BROCADE IP PRIMER

Console49F Link 50F1Power1F 2F 3F 4F325476981110131215141716191821202322252427262928313033323534373639384140434245444746 4849F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 4849F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 48Chapter 10: Routing BasicsICMP Echo Host CICMP Echo Host CSwitch Ae1192.168.1.1/30Switch Be2192.168.2.2/30Switch Cve 10172.16.0.1/24e1192.168.1.2/30ve 20172.17.0.1/24e1192.168.2.1/30ve 30172.18.0.1/24ICMPEchoHost CICMPEcho-ReplyHost AICMP Echo-ReplyHost AICMP Echo-ReplyHost AICMPEcho ReplyHost AICMPEchoHost CHost A172.16.0.10/24Host B172.17.0.10/24Host C172.18.0.10/24What? It worked?! Woo hoo! Ahem... I mean, of course, it did. Why did I takeyou through this long painful exercise? Two reasons, really. One, I wanted youto become familiar with how we configure static routes. And two (and perhapsmore important), I wanted to show you the fundamental rule for troubleshootinga routing problem: Always remember that a packet must know how toreach its destination and how to get back.Too often, network engineers will focus too much energy on trying to troubleshoota packet getting to its destination when they may not realize that it isgetting to its destination. It just can't get back. If you're dealing with a rare protocolthat only works in one-way communication, this concern doesn't reallyapply. However, 90-95% of all protocols you will troubleshoot require a reply.You've got to go through the path both ways.Now we're not tied down to connected routes! We can configure static routesto get anywhere we want to go. In fact, we haven't talked about a special staticroute yet: the default gateway. The default gateway (or “gateway of last resort”)is a way of telling your router, “Hey, if you need to forward a packet to a networkthat's not in my routing table, forward it to this router.”The default gateway works the same way as it does on your workstation. HostA, for example, could function just fine in talking to his neighbors in the172.16.0.0/16 network without a default gateway. The only time Host A wouldneed a default gateway is to talk to a host outside of its subnet. Now, could weadd static routes to the workstation's routing table (remember that all Layer-3capable devices have routing tables)? Sure, but oh, the pain! Wouldn't it justbe simpler to say, “Hey workstation, if you need to go outside your subnet,hand your packets to this guy.” That's the default gateway.That's great for a workstation, but would a router really need a default gateway?They're actually very common. In fact, if you have broadband Internetaccess at home, odds are that your broadband router has a default gateway onit. Internet access is one of the more common uses of default gateways thesedays. In this case, the default gateway would be the IP address of the InternetService Provider's router. You're trusting your ISP's router to know how to forwardyour packet to anywhere on the Internet you wish to go.226 Brocade IP Primer