Posts Tagged ‘ospf’
To pass the BSCI exam and become a CCNP, you have to be aware of the proper use of passive interfaces. You learned about passive interfaces in your CCNA studies, but here we’ll review the basic concept and clear up one misconception regarding passive interfaces and OSPF.
Configuring an interface as passive will still allow the interface to receive routing updates, but the interface will no longer transmit them. While the command itself would make you think this command will be applied at the interface level, that is not the case. Below, we’ll configure ethernet0 as a RIP passive interface.
R1(config)#router rip
R1(config-router)#passive-interface ethernet0
Ethernet0 will no longer send RIP routing updates, but will accept them.
The passive interface concept is clear enough with RIP, IGRP, and EIGRP – all protocols that send routing update packets. But OSPF doesn’t send routing update packets – OSPF sends link state advertisements. It’s the inability of the passive interface command to stop LSAs that lead many to think that passive interfaces cannot be used with OSPF. Read the rest of this entry »
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
If there’s one CCNP / BSCI topic that looks so easy but can lead to a real headache, it’s route redistribution. I’m not even talking about the routing loops and suboptimal routing that can result when route redistribution is done without proper planning – I’m talking about the basic commands themselves. Leaving out one single command option, or forgetting what else needs to be redistributed when redistributing dynamically discovered routes, can leave you with a routing table that looks complete but does not result in full IP connectivity.
In this free CCNP / BSCI tutorial series, we’ll take a look at three common errors in route redistribution configurations, and how to fix them. We’ll use three routers, R1, R3, and R5. R1 and R5 are in a RIPv2 domain and R1 and R3 are in an OSPF domain. R1 will be performing two-way route redistribution.
R5 is advertising its loopback, 5.5.5.5/24, into the RIPv2 domain. R1 sees this route in its RIP routing table:
R1#show ip route rip
5.0.0.0/24 is subnetted, 1 subnets
R 5.5.5.0 [120/1] via 100.1.1.5, 00:00:01, Ethernet0
For R3 to see this route, route redistribution must be configured on R1. We’ll use the redistribute rip command to do so.
R1(config)#router ospf 1
R1(config-router)#redistribute rip
% Only classful networks will be redistributed
The router immediately gives us a message that “only classful networks will be redistributed”. What does this mean? Let’s go to R3 and see if that router is receiving this route.
R3#show ip route ospf
Read the rest of this entry »