Posts Tagged ‘eigrp’
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 »
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
Read the rest of this entry »