Badly need help on understanding int connections EIGRP and BGP please

Answered Question
Apr 17th, 2008



- Please see attach diagram.

- Having multiple networks has confused me. all 4 networks have been used by servers and devices in the datacenter.

I am in the process of congurin two routers 6506 for BGP with two providers but one is only ready now for turn up and the other will be in two weeks, so I will be multihomed.

these two routers will pass over a Layer 1 physical fiber link and connect to two other 6506s - full mesh.

I am planning to run EIGRP between these 4 routers and have three or 4 networks to advertise in BGP and EIGRP. I assume the same networks will be used in the EIGRP config as well.

One of the networks is a class C /24, from which I want use an /28 to use for for Interface connectivity between these 4 routers (Cisco 6506) but not having design an EIGRP network from ground up before I cannot tell the relation between these /28 that I want to use to connect these router together and the othet network blocks that I have - All Public IP blocks.

here is the block from which I want to use an /28 for router's connectivity in EIGRP :


Other network blocks that have been advertised in BGP to Provider.

216.x.x.x - 40 Class Cs

216.x.x.x - 16 class Cs

well, I guess my question is how do I assign IP addresses to those interfaces full mesh?

You see, all the examples that I have seen deals with two different networks between two routers running EIGRP or between 4 or more routers running EIGRP!? and this has confused me given the fact that I will be using IPs from one network to connect all these 4 routers to run EIGRP between them!?

Either, there is no issues at all and I am only confused not having this done from ground up before or I am making a big mistake?

this is for a web hosting compant and downstream (going from servers to these routers) are two 6509 swithces L2 only where I will have PVLANS on them while having the Primary VLAN Interface for these PVLANs on the next hop to these switches i.e. on two of the 6506s (routers) that are participating on the EIGRP. - though the last piece of information might be usefull to clarify my orginal question on EIGRP.

Please see attach diagram.

Please help so that I can understand this scenario.



I have this problem too.
0 votes
Correct Answer by Istvan_Rabai about 8 years 9 months ago

Hi Mike,

You're welcome.

You can continue this thread. I'm registered to this thread and I get an email notification whenever a new post arrives to it.



  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Istvan_Rabai Thu, 04/17/2008 - 22:11

Hi Mike,

If I understand your question well, then the 4 different subnets you may use are the following:

These subnets will be within the range.

You can use these subnets to configure the 4 connections between the 4 EIGRP routers.

Does this answer your question?



m-abooali Fri, 04/18/2008 - 13:08

yes it does, my confusion was/is on the need to have cross conections between the EIGRP speaking routers as well but I don't kno wif it is really required!? if it i sthen I must subnet further in order to get 4 more IPs for the cross connections.

then, I will advertise the whole /24 plus the other networks that we have.

do you think this is requird?



connections between A to D and Between C to B and that bring us to conections between A to B and C to D unless EIGRP doesn't require this kind of a full mesh!?

Please advise.



Istvan_Rabai Sat, 04/19/2008 - 00:05

Hi Mike,

In EIGRP it is not a requirement to have a full mesh neighbor relationship.

However, you can physically cross-connect the routers for resiliency.

Even if 2 routers, like A and D go down, router C still will have a route to the outside world.

In this case you will need to create additional subnets for the transit networks between the EIGRP routers.

Eigrp will handle the situation by placing more than one route in the routing table to the outside destinations.

So there can be more than one successor routes in the routing table.

If this is the case, than EIGRP will per packet load balance between the equal cost routes.

If you do not want this load balancing, only you want to have redundant routes to the outside world, then you will have to play around with the interface costs.

You will have to make some routes have higher costs, thereby less desirable. They can become feasible successor routes and in case the successor route fails, a very fast failover will occur.

If you need help in configuring the above, I will need the running-configs and the outputs of the "show ip route" command from you, so I can orient myself better.



m-abooali Mon, 04/21/2008 - 04:51

Hi Istvan,

Thanks for you deitailed explanation. I am still in the process of installing equipment and writting the configurations for those devices that to include L2 PVLANs and stuff like that, its a whole 9 yard thing!

I will post those configuration wile I have them.

I just wanted to cross-connect those routers and for that I subnetted an /27 to give me more /30s for this interfaces.

I beleive, after these rouers are up and runnig, time comes for playing with the costs and I then need some help for sure!

if I post on this current posting in say two weeks you will be able to pick it up? or I will need a new post?



Correct Answer
Istvan_Rabai Mon, 04/21/2008 - 09:56

Hi Mike,

You're welcome.

You can continue this thread. I'm registered to this thread and I get an email notification whenever a new post arrives to it.



m-abooali Mon, 04/21/2008 - 10:32

now that we are on this discussion and i have all the /30s that I want, I would like to pick on your mind again,

I just got a chance to look at the actual devices and what a surprise!, they are all RJ21s with one RJ45 blade on each one.

the 6509s have two SUP engine WS-720 with Integrated Switching.

now, I am totally confused! I know that all the datacenter servers are on RJ21, and each supports 12 Ether connections, thats fine only makes my PVLAN configurations harder.

but how do I connect these devices as discused for the EIGRP? use RJ 45 ports on that one JJ45 blade? they don't want to spend money and buy SFP and GBIC connectors and i have no choice but to use those ports on the RJ45 blades. what do you recommend?

