Posts Tagged ‘cisco’
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
As a CCNP candidate, as a CCNA, and in getting ready to pass the BSCI exam, you may be tempted to breeze through your static route studies, or even skip them! That’s because static routes are easy enough to configure, and as long as you remember the syntax of the ip route command, you’re in good shape.
But there’s one vital detail regarding static routes that many exam candidates miss. That’s because many CCNA and CCNP books say “the administrative distance of a static route is 1″, but that is not quite accurate.
You know from your CCNA studies that the ip route command is used to create a static route, and that you have the option of configuring a local exit interface or a next-hop IP address at the end of the command. However, the administrative distances are not the same. The AD of a static route that uses a local exit interface is zero! (That’s because the router considers a static route with a local exit interface to actually be a directly connected network.) The AD of a static route with a next-hop IP address is 1.
Therefore, if the router has the following two ip route statements to consider…
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In your CCNA studies, you learned about PortFast and the trouble it can cause if configured on the wrong port! Suitable only for switch ports connected directly to a single host, PortFast allows a port running STP to go directly from blocking to forwarding mode.
A Cisco router will give you a warning when you configure PortFast:
SW1(config)#int fast 0/5
SW1(config-if)#spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a
single host. Connecting hubs, concentrators, switches, bridges, etc…
to this interface when portfast is enabled, can cause temporary
bridging loops. Use with CAUTION
%Portfast has been configured on FastEthernet0/5 but will only
have effect when the interface is in a non-trunking mode.
SW1(config-if)#
Not only will the switch warn you about the proper usage of PortFast, but you must put the port into access mode before PortFast will take effect.
Now, you’d think that would be enough of a warning, right? But there is a chance – just a chance – that someone is going to manage to connect a switch to a port running Portfast. That could lead to two major problems, the first being the formation of a switching loop. Remember, the reason we have listening and learning modes is to help prevent switching loops. The next problem is that there could be a new root bridge elected – and it could be a switch that isn’t even in your network!
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For CCNA exam success, you had better know what split horizon is, how to turn it off, and when to turn it off. Knowing when to turn split horizon off is also important in production networks, because it can cause a hub-and-spoke network to have incomplete routing tables on the spokes.
Split horizon exists for a very good reason – routing loop prevention. The rule of split horizon states that a router cannot send an advertisement for a route out the same interface that it came in on. Split horizon is on by default on all interfaces running RIP, IGRP, and EIGRP.
In this CCNA tutorial, R1 will serve, as the hub and R2 and R3 will be the spokes. We’ll first configure EIGRP over the 172.16.123.0 /24 network, the network connecting the three routers.
R1#conf t
R1(config)#router eigrp 100
R1(config-router)#no auto-summary
R1(config-router)#network 172.12.123.0 0.0.0.255
R2#conf t
R2(config)#router eigrp 100
R2(config-router)#no auto-summary
R2(config-router)#network 172.12.123.0 0.0.0.255
R3#conf t
R3(config)#router eigrp 100
R3(config-router)#no auto-summary
R3(config-router)#network 172.12.123.0 0.0.0.255
Running show ip eigrp neighbor on R1 shows that adjacencies to R2 and R3 are up.
R1#show ip eigrp neighbor
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
1 172.12.123.3 Se0/0 11 00:02:45 1 5000 0 1
0 172.12.123.2 Se0/0 161 00:03:01 1 5000 0 1
Each router will now advertise its loopback address via EIGRP.
R1#conf t
R1(config)#router eigrp 100
R1(config-router)#network 1.1.1.0 0.0.0.255
R2#conf t
R2(config)#router eigrp 100
R2(config-router)#network 2.2.2.0 0.0.0.255
R3#conf t
R3(config)#router eigrp 100
R3(config-router)#network 3.3.3.0 0.0.0.255
Running show ip eigrp route on each router shows that R1 has a route for both R2’s and R3’s loopback. R2 and R3 will only see R1’s loopback address, and not each other’s. Why?
R1#show ip route eigrp
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/2297856] via 172.12.123.2, 00:03:19, Serial0/0
3.0.0.0/24 is subnetted, 1 subnets
D 3.3.3.0 [90/2297856] via 172.12.123.3, 00:03:04, Serial0/0
R2#show ip route eigrp
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:03:40, Serial0/0.123
R3#show ip route eigrp
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:05:17, Serial0/0.31
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