The linked video demonstrates RIPng, our first dynamic routing protocol for IPv6. This is a simple but inefficient routing protocol. The metric is based on number of router hops, with no provision for differentiating between links with drastically different bandwidth (a frame-relay hop has the same cost as a 10-gig-ethernet in RIPng). Each router multicasts its entire routing protocol out each interface every 30 seconds, which wastes router CPU. RIPng routinely takes minutes to reroute around ne...
Mar 14, 2010
In this hands-on exercise, we configure IPv6 addresses on 3 routers in a triangle. Then we configure IPv6 static routes to allow the 6 IPv6 subnets (3 loopback, 3 P2P links) to be accessible on all 3 routers.Static routes are easy to understand. At first glance they appear simple. You just manually configure which next-hop to go to for each subnet destination. But in actual use they are very complex. In our example with 3 routers and 6 subnets, we end up using 12 static route commands to impleme...
Nov 02, 2009
In 2003, I made a VOIP call from home while downloading a large email attachment. The DSL line saturated and my audio quality became horrible while VOIP packets (and email packets) were being dropped. Doubling the bandwidth to my home would not have solved this problem. The email download would simply have been faster, but the VOIP call would still have suffered packet loss.The solution to this problem is 'quality of service' (QOS). Some applications, particularly realtime interactive applicatio...
Sep 27, 2009
The linked video introduces IPv6 theory. IPv6 is the 128-bit address replacement for IPv4. The Internet is expected to run out of it's 4-billion IPv4 addresses in 2012. IPv6 will replace IPv4 at the network-layer of the OSI stack. By replacing one layer in the stack, most applications and most layer-2 network devices will continue to function. IPv6 includes several technical improvements over IPv4. IPv6 uses optional extension headers, so only packets requiring special options will have those he...
Sep 20, 2009
The linked video provides guidance for optimal IOS version selection.The large number of IOS versions makes choosing the best version for your router or switch difficult. You must pick the most reliable version which includes the features you need. Different IOS "packages" have different features. For example, the "LAN base" package includes basic switching code. "IP base" adds access-layer routing features (RIP and EIGRP-stub). "IP services" adds most layer-3 routing protocols (OSPF, EIGRP, BGP...
May 31, 2009
In this video demonstration, we show an example of writing IOS Access Control Lists (ACL's) on a home router. We use the revision control system (RCS) to maintain the master ACL file and push the ACL's to the router via TFTP. This is similar to many production networks, where maintaing comments and old revisions of ACL's is a requirement. We also show examples explaining the "don't care bit" format of IOS ACLs. Many network engineers mistakenly refer to the format as inverse-netmask, but that is...
Mar 24, 2009
IOS routers can act as DHCP clients and DHCP servers. They can also function as Network Address Translation (NAT) devices. In this video we show a demonstration using a 2621 as a DHCP client, server, and NAT translation device for my home network.It's important to understand that most IOS routers have relatively slow CPU's. An IOS router is fine as a DHCP server for a few dozen clients. But if you try to serve thousands of DHCP clients you are likely to fail and suffer an outage.IOS routers can ...
Mar 23, 2009
In this episode we show a video demonstration of the hot standby router protocol. This is a Cisco proprietary redundancy protocol. The purpose is to allow two routers to share one virtual IP address on an access subnet/vlan. Hosts on the subnet can use the virtual IP for their default route. This way if one router goes down the redundant router will assume the virtual IP address, preventing a loss of connectivity to the hosts on the net.HSRP is configured with the "standby ip" group of commands ...
Mar 22, 2009
This video demonstrates layer-2 convergence in less than 2 seconds thanks to rapid spanning-tree.Rapid per-vlan spanning-tree is configured with "spanning-tree mode rapid-pvst".The rapid spanning tree protocol, 802.1w, is the answer to the slow convergence time of the historic 802.1d spanning-tree protocol. Rapid spanning tree replaces timers with triggered updates. Switches almost never wait for a timer to expire. When converging on a new switch-to-switch link they will start with the port in t...
Mar 08, 2009
When you have two different links between the same two switches, normally spanning tree will forward on one and block on the other. This means half of your bandwidth is sitting idle. An etherchannel is a way to bind two links into one logical link with twice the bandwidth. In addition to increased bandwidth, etherchannels fail over in a fraction of a second. So the failure of one physical link in a multi-link etherchannel will not result in a significant outage.The port aggregation protocol (PAg...
