Lan on Aaron's Worthless Words

VLANs on Linux

Thu, 19 Feb 2009 00:00:00 +0000

My home network has a Linux box running IPTables as it’s center point, and, since there are four networks, it has 4 NICs and 4 cables into the switch. I kept running into problems with the NICs (they would reorder depending on what flavor of Linux was installed), so I wanted to consolidate the NICs down to 2 – one for the Internet link and one for the LAN segments with 802.1q tagging.

Configuring Dedicated Trunks for the CSM

Mon, 24 Nov 2008 00:00:00 +0000

Did you catch the article on setting up fault tolerance on the CSM? In that article, I mentioned that Cisco recommends a dedicated trunk for the FT VLAN if you have two HA CSMs in two chassis. Discuss amongst yourselves while I drone on.

Why should you set up a dedicated trunk for this stuff? The most obvious reason is to be sure that normal traffic doesn’t step on the syncing traffic. Since we’re syncing state information as well as configuration, the frames need to arrive in a timely manner. Any errors could potentially disrupt the FT process, which is bad. You surely don’t want the primary to fail only to find out that the standby doesn’t have the complete or current config.

Using CDP To Track Down Physical Connections

Fri, 31 Oct 2008 00:00:00 +0000

We have a location that’s a few blocks down from the main office here, and we were reviewing the circuit size to make sure it was sized properly. Since not one person knows what’s going on and the trending graphs gave us conflicting details, one of our network dudes took me down to the site to do a physical survey to see what’s going on. Well, besides the fact that no one was there, we discovered a hodgepodge of routers and switches that were cross-connected to one another on multiple floors of the building (I really wish I could post pics to emote the effect). It’s kind of hard to figure out what’s going on when you can’t see both ends of the cable, so we had to abandon all hope.

Using MAC Access-lists

Mon, 27 Oct 2008 00:00:00 +0000

We ran into this today, and, though I knew it existed, I never actually saw it in the wild. I’m talking about MAC access-lists.

In the example setup, we have a DMZ off of a firewall that contains a whole mess of servers – email, web, ftp, etc. These should all be in the DMZ for sure, but they shouldn’t talk to each other. If a bad guy was able to own my FTP server, he would have a nice platform to use to attack my email server. That’s not cool, so we’ve put in MAC access-lists to help out.

Back to Basics -- CAM Table Population

Mon, 14 Jul 2008 00:00:00 +0000

At the office, we reprovision servers like it’s going out of style. It happens so often that my cabling documentation rarely matches what’s actually out in field, which is a pretty big problem when you’re trying to find to what switch port a server is connected. I finally relegated myself to asking for the MAC address of the server, having the admin ping something, and then tracing it down through the CAM table entries of the switches. It works, but the guys really don’t know how a switch populates its CAM table, so they always say “Why can’t you just look on the switch? I shouldn’t have to ping anything.” Here’s one just for the aspiring system admin.

Cheat Sheets from Packetlife.net

Wed, 28 May 2008 00:00:00 +0000

My friend Josh over at blindhog.net has found a collection of cheat sheet gems for the network dude(tte). There’s sheets on BGP, OSPF, Subnetting, QoS, connector types, and more. Check it out.

Cheat Sheets - Packetlife.net

Storm Control

Thu, 15 May 2008 00:00:00 +0000

We run a large number of LANs all over the country that are “controlled” by the particular business unit. We manage the gear, but, since they have the money and have to pay for anything we do, they make the final decision on what gets put in. Sometimes that gets out of hand, as you can well imagine.

A good terrible example came up a few months ago. It seems that, at some time in the past, one site needed some more LAN ports, but, instead of calling us and having us send them another switch, one of the “technical people” there brought in a hub from home. It really irks me to see a hub on the switched LAN, but we really have no control over those decisions. They plugged the hub into one of the existing drops somewhere in the building and plugged everyone in. It worked…until somebody moved one of the machines. The machine was at a desk near the hub, and the network cable, still with one end plugged into the hub, was just left lying there. A good Samaritan came by, saw that the hub was not plugged into the network (though it was through another path), and plugged it back in for us – providing a nice second link from the hub to the switch stack in the closet. Take one switch stack, add a hub, insert a switching loop, bake at 350F for a few milliseconds, and you have a broadcast storm. If you don’t know already, broadcast storms are bad and eat switch CPU like the yummy cookies we baked. In this case, several 3750s were taken completely down.

