Time
31 hours 29 minutes
Difficulty
Beginner
CEU/CPE
30

Video Description

Mismatched MTU, Bad Modules, Power Failure and Missing Routes This lesson continues to cover trouble shooting common wire and switch issues and discusses the following:

  • Mis-matched Maximum Transmission Unit (MTU)/Black Hole: packets are too large for a certain 'hop' and are dropped. If no error about the packet is received, essentially we've hit a Black Hole.
  • Bad Modules (SFP, GBIC): Bad port module on device
  • Power Failure: Device may have lost power
  • Bad/missing routes: routing information is incorrect

Video Transcription

00:04
So next up, we have mismatched into you slash black hole. Now, what isn't into you? Well, we talked about how into you stands for a maximum transmission unit and essentially an empty you is the size of a the maximum packet size that a certain
00:22
ah, certain device can handle of maximum packet size that a router can handle,
00:26
or when it sends it on to the next location or when it receives it.
00:30
So if we haven't into you on one device on one router that is sending extremely large sized packets to the next hop and the next top, the next router can't accept those sized into you packets, it may just drop them.
00:46
It may not be able to take those packets and process and across this process them and send them on
00:52
because it's into you. It's maximum transmission. Unit size is too small, so it just drops the packets.
00:59
Now,
01:00
if we don't receive any air back from that hop that it dropped the packet because it was too large, then we can we will refer to that as a black hole. The the router that we're sending those packets to that is then sending them off is it's black. Hope is a black hole because we're sending data to it and it's not going anywhere.
01:19
Just dropping those packets.
01:21
When, when you think of dropped packets on a device, don't think of, you know, letters that don't get delivered. The packets aren't building up in the other router, and just sitting there when a router drops a packet is as if it just
01:34
It's as if your mail carrier took your letters and then just threw him in the fire. It just burns them. When it drops the packets. It just doesn't even take them into account. It's just electrical impulses that it just does nothing with. It doesn't record them. It doesn't save them. It just drops them.
01:52
So that black hole, maybe especially prevalent with packets that we attached on the do not fragment flag to the do not fragment flag essentially tells that next top our black hole. Hop that, Hey, I have this packet and I know it's too big. I know it's really big, but I don't want you to break it up into smaller size chunks.
02:12
If
02:12
our next router receives this packet and it's in the packet sizes too big for its into you to handle. It's bigger than its maximum transmission unit, and it has this do not fragment flag on it, so it can't break the pack it up. It can't send it along. It can't do anything with it. So it's just gonna drop it
02:30
so we can use There's a command that we can use called trace Artie that stands for trace route that can help us to identify these black holes. Using the trace route command will be ableto our trace route Command will simply hop along each hop each router that we hit
02:46
until it hits that router that it's a black hole and then it'll just drop.
02:52
So using our trace route command, we can help to identify. We can help to hunt down where this black hole might exist and what might be dropping these packets to this network that we're trying to get to.
03:02
So take Well, maybe we need to make sure that we avoid having a mismatched into you a mismatch in maximum transmission unit.
03:09
Then we have bad modules are different modules s f p and G B. I see modules stand for small form factor plausible and gigabit interface converter. Now these modules are actual or looked like slide in cards that we actually slide into devices,
03:29
router switches, et cetera,
03:30
especially enterprise level devices where their actual ports that we just slide into a slot. So if we have we have a gigabit port. We haven't Ethernet port that we need to install into this route into this rack into this rack mounted router, and we just slide in this SFPD we smallness.
03:50
We slide in a small form factor. Plausible.
03:53
Now these modules can go bad. Those modules eventually, over time, may go bad, or there may be some. There may be some electrical shock that shocks that module, and it goes bad. But whatever the case may be,
04:05
that module may need to be replaced with a lot of these different bad modules. We may need to just simply slide it out and then replace it with a new module. Keeping Keeping in mind, however, that those new modules may have a noon Mac address,
04:19
and we also need to understand that sometimes we can't just
04:24
slots, take him out and slap in a new module. If we don't have the module to replace it with. So there may be more repercussions other than how it went bad. I just pull it out and slap in a new one. There's additional settings that we may need to change. If there were any settings based specifically on that module,
04:41
there may be we may need to go on order. New module So we mean may need to change our network topology while we're waiting for it to come in.
04:46
So we will need to track down these bad modules, and we'll need to keep in mind that
04:53
as far as physical devices go, sometimes it's not just the cable that's bad. Sometimes it's not just our network interface card. That's bad. And sometimes the whole switch or router isn't bad. But there may be a single module on that switch, a router that's gone bad,
05:08
and then we have power failure. Now, power failure is exactly what it sounds like. It's when a device is lost power,
05:14
so this may be as simple as someone tripped over the power cable or the power went out. Or it may be as complex as there's a short on the internals of the device that's causing it to have some power issues. Or maybe we have electrical wiring issues in our building
