Brouters (Bridge/Routers)

Brouters are protocol dependant devices. When a brouter receives a frame to be forwarded to the remote segment, it checks to see if it recognizes the Network layer protocol. If the Brouter does, it acts like a router and finds the shortest path. If it doesn't recognize the Network layer protocol, it acts like a bridge and forwards the frame to the next segment.

The key advantage to Brouters is the ability to act as both a bridge and a router. It can replace separate bridges and routers, saving money. This is, of course, provided that the Brouter can accomplish both functions satisfactorily.

EGRP - Exterior Gateway Routing Protocol

EGRP was created to solve many of the problems with RIP and has become the default routing protocol across the Internet. EGRP is an enhanced distance vectoring protocol, it uses up to 5 metrics (conditions) to determine the best route:
Bandwidth
Hop Count (Delay) - maximum of 255
Maximum Packet size
Reliability
Traffic (Load)
These routing metrics are much more realistic indicators of the best routes compared to simple hop counts

OSPF - Open Shortest Path First

OSPF is a link state premise, this means that it has several states of routers linked together in a hierarchical routing model:

The top of the root is the Autonomous Router, it connects to other autonomous systems (the Internet). The next is the Backbone Routers, which is the highest area in the OSPF system. Border routers are attached to multiple areas and run multiple copies of the routing algorithm. Last is internal routers which run a single routing database for one area.
Basically, by dividing the network into a routing hierarchy, substantial reduction of routing update traffic and faster route convergence results on a local basis. Each level has a smaller routing table and less to update.

Router Addressing

Routers combine the Network Number and the Node Address to make Source and Destination addresses in routing Network Layer PDUs across an network. Routers have to know the name of the segment that they are on and the segment name or number where the PDU is going to. They also have to know the Node Address: MAC Address for Novell and the IP address for TCP/IP.
For Novell's SPX/IPX (Sequential Packet eXchange/Internetwork Packet eXchange), the Network Layer PDUs address is composed of the Network Address (32 bit number) and the Host address (48 bit - MAC address).

Routing Protocols

Routing Protocols are a "sub-protocol" of the Network Layer Protocol that deal specifically with routing of packets from the source to the destination across an internetwork. Examples of Routing Protocols are: RIP, IGRP and OSPF

RIP - Routing Information Protocol

RIP was one of the first routing protocols to gain widespread acceptance. It is described in RFC1058 which is an Internet standard. RFC stands for request for comment and the RFC1058 is the 1,058 RFC standard published. Commercial NOS such as Novell, Apple, Banyan Vines and 3Com, use RIP as the base routing algorithm for their respective protocol suites.
RIP is a distance vector algorithm. Routers maintain a detailed view of locally attached network segments and a partial view of the remainder of the routing table. The routers contain information on the number of hop counts to each segment. A hop is considered to be one transverse through a router. Pass through a router and the Hop count increases by 1.

The routers are updated every 30 seconds, each router sending out a RIP broadcast. This advertisement process is what enables RIP routing to be dynamic. Dynamic routers can change routing tables on the fly as the network configuration changes. By using the Hop Count information from their routing tables, routers can select the shortest path - the least number of hops to the destination.
Apple uses RTMP (routing table maintenance protocol) which adds a route status indicator: good, bad or suspect depending on the age of the route information.
Novell adds ticks to the RIP algorithm, Ticks are dynamically assigned values that represent the delay associated with a given route. Each tick is considered 1/18 of a second.

LAN segments are typically assigned a value of 1 tick, a T1 link may have a value of 5 to 6 ticks and a 56 Kbps line may have a value of 20 ticks. Larger number of ticks indicate a slower routing path.
Three commonest problems that can occur with RIP are:
Routing loops: the router indicates that the shortest path is back the way the packet came from.
Slow Route Convergence: routers have delay timers that start counting after the RIP advertising packet is broadcasted. This gives the routers time to receive and formulate a proper routing table from the other routers. If the delay timer is too short, the routing table can be implemented with incomplete data causing routing loops
Hop Count Exceeded: the maximum number of hop counts is 15 for RIP. A hop count of 15 is classified as unreachable which makes RIP unsuitable for large networks where hop counts of 15 and above are normal.

Router Segment to Segment Characteristics

Routers that only know Novell IPX (Internetwork Packet Exchange) will not forward Unix's IP (Internetwork Packet) PDUs and vice versa. Routers only see the Network Layer protocol that they have been configured for. This means that a network can have multiple protocols running on it: SPX/IPX, TCP/IP, Appletalk, XNS, etc..

In the following network, Router #3 is a Novell SPX/IPX router, it only sees the Network Layer protocol IPX. This means that any TCP/IP PDUs will not pass through, the router does not recognize the PDUs and doesn't know what to do with them.

Purpose of Routers

The purpose of a router is to connect nodes across an internetwork regardless of the Physical Layer and Data Link Layer protocol used. Routers are hardware and topology independent. Routers are not aware of the type of medium or frame used (Ethernet, Token Ring, FDDI, X.25, etc...). Routers are aware of the Network Layer protocol used: Novell's IPX, Unix's IP, XNS, Apples DDP, etc.

Router OSI Operating Layer

Routers operate on the OSI Model's Network Layer. The internetwork must use the same Network Layer protocol. Routers allow the transportation of the Network Layer PDU through the internetwork even though the Physical and Data Link Frame size and addressing scheme may change

Figure D: The Windows Network Diagnostic Tool is designed to help you troubleshoot connectivity issues

Conclusion

As you can see, the Network Center offers some promising tools for managing network connections within Vista. Right now some of these tools are a little buggy, but hopefully Microsoft will work out some of the kinks before the next beta.

Figure C: The Status screen provides summary information related to the connection

If you look at the bottom of Figure C, you will notice the Configure and Diagnose buttons. Clicking the Configure button simply takes you to the connection’s configuration screen. This screen is very similar to the one that you are used to seeing in Windows XP. What’s more interesting though is the Diagnose button.
Clicking the Diagnose button takes you to a diagnostic tool that you can use to figure out why the connection is not working. This tool is a neat concept, but it’s one of those areas where Microsoft needs to do a little more work.
As I mentioned earlier, IPv6 is kind of a big deal in Windows Vista. IPv6 is Vista’s preferred protocol, but Vista runs IPv4 along side IPv6 for backward compatibility with existing networks. The reason why I say that Microsoft needs to do a little more work on the diagnostic tool is because as you saw in Figure C, my network connection is working fine. The only catch is that I am using IPv4 instead of IPv6. However, when I click the Diagnose button, Windows returns a bunch of errors related to IPv6, which I am not actively using, as shown in Figure D.