| Routing is the process of forwarding a packet based on the destination IP address. Routing occurs at a sending TCP/IP host, which is known as host routing, and at an IP router, which is known as router routing. For the host to send a packet to its destination via the router, it must first determine the address of the router to which to send the packet. This can be accomplished using one of the following methods: The default gateway address is determined and then the local ARP cache is queried to identify the physical address to be used to reach the desired router. An ICMP redirect message is sent by an IP router to the sending host, informing it of a better route to a destination host. The better route becomes a host route in the routing table. A TCP/IP host can "listen" to the routing protocol traffic used by routers. This is known as RIP listening or passive RIP.
With router routing, several variables determine the router to which to forward the packet. These variables include each of the network paths to the destination host, including the number of hops and the cost of each hop. Windows Server 2003 provides router routing through RRAS. Routing tables can be manually configured (static) or created through the use of a dynamic routing protocol. Two dynamic routing protocols that are provided with Windows Server 2003 are RIP and OSPF. Routing can employ either distance-vector routing or link-state routing technologies. The main differences between the two routing protocols are as follows: The routing information that is exchanged How the information is exchanged How quickly the internetwork can recover from a downed link or a downed router
Distance-vector routing is the oldest and most common routing algorithm. Distance-vector routers build their routing information tables based on information received from other routers. Link-state routing protocols exchange information only about the specific routes that have changed. Routers using link-state routing protocols learn about their network environment by "meeting" their neighboring routers. RIP is a distance-vector protocol that uses hop count as its metric for measuring the number of routers that must be crossed to reach the desired network. Following are some details about RIP: RIP is limited to 15 router hops, with 16 hops being infinity. What does this mean? Picture a LAN with a PC that needs to communicate with a server on another LAN that is 15 routers away. The routers keep a list of which subnets are no more than 15 routers away, and if you try to expand the network past the sixteenth router, communication does not happen. Remember: You have a 15-hop minimum between networks, and the sixteenth hop is deemed infinity. RIP has problems with subnetted networks. In Chapter 2, "Implementing, Managing, and Troubleshooting DHCP," we briefly covered how subnetted networks should appear in your design. RIP was created and deployed before networks were subnetted to the degree that they are today, and because they have no direct support for RIP, your networks cannot support it. It really comes down to the information that each router sends to the others and what that packet contains. If the routing update does not contain a field in the packet to allow for subnetted networks, it does not carry over the information you may need it to carry over. In other words, the RIP protocol is configured to look at the prefix of the IP address in the routing update and know whether it's A, B, or Cand nothing more. If you subnet, your subnet will not be supported because RIP understands only that if a packet with an IP address of 10.0.0.1 comes in, it's automatically assigned a subnet of 255.0.0.0, even if you have it subnetted down to 255.255.255.0. RIP is bandwidth intensive, and although that isn't a problem on most networks, a network with small WAN links (such as 32KB Frame Relay link) could feel the effect of constant broadcasts every 30 seconds, especially if the routing table is large. If the routing table is too large, more than one update could be sent every 30 seconds, making RIP even more bandwidth intensive. For the 70-291 exam, remember that, by default, RIP broadcasts to its neighbors every 30 seconds. These broadcasts contain lists of networks and subnets it can reach. RIP is not very secure. RIP contains no security features or configurable parameters to make it secure. Other protocols used today have configurable parameters to make them more secure; however, RIP does not contain any solution to make it secure by default.
RIP2, or RIP version 2, was created for RIP users to overcome problems with security. This version adds an option for authentication to the RIP packet. It also adds support for subnetted networks and allows for variable length subnetted networks to be passed through routing updates. OSPF is a link-state routing protocol that functions by sending LSAs to all other routers within the same hierarchical area. OSPF has the following features: Better convergence than RIP. Calculates routes that are always loop free. Sends updates only when the routing changes, rather than sending routing updates periodically. This makes better use of bandwidth. Allows for logical definition of networks where routers can be divided into areas. Designed to advertise the subnet mask with the network. OSPF supports VLSMs, disjointed subnets, and supernetting. Allows for routing authentication using different methods of password authentication. Routes outside the OSPF autonomous system are advertised within the autonomous system so that OSPF routers can calculate the least-cost route to external networks.
OSPF divides the network (what it considers a routing domain) into areas. An area is a subdivision of the entire network and is given a label. Area 0 (zero) is considered the backbone of an OSPF network. If your network is small enough, you can set up the entire network to use Area 0, which is very important. For example, if the network becomes more subdivided (say you have three areas, including Area 0), you would need to know that if all traffic must travel between areas, the packets are first routed to the backbone, or Area 0. When you plan a network this way, the design keeps subnets consolidated to areas, thus reducing the size of the link-state database that is updated on every router in an OSPF network. This keeps your OSPF network running optimally. The following is some important information to remember about OSPF: OSPF sends out hello packets to each adjacent router connected to the network. OSPF uses hello packets to verify that the network is always ready to work as advertised. These hellos are sent out of every router interface every 10 seconds, but they are so small in size that they do not affect your available bandwidth. Link-state advertisements can help in a situation in which a router does not hear from its neighboring router for more than 40 seconds. In such a situation, the router sends out LSAs, marking the other router as down so that all the other routers can adjust their tables with the change. Hellos can be adjusted. A hello has a timer value that, when configured properly throughout the OSPF network, allows proper and accurate communications to take place. It is important to know that if a hello timer is misconfigured (not all timers are identical), problems can occur. Make sure that when you plan an OSPF network, you ensure that all the hello timers match. They must all be consistent across all routers on a network segment. Because LSAs age, it is important to get a refreshed routing table (or database) from a neighboring router just in case anything has changed. If nothing has changed on a particular router for 30 minutes, the router flushes its information and seeks an updated database from its adjacent routers. This means that the network reconverges every 30 minutesa far cry from the forced reconvergence in a RIP network. That forced reconvergence is every 30 seconds!
Three forms of traffic traverse a network: Unicast When a single node on the network sends data to its destination node under one single packet, it knows where the destination node is and gets the data to it. Remember "uni" as being a single transmission. Broadcast Broadcasting (think of the word "broad") occurs when the destination is not known or cannot be found. In this situation, the node (which needs to find the destination node) sends out packets to all nodes on the network segment to see whether it can find its destination node. This process leads to increased traffic, collisions on hubbed networks, and so on. Broadcasts are at times a necessary evil, but most of the time, you should prevent them from happening often or control their number. Basically, broadcast traffic adds overhead to your network and its devices by creating packets that the majority of hosts shouldn't ever see in the first place. Multicast Multicast is the happy medium, but it still can cause problems. A multicast transmission is based on a group. In simple terms, think of 20 nodes on a network, 5 of which need to communicate at all times and 15 of which do not need to ever know what the other 5 are doing. Say these nodes are OSPF routers. OSPF routers send out updates to each other via a multicast, so you might ask, "What about devices that do not need to know this information?" If you use a multicast address, such as 224.0.0.5 for OSPF-based networks, only those devices will listen for transmissions from other nodes using this service. This reduces the problems caused by broadcast traffic and enables you to cause one sender to get information to multiple (group) nodes without that message going to every node on the network. Consequently, multicasting is good, but as with any other technology, it can cause problems or require an advanced level of administration to make it work properly.
|