05:30
that's causing power fluctuations. That's causing this device to go off or have some power issues.
05:35
So we may need to not only troubleshoot our device, but we may need to use our different tools in order to troubleshoot our actual power in our building and see if there's any way we can mitigate these fluctuations
05:49
by installing things such as a surge protector or a battery backup to help prevent this power fit power failure to our devices
05:57
If we have a device that we plug into if we're going through switches or going through certain devices like crazy and they just seem to be burning out and they seem to be having power issues and we're going through him like crazy and we're plugging them all into the same circuit,
06:12
we may need to check out that circuit. We may need to install a surge protector, install a battery backup for those devices.
06:17
Now
06:19
this power failure when it occurs during normal operation, maybe a traumatic for our network and they shut down our network, but it would be even worse if we were actually in the device making configuration changes and our device was in the middle of a major configuration change and then power it lost power.
06:39
This could completely destroy the device they could
06:42
was completely bricked the device. And that device would no longer be usable because the actual installed firmware on the device now has issues. So we need to make sure that we take power considerations very seriously for our routers and switches. So if we do think that a device, a particular device, is lost power,
07:01
we may want to try and perform a performing a ping or trace route on that device.
07:06
Or you may want to try. If we're trying to get to a particular network, we may want to perform a trace route a trace. Artie, Command s o. Just trace our command. Prompt trace Artie and then an i p address of a different network and then see where our trace routes stops. See how many hops we get to before trace route stops.
07:25
Where once we've hit that stopping point somewhere between our first are hot that we hit last and the hot After that, there's an issue So we need to check any switches, any bridges, any any hubs, any routers that air in between the last good hop and the next hop that we expected
07:45
on. We can check for those power issues and see if it's a simple as plugging it back in
07:49
are turning it back on. And then, lastly, we have just make sure that we're we have battery back, good battery backup set up. And then we're changing out of the batteries in the battery backups because there's battery. Just do need to be cycled out.
08:01
Then, lastly, here we have our batter missing routes we talked about. We talked about several times routing tables within our routers. Essentially let our routers know where to go to route information. They include information such as the I P address of the next router. The next hot they need to go to the include include information such as costs
08:22
and the best route to go to,
08:24
and sometimes this routing information may go bad, so these routing tables may be incorrect. They may include ah, bad next hop information they made include they have bad I P addresses or sub net mask information.
08:37
So We're trying to send a packet to a certain network and our routers sending it incorrectly, it sending it to the incorrect address.
08:43
So we need to check out those routing tables, and especially if we're on Lee not able to get to certain network segments. But other people are from other routers than it may be that a certain hop in between us and that router has miss configured routing table information.
08:58
Now, this doesn't mean that we just go on our router and we just blow away the routing table configuration
09:05
that is a giant No, no. Um, we don't want to just go in and blow out the routing table configuration and just say, Oh, we have dynamic routing set up. It'll just reconfigure everything.
09:16
That's a resume creating event,
09:18
Um,
09:20
and not in a good way.
09:22
So we want to make sure that we if we are going in and we're making major routing table modifications, especially to our enterprise level backbone routers that we are conferring with, we are confirmed with our next level up were conferring with our team. Members were making sure that it's what we need to do, and
09:41
we are making very careful, methodical, documented
09:45
changes to our routing tables. And again, especially because if there are backbone back in routers, we make an incorrect change there. Or we just wipe out a routing table than we now have.
09:58
Hundreds, maybe even thousands of users that can't connect to a particular resource. Because once the data from all those different users gets to that hop,
10:07
that router doesn't know where to go next. So we need to take that into consideration. So we'll want to check and make sure that we don't have miss configured manual or dynamic routes. Sometimes those routes may our dynamic configuration, maybe incorrect, and we need to statically
10:26
statically configure a route for particular network.
10:28
Or maybe someone went in and created a static route that has now changed. And so are router isn't creating a dynamic table for that particular route because it has a static route that it thinks it's supposed to go to? But that information has now changed. So check those routing tables on make sure that you're being very, very careful when you're modifying them,
10:48
and you're being very diligent when you're actually setting them up to begin with
10:52
so you don't run into issues later

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CompTIA Network+

This CompTIA Network+ certification training provides you with the knowledge to begin a career in network administration. This online course teaches the skills needed to create, configure, manage, and troubleshoot wireless and wired networks.

Instructed By

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Anthony Harris
Systems Analyst and Administrator at SAIC
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