Cisco CCNP Certification / BSCI Exam Tutorial: Floating Static Routes

Passing the BSCI exam and earning your CCNP certification demands that you add greatly to the networking skills foundation you created when you studied for your CCNA certification. You learned quite a bit about static routing and default static routing when you passed the CCNA test, and it does seem like that should be all you need to know about static routing, right?

One thing you'll learn as you continue to earn Cisco certifications is that there's always something else to learn! You may have heard the term "floating static route", which does suggest some interesting mental pictures. "Floating"? Floating on what?

In a way, a floating static route is "floating" in your routing table. A floating static route is a route that will be used only if routes for the same destination but with a lower administrative distance are removed from the table. For example, you could be using an OSPF-discovered route as your primary route to a given destination, and the floating static route would serve as a backup route that would be used only if the OSPF route leaves the routing table.

Now, how can that happen? After all, OSPF has an administrative distance of 110 and static routes have ADs of one or zero, depending on whether it's configured with a next-hop IP address or a local exit interface. One way or the other, 1 and 0 are still less than 110!

When you want to configure a floating static route, you must assign the route an AD higher than that of the primary route. In this case, we've got to create a static route with an AD higher than 110. We do this by using the "distance" option at the end of the "ip route" command.

R1(config)#ip route 110.1.1.0 255.255.255.0 172.12.123.1 ?

<1-255> Distance metric for this route

name Specify name of the next hop

permanent permanent route

tag Set tag for this route



R1(config)#ip route 110.1.1.0 255.255.255.0 172.12.123.1 111

The number entered at the very end of the "ip route" command is the AD of that route. If there is an OSPF route for 110.1.1.0 /24, that will be the primary route, and the floating static route will not be used unless the OSPF route is taken out of the routing table.

Floating static routes aren't just a good thing to know for the BSCI exam and your CCNP certification pursuit - they're very practical in the real world as well.

Cisco CCNP / BSCI Exam Tutorial: Not All Static Routes Are Created Equal

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...

Router(config)#ip route 172.1.1.1 255.255.255.255 fast0

Router(config)#ip route 172.1.1.1 255.255.255.255 210.1.1.1

... the prefix lengths are the same, so the static route using the local exit interface fastethernet0 will be preferred due to its lower AD, and will be installed into the routing table.

Keep the details in mind on the job and in the exam room, and you’re on your way to CCNP exam success!

Cisco CCNA Certification Exam Tutorial: Frame Relay DLCIs And Mappings

Passing the CCNA is tough, and one of the toughest parts is keeping all the acronyms straight! Frame Relay has plenty of those, and today we're going to examine what DLCIs do and how they're mapped on a Cisco router.

Frame Relay VCs use Data-Link Connection Identifiers (DLCI - pronounced "del-see") as their addresses. Unlike other Cisco technologies, VCs have only a single DLCI in their header. They do not have a source and destination.

DLCIs have local significance only. DLCI numbers are not advertised to other routers, and other routers can use the same DLCI numbers without causing connectivity issues.

Cisco uses the term global addressing to describe a technique by which a router in a frame relay network is reached via the same DLCI number from each router in the network. For example, in a 25-router network, the same DLCI number would be used to reach “Router A” by each router.

Global Addressing is an organizational tool that does not affect the fact that DLCIs have local significance only.

The locally significant DLCI must be mapped to the destination router’s IP address. There are two options for this, Inverse ARP and static mapping.

In both of the following examples, the single physical Serial interface on Router 1 is configured with two logical connections through the frame relay cloud, one to Router 2 and one to Router 3.

Inverse ARP runs by default once Frame Relay is enabled, and starts working as soon as you open the interface. By running show frame-relay map after enabling Frame Relay, two dynamic mappings are shown on this router. If a dynamic mapping is shown, Inverse ARP performed it.