Getting Started with EtherChannel

Fri, 18 Apr 2008 00:00:00 +0000

In my professional life at some point, I came across someone who had a stack of Catalyst 2950 switches all trunked together with their Internet routers connected to the top of the stack. This was all well and good until they kept adding hosts to the “middle” of the stack, then they had all sorts of latency and packet loss.

The old adage of your chain only being as strong as your weakest length holds true in this case. Here, the weakest link is actually the most-congested trunk, though. Let’s step through to see. A 2950 is a 10/100 switch, so a single trunk can handle 100Mbps of traffic. We have 10 of these guys, Switch1 to Switch10, all trunked to the one above and below. If a server in the center of the stack on Switch5 is sending a lot of data to the Internet routers on Switch1, the trunks off of Switch5 will start to get saturated. Switch4 has a few hosts doing the same thing, so traffic from both Switch4 and Switch5 heads towards Switch1, further filling the trunks. Same for Switch3. Same for Switch2. Next thing you know, there’s 184Mbps or so trying to go across a 100Mbps link.

VTP and You

Wed, 16 Apr 2008 00:00:00 +0000

VLAN Trunk Protocol (VTP) is a little gem on Cisco switches that allows you configure VLANs in one place and have them appear on all of your switches. This is great for large enterprises with 8457839 switches all trunked together because who wants to configure the new VLAN for that one-off application on all 8457839 switches?

VTP works by having designated VTP servers (not real servers like your Linux box, but a switch) tell the rest of the switches in the network with what VLANs they should be configured. All the designated VTP clients say “OK” and configure themselves with those VLANs. When you take a VLAN out of the server, all the clients take it out; when you add a new VLAN, all the clients add it as well. The server and client designation is known as the VTP mode, and there’s one more to mention. When a switch is in VTP transparent mode, he will see VTP from the servers but will ignore them and pass them on to the next switch as if nothing ever happened.

DHCP on the ASA 5505

Thu, 03 Apr 2008 00:00:00 +0000

Let’s keep going with our example setup on the ASA 5505 and set up DHCP on this guy. You can set it up to either forward (relay) DHCP requests to a DHCP server somewhere or have it be the DHCP server. Let’s do it.

To set up DHCP forwarding, you have to configure where the DHCP server is and then enable the relaying on the proper interfaces. Let’s say we have a DHCP server on the inside interface at 192.168.14.11 and we want it to serve IPs to the guests network. Setting up the DHCP server is beyond the scope here, so you’ll have to look elsewhere on how to set that up.

Configuring GLBP

Sat, 22 Mar 2008 00:00:00 +0000

Believe it or not, I got a request for an article on how to configure GLBP. I’m as shocked as you are, so here it goes.

The Gateway Load Balancing Protocol (GLBP) is another Cisco-proprietary protocol for providing highly-available gateways on a network…but there’s a twist. GLBP, as you can figure out from the name, load-balances the traffic going through the participating routers. With HSRP and VRRP, one host is the active peer and handles all the traffic until it dies, then another peer takes over. With GLBP, all the routers accept traffic.

Trunking on a Catalyst Switch

Fri, 21 Mar 2008 00:00:00 +0000

If you didn’t now already, trunks are connections between switches that carry traffic for all VLANs. It allows you to have, say, VLAN 10 and VLAN 20 on two switches appear as the same network. Unless you’re a really small shop, you’ve already dealt with trunks, so there’s no need for an introduction.

Let’s say we have a Catlyst 2950 switch with multiple VLANs connected to another 2950 configured with those same VLANs. We’ll say we have VLANs 10, 20, and 30 and that the switches are connected to port F0/24 of each switch. First, let’s turn on the trunk.