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Core Cisco switch replacement with EIGRP question

Planning on replacing a 4710 Cisco switch in a customer's environment. I am reviewing the config of the switch to be replaced and there are some EIGRP commands. I am not very experienced with EIGRP so I had a couple of questions.

Is it correct to assume that these routes are being advertised by this switch? I don't see any actual route-maps configured so I don't think the switch is doing any routing itself.

My initial idea was to copy this configuration to the new switch and run it alongside the old switch so I could test connectivity in a controlled manner prior to cutover. However, now I'm worried if I have both switches advertising these subnets at the same time, it would fuck up the routing within the entire environment.

As I mentioned, I'm not well-versed in EIGRP. Just looking for a little clarity if anyone would be kind enough to offer it.

router eigrp 1000

default-metric 1000000000 1 255 100 65535

network 10.2.1.0 0.0.0.255

network 10.2.12.0 0.0.0.255

network 10.2.11.0 0.0.0.255

network 10.2.13.0 0.0.0.255

network 10.2.14.0 0.0.0.255

network 10.2.15.0 0.0.0.255

network 10.2.16.0 0.0.0.255

network 10.2.17.0 0.0.0.255

network 10.2.18.0 0.0.0.255

network 10.2.19.128 0.0.0.15

network 10.6.20.244 0.0.0.3

redistribute connected

eigrp router-id 10.2.13.1

eigrp stub connected

12 comments
76% Upvoted
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Unlike with BGP, when you work with EIGRP and you see a network statement, it's not advertising said network. It's allowing EIGRP to run on any and all interfaces that are on that subnet.

In your example, you see the first network statement being 10.2.1.0 0.0.0.255. This would mean that if interface G1/0/2 had an IP address of 10.2.1.12 255.255.255.0, it would participate in EIGRP.

If you're concerned with seeing what routes are being advertised by this switch, you can run the sh ip protocols command and look for the EIGRP autonomous system 1000 and see what networks it's routing for. Hope this helps.

This. ^ If you see eigrp neighbor config with a wildcard matching an interface subnet and not all 0.0.0.0 and the exact IP of the interface, either it's lazy or the guy doesn't know the difference between a network statement IGP vs BGP. But this doesn't help you, see how many neighbors are up. Look out for useless vlan interface neighbors on the other end of trunks (fake load balancing)

Data Center Everything5 points·5 days ago

EIGRP is still a dynamic routing protocol. If you're familiar w/ any dynamic routing protocol in IOS, you know by looking at this if it's squared away or not.

You're going to need some help.

The most important thing is if there are any eigrp neighbors. What you see there is just the interfaces that EIGRP is participating in. For instance, if there's a NIC with an IP of 10.2.14.5, then the network command says "For any interfaces that have an IP address in this range, send EIGRP hellos and see if anyone is there". In cisco this is the same if it's EIGRP, whatever.

Run a show ip eigrp neigh. Anything that's attached, you'll need to come up with an alternate solution of exchanging routes (or just use EIGRP if swapping "like" cisco gear)

You can do a parallel replacement as long as you have enough "neighbor" ports on the other systems, but a cutover replacement is the best way to go to be safe unless you're experienced in these sort of things.

For the love of god get a consultant or get Cisco TAC on the line if you don't know what you're doing and save yourself some headache, that's what they're there for.

-6 points·5 days ago(3 children)
Generalist5 points·5 days ago

ay it's the interfaces, I'd say subnets. There are no interfaces in this config snippet. Also, OP, just because there are no route-maps doesn't mean anyth

I would say it's the interfaces, because regardless of the lack of interface in the snippet that's what the network statement is for. The network statement "network 10.2.1.0 0.0.0.255" does not advertise a /24. It checks for any interface in the 10.2.1.0/24 range and advertises the subnet associated with that matching interface.

But it also doesn’t advertise secondary addresses on that interface that don’t match a network statement.

To be clear I meant the interfaces covered by the "subnet" statements.

CCIE3 points·5 days ago
  1. Look for current neighbors. Make sure the table looks the same after the change. Also get a count of the routes for initial check.

  2. Why do engineers not actually RTFM anymore? 2a) EIGRP is dog simple. https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html

You can safely assume that any interface that falls within any of the “network” statements are being advertised. Any directly connected network that is not listed with the “network” command is also being advertised (I believe as an external-type route) meaning those routes will have a higher AD when received by a neighbor.

Others on the thread mentioned this but to reiterate, this is all in play only if you have a neighborship formed with another router in the existing environment.

You should not be simply staring and comparing. You should understand the protocol and features on the old switch and migrate them to the new switch using current best practices.Look for eigrp neighbors on the old switch. Write those interfaces that have active neighbors and prepare the new switch accordingly. Is eigrp stub necessary? You should understand your topology and make a determination if stub is correct.

Keep in mind that the tech that may have set this up initially may have had less understanding than you and copied-n-pasted from a blog post. If nothing else this is your opportunity to learn something new and do it right. EIGRP is a great protocol to learn. It is intuitive but you need to understand some basics. Fortunately for you there must be 10,000 videos and even more blog posts out there nowadays if you don't understand the config guide. Heck, you can even read RFC 7868 to gain a developer level of understanding for the protocol.

You should've enabled port-security1 point·3 days ago

Fyi you dont want to use EIGRP stub on the core. That feature is for remote sites that either A. Only have connected networks or B. Are dual homed and shouldn't be used as transit. If you left it how it is now those redistributed routes wouldn't be advertised either because you'd have to have your syntax look like "eigrp stub connected redistributed".

Generally with EIGRP you'd only want to redistribute if you had to (example bgp default route) or for using backup path (dual homed with backup being redistributed) where the redistributed backup routes will have AD of 170.

I recommend to run passive interface default then use specific network statements (ie 10.1.1.1 0.0.0.0)on a per interface basis and run no passive per interface you want the nrighborship on. How deterministic do you want your routing? If you have lots if equal paths I recommend to run max paths 1 or use delay adjustments to provide deterministic forwarding. Remember also EIGRP can do unequal cost forwarding which I've seen cause async routing before.

Source: I've been involved with EIGRP in the past at a medium enterprise. Good luck

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