Discovering the speed of a serial link

Tricon7 · May 2007

I'm configuring EIGRP between two routers, and I have two serial links between them. One link in the example is a T1 and the other is a T3. The two hosts are separated by a discontiguous network, and one router is supposed to pick the T3 serial link over the T1 link, as it's faster. I've set up EIGRP and it's working fine and both hosts can ping each other, but they're using the T1 link, not the T3 link. I've put in the "no-auto summary" command, but the class problem says I have to find out the speeds of the T1 and T3 links so the one link will be preferred over the other.

I don't remember going over how to find link speeds, nor do I know how to configure them to choose one over the other. Can anyone offer any suggestions?

blackmage439 · May 2007

To answer your first question, the "bandwidth" line in a show interface command will show you the bps, I think.

If I remember correctly, one of EIGRP's metrics is bandwith. By making this value on the "T3" interface greater than the "T1" interface, you will force EIGRP to route packets on that link.

Router(config)# interface serial x/x
Router(config-if)# bandwith xxxxx

I'm too lazy to look up the answers right now... someone correct me if I'm wrong...

[/code]

Tricon7 · May 2007

blackmage439 wrote:
To answer your first question, the "bandwidth" line in a show interface command will show you the bps, I think.

If I remember correctly, one of EIGRP's metrics is bandwith. By making this value on the "T3" interface greater than the "T1" interface, you will force EIGRP to route packets on that link.
Router(config)# interface serial x/x
Router(config-if)# bandwith xxxxx
I'm too lazy to look up the answers right now... someone correct me if I'm wrong... [/code]

Yes, that sounds like it would work. I've played with your suggestion, but now I'm looking for three things:

- how to *find* the BPS speed on a serial interface?
- how to *change* the BPS speed on a serial interface?
- do the changes need to be made on each router, or just one of the two?

tech-airman · May 2007

Tricon7 wrote:

I'm configuring EIGRP between two routers, and I have two serial links between them. One link in the example is a T1 and the other is a T3. The two hosts are separated by a discontiguous network, and one router is supposed to pick the T3 serial link over the T1 link, as it's faster. I've set up EIGRP and it's working fine and both hosts can ping each other, but they're using the T1 link, not the T3 link. I've put in the "no-auto summary" command, but the class problem says I have to find out the speeds of the T1 and T3 links so the one link will be preferred over the other.

I don't remember going over how to find link speeds, nor do I know how to configure them to choose one over the other. Can anyone offer any suggestions?

Questions:

When you say "..., and I have two serial links between them," do you mean the first router is directly connected to the second router with two back to back DCE/DTE cables?
When you mention "One Link in the exam is a T1," from which router's perspective are you talking about? The first or second router?
When you mention "...and the other is a T3," from which router's perspective are you talking about? The first or second router?
Be careful with the use of the term "hosts." So when you mention "The two hosts..." are you talking about the two routers?
When you mention "...are separated by a discontinguous network,..." then that means we're not talking about a single Autonomous System, therefore an IGP like EIGRP cannot route packets from different Autonomous Systems. You will need to use an EGP to interconnect the two Autonomous Systems.
Which Autonomous System number are you using on the first router?
Which Autonomous System number are you using on the second router?
When you mention "I've setup EIGRP and it's working fine..." how did you determine that?
Is the sub/network for the "T3 link" included in the EIGRP router process as a network statement?
When you mention "...and both hosts can ping each other..." are you talking about pinging from the first router to the second router and from the second router to the first router? Was this done through the Console port or the Ethernet/LAN port of the router?

To find out what speed T1 is, check out the Cisco Networking Academy Program - CCNA Semester 1, Module 5, Section 5.2.2, Figure 1. Go to http://cisco.netacad.net to review the curriculum.

As far as configuring the router for EIGRP, you need to configure the "bandwidth" statement in the interface configuration mode.

I hope this helps.

Paul#4 · May 2007

show interface serial...

Tricon7 · May 2007

Paul#4 wrote:

show interface serial...

D'oh!

Tricon7 · May 2007

tech-airman wrote:

Tricon7 wrote:

I'm configuring EIGRP between two routers, and I have two serial links between them. One link in the example is a T1 and the other is a T3. The two hosts are separated by a discontiguous network, and one router is supposed to pick the T3 serial link over the T1 link, as it's faster. I've set up EIGRP and it's working fine and both hosts can ping each other, but they're using the T1 link, not the T3 link. I've put in the "no-auto summary" command, but the class problem says I have to find out the speeds of the T1 and T3 links so the one link will be preferred over the other.

I don't remember going over how to find link speeds, nor do I know how to configure them to choose one over the other. Can anyone offer any suggestions?

Questions:
When you say "..., and I have two serial links between them," do you mean the first router is directly connected to the second router with two back to back DCE/DTE cables?

When you mention "One Link in the exam is a T1," from which router's perspective are you talking about? The first or second router?

When you mention "...and the other is a T3," from which router's perspective are you talking about? The first or second router?

Be careful with the use of the term "hosts." So when you mention "The two hosts..." are you talking about the two routers?

When you mention "...are separated by a discontinguous network,..." then that means we're not talking about a single Autonomous System, therefore an IGP like EIGRP cannot route packets from different Autonomous Systems. You will need to use an EGP to interconnect the two Autonomous Systems.

Which Autonomous System number are you using on the first router?

Which Autonomous System number are you using on the second router?

When you mention "I've setup EIGRP and it's working fine..." how did you determine that?

