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Excerpt: Back to the basics today. I have seen this pop up a few times and wanted to offer some clarification on what seems to be a cloudy issue for CCNP (and some CCIE) candidates. I’ve seen quite a few times now where engineers see a route to Null0 in a Cisco router and assume instantly that the router is “black holing” traffic. Sometimes, a route to Null0 is inserted into the routing table when performing summarization with nearly every routing protocol in common use today. Take this example topology, for instance: After initial configuration, the routing table on R1 looks like this: R1#show ip…
The Route to Null0 is OK
April 15, 2013 by Leave a Comment
A Contest of Protocols: EIGRP or OSPF?
March 18, 2013 by 1 Comment
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Excerpt: Ah, the age old question that nearly every CCNA and CCNP candidate asks of themselves and others at some point. We see a minimum of 4 routing protocols in our networking studies, more if you decide to take on the Service Provider track. What makes one routing protocol better than another? I think it’s clear why (at least mostly) these particular two protocols are different, keeping in mind that one is distance vector (yes, distance vector, not hybrid distance vector) and the other is link state. At this point, we at least have a fundamental understanding of what’s different between those…
Link-State vs. Distance Vector – The Lowdown
October 25, 2011 by 2 Comments
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Excerpt: I’ve been trying to get more into networking message boards like Networking Forum and TechExams.net lately. It’s a great way to get in touch with fellow packet lovers and gain some interesting perspectives along the way. In fact, it’s great for anyone in networking, whether you’re a hardened veteran or a newbie – there’s usually a place for you in at least one of these sites. As a result, I’ve seen quite a few posts asking about fundamental concepts, which is great because it shows that new networkers are getting out there and learning new things proactively. I’d like to address one such post…
EIGRP Unequal-Cost Load-Balancing
September 22, 2011 by Leave a Comment
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Excerpt: In a previous post, I explored the basics of IP routing, and in the process, we discovered an interesting default feature of OSPF. When there were two OSPF routes in the routing table to a network, and both routes had the same cost, the router performed load balancing between the two. Take, for instance, the following route: 172.16.2.0 [110/12] via 1.1.1.13, 00:09:24, FastEthernet0/0 [110/12] via 1.1.1.2, 00:09:24, FastEthernet0/1 In this example, every packet sent would take one of two routes. The next hop at this particular…
EIGRP over NBMA Networks
September 19, 2011 by 4 Comments
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Excerpt: Commonly used routing protocols like OSPF and EIGRP utilize multicast addresses to distribute hello messages, and routing information. In a broadcast-capable layer 2 network like Ethernet, EIGRP will send a packet containing a hello message to the address 224.0.0.10, which results in a corresponding layer2 destination 01:00:5e:00:00:0a. Something I used to wonder about all the time is how routing protocols work over Non-Broadcast Multi-Access networks like Frame Relay. In these networks, there are no broadcasts or multicasts. With Frame Relay, a service provider will set up PVCs for an organization which act like virtual layer 2 point-to-point connections. Each PVC will…
EIGRP Feasible Successors
July 31, 2011 by 3 Comments
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Excerpt: Link state routing protocols maintain topology tables to determine the best candidate to place in the routing table. EIGRP is no different – it uses this topology table to build a vision of the network from the perspective of each participating router. This topology table is reviewed by the routing algorithm (in the case of EIGRP, it is DUAL) and decisions are made regarding what gets placed into the routing table. Since EIGRP uses bandwidth and delay as metrics in this decision making process, they are used to decide which link out of several redundant connections to use to get…
