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Community Member

Bandwidth Command on Serial Interface

Hi all,

The router at my cleint location using EIGRP routing protocol is having T1 link, but bandwidth(S0) is configured as 1544000. They will be getting soon T3 between the same locations and may want to use the existing T1 link as Backup and with HSRP between both the outers(T3&T1- with T3 router High priority). Now what is going to happen to EIGRP with the bandwidth command in place?

Thanks a mil.



Re: Bandwidth Command on Serial Interface

EIGRP is going to use the link with the highest bandwidth/lowest delay, as configured at the interface level. It sounds like the bandwidth may be set right, just possibly too many 0's (are you taking the configured bandwidth from the show interface, or the show run?).

I would configure the bandwidths to what they really are, and then adjust the delay as necessary over the backup link to get the routing you want. Remember the metric is applied inbound with EIGRP, not outbound, so the transmitting router sends an update just as it has it in its local table, the receiving router takes the bandwidth and delay listed in the update and adjusts the metrics based on the interface the update was received on.


Community Member

Re: Bandwidth Command on Serial Interface

Thanks Russ.The Bandwidth is from SH RUN.


Re: Bandwidth Command on Serial Interface

I just checked, and it's the delay I'm thinking of that has a one 0 difference between show run and the show output. For instance, on a 2600, I have this:


interface Serial0/1

bandwidth 100

ip address

encapsulation frame-relay

delay 100

clockrate 1000000

frame-relay interface-dlci 36


And the show output shows this:

2651A#show ip eigrp topo

IP-EIGRP (AS 100): Topology entry for

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 25625600

Routing Descriptor Blocks: (Serial0/1), from Connected, Send flag is 0x0

Composite metric is (25625600/0), Route is Internal

Vector metric:

Minimum bandwidth is 100 Kbit

Total delay is 1000 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 0

Note that delay shows as 100 in the show run, and 1000 in the show ip eigrp topology....

Anyway, I hope that setting the bandwidth and delay gets your load sharing where you want it to be. Note that I would normally recommend not setting your bandwidth to engineer traffic flows, because EIGRP paces packets based on the configured bandwidth. I don't think it's going to make much difference in this case, however--it's generally more problematic when you have thousands of routes over low speed links (T1 and below) on large hub and spoke implenations.



Community Member

Re: Bandwidth Command on Serial Interface

Thanks again Russ..

But If I do not have Delay configured on the interface, then with the existing Scenario(T3 coming in pic.) whats going to be the result?

interface Serial3

description "link to Branch#1"

bandwidth 1544000

ip address < > < >

no ip directed-broadcast

ipx network B3E4



Re: Bandwidth Command on Serial Interface

You would need to know the bandwidth and the delay on both interfaces from a show interface to know. BTW, that bandwidth should still have the two 0's knocked off--bandwidth is entered in k-bits/sec, rather than bits/second, so if this is a T1, it shoudl be 1544. In fact, for a serial link, T1 is default, so if it's a T1, you shouldn't have to enter a bandwidth command.

What's the bandwidth set to on the T3 side? Can you post a show interface with the bandwidth and delays for the two interfaces?



Community Member

Re: Bandwidth Command on Serial Interface

Serial2 is up, line protocol is up

Hardware is HD64570

Description: "Branch"

Internet address is < >

MTU 1500 bytes, BW 1544000 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Encapsulation HDLC, loopback not set, keepalive set (10 sec)

Last input 00:00:01, output 00:00:02, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: weighted fair

Output queue: 0/1000/64/0 (size/max total/threshold/drops)

Conversations 0/3/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

--- T3 is not yet UP, but the BW configured on T3 interface is 44000.



Re: Bandwidth Command on Serial Interface

I would either set the bandwidth on the T1 for 4400, or set the bandwithd on both of the links to 1544. The delay should take care of itself once this is done.



Community Member

Re: Bandwidth Command on Serial Interface

Thanks alot Russ. I consider changing the BW back top 1544 on Serial interface.

Also, How can I remove the previous questions posted in NetPro..?

Thanks a mil



Re: Bandwidth Command on Serial Interface

Okay--that should do the trick.... I don't know how to remove questions from netpro--it's something I've asked before, and I think the answer is generally that you can't.



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