Mar 03, 2009
VTP is the VLAN trunking protocol. It's used to disseminate uniform vlan information between switches over 802.1q or ISL trunks. It can also "prune" vlans, dynamically removing unneeded VLANs from trunks. This decreases unneeded frame flooding.VTP can eliminate outages thanks to the uniform VLAN configuration. But it can also cause outages if incorrect VLAN information is uniformly distributed.We also attempt a loopguard demonstration, but it doesn't work out well. We'll have to revisit the docu...
Mar 01, 2009
Switches can have multiple vlans. When we connect switches together we use 802.1q trunks (or older ISL trunks) to run multiple vlans over one physical link. With either trunking protocol, a tag is added to the ethernet frame with the vlan information. ISL is an older Cisco-proprietary trunking protocol. Newer switches do not even support ISL. Newer switches use the 802.1q vendor-indepentend trunking protocol. Cisco switches also speak the dynamic trunk protocol (DTP) to dynamically negotiate whe...
Feb 24, 2009
We cover intermediate spanning tree concepts. The importance of specifying your root bridge and backup root bridge with spanning-tree priority. Using portfast to allow host ports to start forwarding without waiting for 30 seconds. Using bpduguard to disable portfast-enabled ports where someone erroneously plugs in a switch. Using errdisable timeout to automatically reenable those ports after 15 minutes. Using rootguard to prevent improper switches from becoming your spanning-tree root. The dange...
Feb 23, 2009
VLANs are a feature of ethernet switches which makes them act like multiple "virtual switches". Each VLAN is a separate broadcast domain and could be configured with a separate subnet. That way could could have separate subnets for separate purposes (IT, accounting, network management) on one physical switch. This saves money and cabling while decreasing complexity. Spanning tree is a protocol which allows you to build redundant loops out of ethernet switches without suffering a bandwidth outage...
Jan 31, 2009
So far we've talked about how IPv4 encodes data into a packet, and how routers learn which direction to forward those IPv4 packets based on the destination IP address and the route table. But in the end, routers and hosts need to encode the IPv4 packet onto a physical medium. Examples of physical mediums include fiber, twisted pair, coax, radio waves, lasers, and microwaves. Each encoding rate and medium requires a specification or protocol definition. Ethernet is a family of similar encoding sp...
Jan 25, 2009
In BGP, MED stands for Multi Exit Discriminator. It is a well-known optional attribute which allows one autonomous system to inject it's IGP route metrics into its BGP advertisements to another BGP autonomous system. This allows the second autonomous system to make intelligent routing decisions regarding which of multiple paths to take to send traffic to a particular destination in the first autonomous system. Because different AS's use different IGP's and can calculate metrics in different ways...
May 29, 2008
We filter BGP routes in 4 different ways.
Apr 11, 2008
We put together what we learned about eBGP, iBGP, and OSPF.
Mar 22, 2008
An iBGP example with 1 autonomous system with 3 routers.
Mar 20, 2008
An eBGP example with 3 autonomous systems with 1 router each.
Mar 19, 2008
An introduction to BGP theory.
Mar 15, 2008
OSPF routes should be filtered or summarized at area border routers.
Oct 21, 2007
OSPF's fundamental design is that all routers in an area have the same exact view of the network topology. This is fundamentally incompatible with filtering routes within an area. As a result OSPF distribute lists do not have the same effect as RIP distribute lists. In fact, using distribute-lists within an OSPF area is dangerous.
Oct 07, 2007
OSPF totally stubby areas and not-so-stubby areas are ways to improve OSPF scalability. But they can be very confusing.
Sep 30, 2007
We cover OSPF stub areas and neighbor synchronization.
Sep 23, 2007
We redistribute from RIPv2 into OSPF and introduce autonomous system boundry summary LSAs and external summary LSAs.
Jul 31, 2007
We introduce multiple OSPF areas and network summary LSA's.
Jul 16, 2007
We show a single area OSPF network and go into router and network link state advertisements (LSA's) in detail.
Jul 01, 2007
A brief tutorial on Dijkstra's Shortest Path First Algorithm. This algorithm is used by most link state routing protocols, including OSPF and IS-IS.
Jun 04, 2007
RIP version 2 includes subnet information in the route advertisement. It also improves efficiency by multicasting to RIPv2 routers instead of broadcasting to all hosts.
Sep 22, 2006
