Category: LAN
… updated on Saturday, December 26, 2020 14:04 UTC
Multihomed IP Hosts
IP host (workstations, servers or communication equipment) is multihomed if it has more than one IP address. An IP host can be multihomed in numerous ways, using one or more layer-3 interfaces for network connectivity. Some multihoming scenarios are well understood and commonly used, while others (multiple physical layer-3 interfaces in the same IP subnet) could be hard to implement on common operating systems.
Blurt from the past: ATM LANE module for Catalyst 3000
I've found the following "gem" in the Catalyst 3000 LANE module data sheet:
The module "provides legacy LANs with access to ATM-based services in an ATM campus backbone".
The legacy LAN was switched Ethernet (which is still around after 15 years) and ATM campus backbones have joined the dinosaurs.
In case you've never seen a Catalyst 3000: it was a switch that Cisco got through one of its first acquisitions and although it was a good Ethernet switch, it was a nightmare to configure and the later additions (for example, the LANE module) were a disaster. Luckily, it was allowed to die a quiet death a few years later.
VPLS Is Not Aspirin
If you’re old enough to remember the days when switches were still called bridges and were used to connect multiple sites over WAN links, you’ve probably experienced interesting network meltdowns caused by a single malfunctioning network interface card. Some of you might have had the “privilege” of encountering another somewhat failed attempt at WAN bridging: ATM LAN Emulation (LANE) service (not to mention the “famous” Catalyst 3000 switches with LANE uplink).
It looks like some people decided not to learn from others’ mistakes: years later the bridging-over-WAN idea has resurfaced in the VPLS clothes. While there are legitimate reasons why you’d want to have a bridged connection across the Service Provider network, VPLS should not be used to connect regular remote sites to a central site without on-site routers, as I explained in the VPLS: A secure LAN cloud solution for some, not all article I wrote in 2009 (republished below).
Carrier Ethernet: the big picture
My regular readers are probably not surprised to learn that I dislike buzzwords and marketectures. I’ve heard so much about Carrier Ethernet in the last few years (not including confusing definitions of what it’s supposed to be) that I decided to write an article about it. It was recently published by SearchTelecom as “Carrier Ethernet: Big picture issues for carrier deployment”.
Generating layer-2 broadcast from a regular IP packet
The Wake-on-LAN discussion we had a while ago brought us nowhere; there's simply no way to generate UDP packets on the router. I thought I could use Application Performance Monitor's Tcl scripts to generate the packet, but it looks like APM has been removed from recent IOS releases (and it's not clear whether you can use APM without a peer router).
The discussion nonetheless had an interesting side effect. Robert Turnšek sent me an interesting trick: with static ARP you can generate layer-2 broadcasts with a layer-3 unicast packet.
Blocking rogue DHCP servers
The reader who was concerned about making a loop while connecting a switch to itself was also facing “customer-installed” DHCP servers in his LAN. He wrote:
Some users have installed their own Linksys routers and plug our cable in router's LAN ports, so there is DHCP servers fight in our LAN. How can I sort this out (I cannot physically find the location of the Linsys routers)?
The ideal solution is DHCP snooping (assuming your switch supports it), well documented on www.cisco.com. The basic configuration takes only a few minutes:
Connecting a switch to itself: does it hurt?
I’ve got an unusual question a few days ago:
Does a loop (cable returning back to same switch) in one switch affect other switches? How can I detect that there is such a problem in a particular switch?
The correct answer to the first question is obviously it depends. To start with, it depends on whether the two ports will be able to communicate. With a crossover (switch-to-switch) cable (and assuming there are no negotiation issues), the physical layer will work correctly. If you’re using a standard RJ-45 patch cable, you’re “out of luck” unless the switch is too smart and has auto-MDI sensing (like the Linksys switches, now well hidden under obscure part numbers like Cisco SRW248G4). In this case, the two ports will become active even connected with a patch cable.
Should VTP be disabled by default?
One of my readers sent me a question that triggered one of my old grudges:
In my experience, when you first add a new switch (having a NULL domain) on an existing VTP Domain, it inherits the domain name, regardless of it being a VTP Server. I was wondering if this is a feature (i.e. has proved to be a solution in most cases) or a bug (i.e. has proved to cause problems in most cases). I know it's proved to be the latter for us!
In my personal opinion Cisco at one point in time wanted too much plug-and-play and someone had a great idea that you can just plug another switch into your network and it would autoconfigure itself. We've been suffering because of that "insight" ever since (and the CCIE written test has material for a few more interesting questions :).
Internet Access Russian Dolls
When the local Telco installed my blindingly fast 20 Mbps Internet-over-fiber-cable service, I was expecting to use DHCP on the router’s outside interface to connect to the Internet. After all, they’re running switched Ethernet VLANs over the fiber cable, and using DHCP seemed a logical choice. Imagine my surprise when I had to configure PPP-over-Ethernet (PPPoE) – it was as if I would be using a DSL connection, not a fiber-optic cable.