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Bardlebee posted:The question asks if a ping is sent to PC2 from PC1, what would PC1's arp cache look like. My thought is that it would have an IP of PC2 and a MAC of the Router (CCC). But the question marked it wrong, stating it was both MAC and IP of PC2 that would show up. I am pretty convinced the practice test is wrong. Did they say what the subnet masks were? Were they trying to ask about proxy arp? It would in fact be PC2 IP with the Router MAC in PC1's arp table if you were using proxy arp. If PC2 thought that PC1 was in its subnet range (for example, stupid network drivers that expected classful boundaries), or on some some old OSes, a lack of a default gateway or a route to the network on PC1, you'd have PC1 arping for PC2's MAC address. And with proxy arp enabled on the router, it'd respond back for PC2's IP address with it's own MAC address. At least, that's how I remember it working; it's been such a wonderfully long time since I've had to work with proxy arp. But yes, in the post-CIDR modern not-using-a-bad-config world, it'd be router IP and router MAC for the path to PC2.
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Feb 1, 2011 00:41
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Martytoof posted:I almost wish the consequences were more dire so we'd hasten IPv6 adoption. I'm actually more afraid of no NAT. So many people out there depend on NAT's pseudo-firewall effects that once the ISPs start assigning /64's (or /48's, or /56's, or...) and every system gets a globally routable IP, there's going to be some serious issues unless the consumer router manufacturers step up their stateful firewall skills. Hell, my Linksys 610N (sad attempt to bring this back to Cisco) last year came preconfigured to do Router Advertisements for the IPv6 6to4 prefix associated with my external IPv4 address, and allow all inbound IPv6 connections with no filtering. And no way to turn off either the RAs or the passthrough. They did fix the passthrough finally.
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Feb 1, 2011 03:34
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ragzilla posted:How do you think NATs are implemented in current software? Well, yes, any port-based NATing is by definition stateful. I just don't have huge confidence that the consumer router makers won't gently caress it up, as I described in my example. I think getting rid of NAT is a great thing, but I think we're going to see a period of increased issues given the fact that we have fairly new and/or untested code on hosts protected by fairly new and untested code on the routers along with the fact we're letting people track systems by IP, although RFC3041 will limit the timeframe you could use information about OS or apps running on a host against it.
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Feb 1, 2011 07:26
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Peanutmonger posted:I'm kind of curious what will happen when your ISP gives you a /64, re-arranges their internal network, and then tries to give you a different /64. They do it these days with their v4 networks via DHCP but, if your router uses that /64 to hand out addresses to your internal hosts, you suddenly need to re-address everything in a hurry. The theory? Using Stateless Address Autoconfiguration (SLAAC) and Router Advertisements should provide a clean path to migrate hosts from one network prefix to another without a hit if you can use both prefixes briefly, or quickly come up on a new prefix if it's a hard cut. RAs will tell the hosts to create an IPv6 address for the prefix and whether the router should be used as a default route.
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Feb 1, 2011 16:45
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jbusbysack posted:On a different track, my CCIE R&S is booked for mid-April...anyone taken it and have thoughts on the v4 curriculum? Written or Lab?
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Feb 2, 2011 16:37
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jbusbysack posted:Written. Ballparking lab for nov 2011. I took the R&S written back in November to renew. I primarily used the Cisco Press book to study from; it covered generally everything you need to know. Make sure you know how EIGRP works fairly well as it is Cisco's favorite son (feasible successors, generating EIGRP metrics, going active, stub, etc). Also Multicast in its various forms and the bazillion different forms of Spanning Tree and the various states they can get into (and the commands that impact them like bpduguard, portfast and the like).
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Feb 2, 2011 20:48
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Powercrazy posted:Anyone have any good websites that talk about troubleshooting multicast? Especially Cisco-centric Commands as well as parse what the output means. I know the basics of what the numbers mean, but how do they help me understand the environment. If you're convinced it's the host not joining, focus on the IGMP. Have them join the group, do a "show ip igmp groups" and "show ip igmp interface gi x/x" on the router. Also consider using wireshark either on the host or via a SPAN port to see if you're getting the IGMP joins. Do you have IGMP snooping or CGMP on your switches? From a switch level, you can search your CAM table for the 01-00-5E Ethernet address associated with that multicast IP address (steps to convert from IP to Ethernet are here). e: this will only really work if you are using IGMP snooping/CGMP to prevent the switch from treating multicasts like broadcasts.
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Feb 10, 2011 19:22
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Bardlebee posted:This site is full of lies. I looked this up and I saw a cute girl on the front page plugging in a fiber cable. She could unplugging the fiber cable breaking connectivity for the remote site she's at if it makes you feel any better.
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Feb 18, 2011 19:49
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Could use something like Gateway Load Balancing Protocol if your IOS supports it without doing different VLANs, but those lucky souls who get balanced to the T1 will be complaining about the speed. Well, that's assuming you have your own Provider Independent IP space that you're advertising to both providers. But if you don't have PI space it's going to be an ugly failover in any case...
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Mar 14, 2011 16:54
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jwh posted:It's funny you mention that, because the best question I've learned to ask to really gauge someone's level of competency is: Funny, my personal "they probably know their poo poo" question is: "How does traceroute work?" with bonus points if they mention the ICMP/UDP difference between Windows and Cisco/Unix. Amazing how many people don't understand the basics of the tools they use (and what it means when they fail).
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Mar 16, 2011 13:21
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Bardlebee posted:They mentioned something about sometimes students will switch out the ports on their switch or something. But, I just don't understand why you WOULDN'T have spanning-tree running. Because you're running TRILL? Because the network is dead simple and architected so you don't see redundant links? ummm, because someone misconfigured something causing an outage, blamed it on spanning tree, so the boss said never run that again?
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Mar 17, 2011 13:05
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Bob Morales posted:Is there a site I can look up a MD5 password hash from a device? I know the simpler password system is easily decrypted. For practical purposes, MD5 is not reversable. I was going to write a bunch of stuff about rainbow tables to break hash functions, but really, you're just going to need to do a password recovery on it.
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Mar 20, 2011 03:53
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inignot posted:Does anyone have any thoughts on using the gig interfaces onboard a sup 720 to link two 6509s together? In particular etherchannelling both sup ports. As addition fuel for the "move it to a linecard" arguement, one issue I've run into with this is that you're likely not going to be able to do a port channel utilizing a port on the sup card and one on something like a 6748 card since the queues/buffers aren't the same structure between the physical ports (1p2q2t vs 1p3q8t). So if you expand beyond the needs of what you have on the sup card, you'll have to move them all off to a linecard instead (which is probably a good idea anyways, at least if the linecard has a DFC on it).
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May 3, 2011 19:26
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Ninja Rope posted:tcpdump is installed almost everywhere by default. If you've got tshark sure, use it. Also, tcpdump lacks many of the security-issue-prone protocol dissectors makes which makes it slightly safer. Yup. If just grabbing a capture file for later analysis or if I don't need detailed protocol breakdowns I'd probably just use tcpdump.
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May 6, 2011 05:04
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