Please advise.

I am going to rate your on this - just wanted to say that I havenot forgotten that!



m-abooali Mon, 04/21/2008 - 17:39

Hi Istvan,

Ok, It seems that I need to know a few things about Cisco 6506, SUP 720 with Integrated Switching fabric and Cisco 6509 with SUP 720 Integrated Switcing Fabric.

I have not worked with this type of router switch in the past and especialy with this SUP 720.

here what I did today: i configured a MUX with a WDM module and I was able to put light into the dark fiber connecting two sites , say Site 1 and Site 2, Layer one physical dark fiber connection.

I crossed conned two MUX and used a tranciver and connected them to two Cisco 6506 on the ports gi 5/1 (the only RJ45 blade on those router) the rest are RJ 21 and they don't want to buy SFPs and GBICs.

so, I gave private address to inte gi5/1 to both and I was able to ping the other router interface through the MUX so the MUX worked.

what didn't work was access to the managenet port of the MUX. I tried a default gateway on one of the 6506s and tried an IP route statemnet but it didn't accept my IP route statement saying the destination bad Mask prefix!

i used:

Ip route subnet mask enter

but it kept saying bad distination mask!

I did all of these on the bench not connected to actual network.

these 6506s will run BGP with two providers and then hit the MUX, then FIber transit to site 2 and MUX and then two 6506s connected to 2 6509s (hosting private vlans) and for custmers of this web hosting provider.

question: doesn't 6509 with SUP 720 with integrated switchig fabric capable of L3 routing?

2- how can i switch between router and switch on these 6506s and 6509s?

i will be running EIGRP between 4 6506s deparated by the dark fiber transit and then in site 2, two 6506s will

be connected to the two 6509s, so without L3 capability, creating and routing these PVLANs over EIGRP network,

out through BGP to the UP stream provider is really difficult and I cetainly have no clues of how to do that?

can you please advise on how to work with 6506 and 6509 router/switch andhow to switch back between

router function and switch function?

I don't know if the 6509s have MSCF cards but I know they have SUP 720 as explained above.

any insight into this whole thing will really make ly life easier given that I only have one more week to get all of these working!!?

and I have been here only for ne week.

Please advise. any amount of help will help me a great deal at this point.



Istvan_Rabai Tue, 04/22/2008 - 09:27

Hi Mike,

A 6509 with SUP720 is definitely capable for layer3 routing.

The MSFC must be on the SUP720 module itself.

You can use the "show module" command to see what modules are in the switch and what submodules are on each module.

To make a 6500 switch to do layer3 routing you need to do the following:

In global config mode, make sure the following commands are entered:

ip routing

ip cef

Then, to make any physical interface a routed port, you need to apply the

"no switchport"

command on the interface, though the physical interfaces should be layer3 interfaces by default on the 6500 series.

After this, you need to apply the ip addresses to the respective interfaces.

Then you can configure the routing protocol on each switch where routing has to be perfomed.

For switches where you need layer2 vlans, you have to apply the "switchport" command to make the interfaces layer2.


conf t

vlan 5


interface range gig5/6 - 15


switchport access vlan 5

switchport mode access

This will put all ports in the given range into vlan 5.

Concerning your MUX:

If your MUX can be configured with an ip address then I assume you need to configure a default gateway for the MUX itself.

Without this the MUX will not know where to send the reply packets.



m-abooali Tue, 04/22/2008 - 09:41


Thanks very much for your explanation and I am sure that this will help me alot.

if you have seen the drawing I posted, Please see if my cofusion is ligitimate or noe:

two 6506s and two 6509s (I have been told that 6509s are L2) i need to power them up today and find out if MSFC is present.

in case 6509s are purely L2, and I want PVLANS on it, can I have my promary VLAN interface for these PVLANs on the one of the or two of 6506s, crerate a VTP domian and have PVLANs created on 6509s for customers connectivity and our other devices (promescouse ports for devices)? ports are RJ-21s?

Please help me undestand this part as it has taken much of my time and I haven not seen exmaples that refere to a scenario like this.



Istvan_Rabai Tue, 04/22/2008 - 10:00

Hi Mike,

Private vlans need to be created manually. They will not be propagated by VTP.

Based on what I can see on your drawing I suggest the following:

The port on the 6506 should be a routed (layer3) port (that connects to the 6509).

The port on the 6509 should be a promiscuous port (layer2) that will serve the different private vlans and will connect to the 6506 routed port.

It would be also good if you had a redundant connection to the other 6506 router.

In this case the redundant port will be a promiscuous port as well.

You would be able to configure HSRP for gateway redundancy and load balancing for the users.

If you need to create the private vlans, describe the configuration and I will try to make a list of commands that will be needed to configure it.

I know you have to be fast with this, but first you have to configure the basic connectivity between the switches.

If that works then we can go further.




This Discussion