R1#show frame map

Serial0 (up): ip 200.1.1.2 dlci 122(0x7A,0x1CA0), dynamic,

broadcast,, status defined, active

Serial0 (up): ip 200.1.1.3 dlci 123(0x7B,0x1CB0), dynamic,

broadcast,, status defined, active


Static mappings require the use of a frame map statement. To use static mappings, turn Inverse ARP off with the no frame-relay inverse-arp statement, and configure a frame map statement for each remote destination that maps the local DLCI to the remote IP address. Frame Relay requires the broadcast keyword to send broadcasts to the remote device.

R1#conf t

R1(config)#interface serial0

R1(config-if)#no frame-relay inverse-arp

R1(config-if)#frame map ip 200.1.1.2 122 broadcast

R1(config-if)#frame map ip 200.1.1.3 123 broadcast


The syntax of the frame map statement maps the remote IP address to the local DLCI.
Broadcasts will not be transmitted by default; the broadcast option must be configured.


R1#show frame map

Serial0 (up): ip 200.1.1.2 dlci 122(0x7A,0x1CA0), static,

broadcast,

CISCO, status defined, active

Serial0 (up): ip 200.1.1.3 dlci 123(0x7B,0x1CB0), static,

broadcast,

CISCO, status defined, active


Hands-on practice is the best way to prepare for CCNA exam success. Working with Frame Relay in a lab environment practically guarantees that you'll truly master the concepts shown here - and then you're on your way to the CCNA and becoming a master network engineer.

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!

Cisco CCNA Certification: Static Routing Tutorial

In studying for your CCNA exam and preparing to earn this valuable certification, you may be tempted to spend little time studying static routing and head right for the more exciting dynamic routing protocols like RIP, EIGRP, and OSPF. This is an understandable mistake, but still a mistake. Static routing is not complicated, but it's an important topic on the CCNA exam and a valuable skill for real-world networking.

To create static routes on a Cisco router, you use the ip route command followed by the destination network, network mask, and either the next-hop IP address or the local exit interface. It's vital to keep that last part in mind - you're either configuring the IP address of the downstream router, or the interface on the local router that will serve as the exit interface.

Let's say your local router has a serial0 interface with an IP address of 200.1.1.1/30, and the downstream router that will be the next hop will receive packets on its serial1 interface with an IP address of 200.1.1.2/30. The static route will be for packets destined for the 172.10.1.0 network. Either of the following ip route statements would be correct.

R1(config)#ip route 172.10.1.0 255.255.255.0 200.1.1.2 (next-hop IP address)

OR


R1(config)#ip route 172.10.1.0 255.255.255.0 serial0 ( local exit interface)

You can also write a static route that matches only one destination. This is a host route, and has 255.255.255.255 for a mask. If the above static routes should only be used to send packets to 172.10.1.1., the following commands would do the job.

R1(config)#ip route 172.10.1.1 255.255.255.255 200.1.1.2 (next-hop IP address)

OR


R1(config)#ip route 172.10.1.1 255.255.255.255 serial0 ( local exit interface)

Finally, a default static route serves as a gateway of last resort. If there are no matches for a destination in the routing table, the default route will be used. Default routes use all zeroes for both the destination and mask, and again a next-hop IP address or local exit interface can be used.

R1(config)#ip route 0.0.0.0 0.0.0.0 200.1.1.2 (next-hop IP address)

OR


R1(config)#ip route 0.0.0.0 0.0.0.0 serial0 ( local exit interface)

IP route statements seem simple enough, but the details regarding the next-hop IP address, the local exit interface, default static routes, and the syntax of the command are vital for success on CCNA exam day and in the real world.

Cisco CCNA / CCNP Certification Exam Tutorial: Floating Static Routes

To pass the Cisco CCNA and CCNP certification exams, as well as becoming a world-class networker, you've got to know how and when to use floating static routes. And if you're wondering what makes them "float" -- read on!

In this example, R1 and R2 are running OSPF over a Frame Relay network, 172.12.123.0 /24. They're also connected by a BRI ISDN link, 172.12.12.0 /24. R1 is advertising a loopback network, 1.1.1.1 /32, via OSPF. We want R2 to have a route to that loopback even if the frame goes down - and here, we'll use a floating static route to make that happen.