Is the sub/network for the "T3 link" included in the EIGRP router process as a network statement?

When you mention "...and both hosts can ping each other..." are you talking about pinging from the first router to the second router and from the second router to the first router? Was this done through the Console port or the Ethernet/LAN port of the router?

To find out what speed T1 is, check out the Cisco Networking Academy Program - CCNA Semester 1, Module 5, Section 5.2.2, Figure 1. Go to http://cisco.netacad.net to review the curriculum.

As far as configuring the router for EIGRP, you need to configure the "bandwidth" statement in the interface configuration mode.

I hope this helps.

- Yes, both routers are directly connected with two DCE/DTE cables
- per the example, one serial link is a T1 and the other serial link is a T3, which is all the info I have
- when I mentioned "hosts," I meant the users/computers, not the routers
- regarding the AS, I merely pulled a number out of the air to use for the same AS (shared by both routers) separated by the different network in the middle
- I set up EIGRP on both routers and was able to ping from host 1 to host 2 through the routers, but only through the T1 link, not the T3 link which the signal should be going through
- I also included the T3 link in the network statements
- I could not ping from one router to the other on the serial inferface with the T3 set up

I had set up no auto-summary, but I didn't find out the bandwidth speeds on both serial interfaces, nor did I change the bandwidth speed on the T3 to make it more attractive to EIGRP, which I hope to rectify in the lab this afternoon to see if it works. I'm assuming my prof. isn't making this example too complex for us, but we'll see. Thanks again.

tech-airman · May 2007

Tricon7 wrote:

tech-airman wrote:

Tricon7 wrote:

I'm configuring EIGRP between two routers, and I have two serial links between them. One link in the example is a T1 and the other is a T3. The two hosts are separated by a discontiguous network, and one router is supposed to pick the T3 serial link over the T1 link, as it's faster. I've set up EIGRP and it's working fine and both hosts can ping each other, but they're using the T1 link, not the T3 link. I've put in the "no-auto summary" command, but the class problem says I have to find out the speeds of the T1 and T3 links so the one link will be preferred over the other.

I don't remember going over how to find link speeds, nor do I know how to configure them to choose one over the other. Can anyone offer any suggestions?

Questions:
When you say "..., and I have two serial links between them," do you mean the first router is directly connected to the second router with two back to back DCE/DTE cables?

When you mention "One Link in the exam is a T1," from which router's perspective are you talking about? The first or second router?

When you mention "...and the other is a T3," from which router's perspective are you talking about? The first or second router?

Be careful with the use of the term "hosts." So when you mention "The two hosts..." are you talking about the two routers?

When you mention "...are separated by a discontinguous network,..." then that means we're not talking about a single Autonomous System, therefore an IGP like EIGRP cannot route packets from different Autonomous Systems. You will need to use an EGP to interconnect the two Autonomous Systems.

Which Autonomous System number are you using on the first router?

Which Autonomous System number are you using on the second router?

When you mention "I've setup EIGRP and it's working fine..." how did you determine that?

Is the sub/network for the "T3 link" included in the EIGRP router process as a network statement?

When you mention "...and both hosts can ping each other..." are you talking about pinging from the first router to the second router and from the second router to the first router? Was this done through the Console port or the Ethernet/LAN port of the router?

To find out what speed T1 is, check out the Cisco Networking Academy Program - CCNA Semester 1, Module 5, Section 5.2.2, Figure 1. Go to http://cisco.netacad.net to review the curriculum.

As far as configuring the router for EIGRP, you need to configure the "bandwidth" statement in the interface configuration mode.

I hope this helps.

- Yes, both routers are directly connected with two DCE/DTE cables
- per the example, one serial link is a T1 and the other serial link is a T3, which is all the info I have
- when I mentioned "hosts," I meant the users/computers, not the routers
- regarding the AS, I merely pulled a number out of the air to use for the same AS (shared by both routers) separated by the different network in the middle

Questions:

Was the number you pulled out of the air a number 1 through 65535?
Which sub/network address and subnet mask is used on the Ethernet interface of the first router?
Which sub/network address and subnet mask is used on the Ethernet interface of the second router?
Which sub/network address and subnet mask is used on the T1 serial link?
Which sub/network address and subnet mask is used on the T3 serial link?

Tricon7 wrote:

- I set up EIGRP on both routers and was able to ping from host 1 to host 2 through the routers, but only through the T1 link, not the T3 link which the signal should be going through
- I also included the T3 link in the network statements
- I could not ping from one router to the other on the serial inferface with the T3 set up

I had set up no auto-summary, but I didn't find out the bandwidth speeds on both serial interfaces, nor did I change the bandwidth speed on the T3 to make it more attractive to EIGRP, which I hope to rectify in the lab this afternoon to see if it works. I'm assuming my prof. isn't making this example too complex for us, but we'll see. Thanks again.

Which model(s) are the two routers?

Remember, that the metric used by EIGRP is based on: 1) Bandwidth 2) Delay 3) Load 4)Reliability. So instead of "chang[ing] the bandwidth speed on the T3 to make it more attractive to EIGRP..." how about you change the bandwidth for the T1 link to make it less attractive to EIGRP thereby indirectly making the T3 link "more attractive?" Keep in mind that "clock rate" will determine the actual operating speed of the serial link, but if "bandwidth" is configured on the interface, the bandwidth value will be used in the EIGRP metric calculation instead of the "clock rate" configured value.

I hope this helps.

Discovering the speed of a serial link

Comments