Cisco CCNP / BSCI Exam Tutorial: Ten IP Routing Details You Must Know!

To pass the BSCI exam and earn your CCNP, you've got to keep a lot of details in mind. It's easy to overlook the "simpler" protocols and services such as static routing and distance vector protocols. With this in mind, here's a quick review of some details you should know for success in the exam room and real-world networks!

When packets need to be routed, the routing table is parsed for the longest prefix match if multiple paths exist with the same prefix length, the route with the lowest AD is preferred. If there are still multiple valid paths, equal-cost load-sharing goes into effect.

The ip route command is used to create static routes the command ip route 0.0.0.0 0.0.0.0 < next-hop-IP or local exit interface> creates a default static route.

A static route with a next-hop IP address has an AD of one, while a static route with a local exit interface has an AD of zero.

A floating static route is a static route with an AD higher than that of the dynamic routing protocols running on the router, ensuring that the static route can only be used if the routing protocol goes down.

On-Demand Routing (ODR) is only appropriate in a hub-and-spoke network. The spokes effectively become stub routers. ODR uses Cisco Discovery Protocol (CDP) to send route information.

To propagate a default route with IP routing, use the ip default-network command. To do so with IP routing disabled, use ip default-gateway. You can also redistribute a static route into most protocols, but not IGRP. IGRP does not understand a static route to 0.0.0.0.

The ip helper-address command takes certain broadcasts and translates then into unicasts in order to allow the router to forward them. These default ports are:

TIME, port 37

TACACS, port 49

DNS, port 53

BOOTP/DHCP Server, port 67

BOOTP/DHCP Client, port 68

TFTP, port 69

NetBIOS name service, port 137

NetBIOS datagram services, port 138

To name other ports, use the ip forward-protocol command. To remove any of these ports from the default list, use the no ip forward-protocol command.

ICMP Router Discovery Protocol (IRDP) hosts hear multicast Hellos from routers, allowing host-router discovery. HSRP routers create a virtual router that hosts think is a real router. Both protocols help networks cut over to a functional router quickly when their primary router goes down.

Cisco CCNA Certification Exam Tutorial: Distance Vector Command Review

Part of studying for CCNA exam success is keeping all these new commands straight in your head! And let's face it, there are a lot of commands you need to know in order to pass the CCNA exam and earn that certification. Here's a review of some very important distance vector and static routing commands you need to know, along with their proper usage and console output.

Bandwidth
IGRP makes a default assumption that any Serial interface running IGRP is connected to a T1 line, which runs at 1544 KBPS. With equal-cost load-balancing enabled by default, this may be an undesirable assumption.

To alter IGRP’s assumption, use the bandwidth command on the serial interface in question. Note that this command does NOT actually affect the bandwidth available to the interface; it merely changes IGRP’s assumption of the bandwidth.
R2#conf t

R2(config)#int s0

R2(config-if)#bandwidth 512




Clear ip route *

This command clears your routing table of all non-static and non-connected routes. In a lab environment, it’s very handy because it forces your routers running routing protocols to send and request updates, rather than waiting for the regularly scheduled updates.
R2#clear ip route *



Debug ip igrp events

Debug ip igrp events allows you to see IGRP updates being sent and requested. Here, the debug is run and then the routing table is cleared. The router immediately broadcasts update requests via the IGRP-enabled interfaces.

R2#debug ip igrp event

IGRP event debugging is on

R2#clear ip route *

06:02:51: IGRP: broadcasting request on BRI0

06:02:51: IGRP: broadcasting request on Serial0.123




Debug ip igrp transactions

To configure IGRP unequal-cost load-sharing with the variance command, you’ve got to know the metric of the less-desirable routes. EIGRP keeps these in its topology table; IGRP has no such table.

To get the metrics of routes not in the routing table, run debug ip igrp transactions. To force IGRP updates, the routing table below was cleared with clear ip route *.

R2#debug ip igrp transactions


IGRP protocol debugging is on

R2#clear ip route *

06:05:33: IGRP: received update from 172.12.123.1 on Serial0.123

06:05:33: subnet 172.12.123.0, metric 10476 (neighbor 8476)

06:05:33: network 1.0.0.0, metric 8976 (neighbor 501)

06:05:33: IGRP: edition is now 3

06:05:33: IGRP: sending update to 255.255.255.255 via BRI0 (172.12.12.2)

06:05:33: network 1.0.0.0, metric=8976

06:05:33: IGRP: sending update to 255.255.255.255 via Serial0.123 (172.12.123.2) - suppressing null update

06:05:34: IGRP: received update from 172.12.12.1 on BRI0

06:05:34: subnet 172.12.13.0, metric 160250 (neighbor 8476)

06:05:34: network 1.0.0.0, metric 158750 (neighbor 501)




Debug ip rip

R2#debug ip rip

IP protocol debugging is on

R2#clear ip route *

6:14:53: RIP: received v2 update from 172.23.23.3 on Ethernet0

6:14:53: 1.0.0.0/8 via 0.0.0.0 in 16 hops (inaccessible)

6:14:53: 1.1.1.1/32 via 0.0.0.0 in 2 hops

6:14:53: 172.12.0.0/16 via 0.0.0.0 in 16 hops (inaccessible)

6:14:53: 172.12.12.2/32 via 0.0.0.0 in 2 hops

6:14:53: 172.12.13.0/30 via 0.0.0.0 in 1 hops

6:14:53: 172.12.123.0/24 via 0.0.0.0 in 1 hops

6:14:53: 172.23.0.0/16 via 0.0.0.0 in 16 hops (inaccessible)


Run debug ip rip to troubleshoot routing update problems, RIP authentication problems, and to view the routing update contents. Clear ip route * was run to clear the routing table and to force a RIP update.

Ip route
R2#conf t

R2(config)#ip route 1.1.1.1 255.255.255.255 172.12.123.1

OR

R2(config)#ip route 1.1.1.1 255.255.255.255 serial0

To configure a static route to a given destination IP address, use the ip route command. The destination is followed by a subnet mask, and that can be followed by either the next-hop IP address or the exit interface on the local router.

Ip route 0.0.0.0 0.0.0.0

R2#conf t

R2(config)#ip route 0.0.0.0 0.0.0.0 172.12.123.1

OR


R2(config)#ip route 0.0.0.0 0.0.0.0 ethernet0

To configure a default static route, use either of these two commands.

You could have any number for the first “0.0.0.0", since the second set of zeroes is the subnet mask. This means that any destination will match this route statement.

That's a good review to get started with! I'll be back tomorrow with Part II of this CCNA exam command review!