R2 sees the route to the loopback interface via OSPF, and can ping that interface successfully.

R2#show ip route ospf

1.0.0.0/32 is subnetted, 1 subnets

O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:02, Serial0


R2#ping 1.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 68/68/68 ms

This is when it's important to know your administrative distances.... or at least know where to look to see them! The AD of OSPF is 110, which means we can configure a static route to 1.1.1.1 /32, and as long as the AD of the static route is higher than 110, it won't be used unless the OSPF route leaves the routing table. That's why this kind of route is called a "floating" static route - the route "floats" in the routing table and isn't seen unless the primary route leaves the table.

You learned how to write a static route in your CCNA studies, but you also remember that the default AD of a static route is either 1 or 0... and both of those values are less than 110! To change the AD of a static route, configure the desired distance at the end of the ip route command.

R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 ?

<1-255> Distance metric for this route

A.B.C.D Forwarding router's address

name Specify name of the next hop

permanent permanent route

tag Set tag for this route


R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 111

The static route has an AD that's only one higher than that of the OSPF route, but that's enough to make the route "float" and not yet be seen in the routing table.

R2#show ip route

1.0.0.0/32 is subnetted, 1 subnets

O 1.1.1.1 [110/65] via 172.12.123.1, 00:06:44, Serial0

172.12.0.0/24 is subnetted, 2 subnets

C 172.12.12.0 is directly connected, BRI0

C 172.12.123.0 is directly connected, Serial0

Let's see the effect on the routing table when the Serial0 interface is closed.

R2(config)#int s0

R2(config-if)#shutdown


12:04:53: %OSPF-5-ADJCHG: Process 1, Nbr 172.12.123.1 on Serial0 from FULL to DOWN, Neighbor Down: Interface down or detached


12:04:55: %SYS-5-CONFIG_I: Configured from console by console

12:04:55: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down


12:04:56: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down


R2#show ip route

1.0.0.0/32 is subnetted, 1 subnets

S 1.1.1.1 is directly connected, BRI0

172.12.0.0/24 is subnetted, 1 subnets

C 172.12.12.0 is directly connected, BRI0

The floating static route appears in the table, but the ISDN link will not come up until the BRI interface has traffic to send. Let's ping 1.1.1.1 and see what happens. debug dialer was configured on R2 before sending the ping.

R2#ping 1.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

12:16:01: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=1.1.1.1)

12:16:01: BR0 DDR: Attempting to dial 8358661

12:16:01: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.!!

12:16:01: BR0:1 DDR: dialer protocol up!!

Success rate is 80 percent (4/5), round-trip min/avg/max = 36/37/40 ms

The link comes up and traffic can still reach 1.1.1.1. Once R2 becomes an OSPF neighbor of R1 again, the OSPF route will again become the primary path and the floating static route leaves the routing table.

R2(config)#int s0

R2(config-if)#no shut

R2#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

172.12.123.1 1 FULL/DR 00:01:57 172.12.123.1 Serial0


R2#show ip route

1.0.0.0/32 is subnetted, 1 subnets

O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:16, Serial0

172.12.0.0/24 is subnetted, 2 subnets

C 172.12.12.0 is directly connected, BRI0

C 172.12.123.0 is directly connected, Serial0

A floating static route is an excellent "back door" that will keep the ISDN link down while allowing that link to serve as a backup route. Just make sure the ISDN link comes down when you expect it to - always check that with show isdn status!

Cisco CCNP / BSCI Exam Tutorial: Not All Static Routes Are Created Equal

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...

Router(config)#ip route 172.1.1.1 255.255.255.255 fast0

Router(config)#ip route 172.1.1.1 255.255.255.255 210.1.1.1

... the prefix lengths are the same, so the static route using the local exit interface fastethernet0 will be preferred due to its lower AD, and will be installed into the routing table.

Keep the details in mind on the job and in the exam room, and you’re on your way to CCNP exam success!