting those routes into another routing domain. (Static and connected routes can also be redistributed.) When a router running OSPF takes routes learned by another routing protocol and makes them available to the other OSPF-enabled routers it's communicating with, that router becomes an Autonomous System Border Router (ASBR).
Let's work with an example where R1 is running both OSPF and RIP. R4 is in the same OSPF domain as R1, and we want R4 to learn the routes that R1 is learning via RIP. This means we have to perform route redistribution on the ASBR. The routes that are being redistributed from RIP into OSPF will appear as E2 routes on R4:
R4#show ip route ospf
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
E2 is the default route type for routes learned via redistribution. The key with E2 routes is that the cost of these routes reflects only the cost of the path from the ASBR to the final destination; the cost of the path from R4 to R1 is not reflected in this cost. (Remember that OSPF's metric for a path is referred to as "cost".)
In this example, we want the cost of the routes to reflect the entire path, not just the path between the ASBR and the destination network. To do so, the routes must be redistributed into OSPF as E1 routes on the ASBR, as shown here.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#redistribute rip subnets metric-type 1
Now on R4, the routes appear as E1 routes and have a larger metric, since the entire path cost is now reflected in the routing table.
O E1 5.1.1.1 [110/94] via 172.34.34.3, 00:33:21, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E1 6.1.1.1 [110/100] via 172.34.34.3, 00:33:21, Ethernet0
172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
O E1 172.12.21.0/30 [110/94] via 172.34.34.3, 00:33:32, Ethernet0
O E1 7.1.1.1 [110/94] via 172.34.34.3, 00:33:21, Ethernet0
15.0.0.0/24 is subnetted, 1 subnets
O E1 15.1.1.0 [110/94] via 172.34.34.3, 00:33:32, Ethernet0
Knowing the difference between E1 and E2 routes is vital for CCNP exam success, as well as fully understanding a production router's routing table. Good luck in your studies!
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