The network layer adds a header containing the logical source and destination addressing, and then hands the packet to the Data-Link layer. The Data-Link layer processes its header, removes it, and then hands it over to the Network layer.
Ethernet Technologies -
100baseTX uses two of the four pairs in a twisted pair cable and requires Category 5 cable for reliable performance. The layout or knot of the wires in the RJ45 connector dictates the function of the cable.
Spanning Tree Protocol -
The root port of each switch has the lowest root path cost to access the root bridge. Root Port – The port on each switch that has the best path cost to the root bridge.
IPv4 Addressing and Subnetting -
The exponent '' identifies the number of bits to steal from the host portion of the subnet mask. Because it is never possible to assign a host an address with all 0 or all 1 bits in the host portion of the address. The ip subnet-zero commands allow for the use of networks with all 0 or all 1 bits in the stolen network portion of the address.
Fragment offset 13 bits Identifies the fragment relative to the start of the packet Time to Live 8 bits Decreased with each router traversed. Fragment offset 13 bits Identifies the fragment relative to the start of the packet Time to Live 8 bits Limits the lifetime of a packet. Since HostA and HostB are on the same network, HostA will then send out an ARP request, asking for the MAC address of the .
HostA will then issue an ARP request asking for the MAC address of 10.1.1.1.
TCP and UDP -
Note, however, that this sequence number identifies the data (bytes) within the segment, not the segment itself. HostB includes an initial sequence number with its SYN message as well – 4500 in the example above. Using the example above again, the first data byte of HostA will be assigned a sequence number of 1001.
Note that each individual byte of data is not assigned a sequence number and independently validated, as this would incur enormous overhead. Recall that during TCP connection setup, the acknowledgment number was set to sequence number + 1. In the example above, the initial TCP window size is set to 50 bytes, and the first byte in the stream is assigned a sequence number from 1001.
Once accepted, HostA then sends another 50 bytes of data, identifying the first byte with a sequence number of 1051.
IPv6 Addressing -
IPv6 unicast typically allocates the first 64 bits of the address to identify the network (prefix) and the last 64 bits to identify the host (referred to as the interface ID). The first 24 bits of the MAC (OUI) become the first 24 bits of the EUI-64 formatted interface ID. The last 24 bits of the MAC (host ID), become the last 24 bits of the interface ID.
The class of an IPv4 address was identified by the high-order bits in the first octet: This requested node address is created by adding the last 24 bits of the interface ID to the following prefix: FF02::1:FF/103. Packet Too Large (ICMP Packet Type 2) – Indicates that the packet is larger than the MTU of the link.
This RA (ICMP packet type 134) comes from the link-local address of the sending router and sent to the link-scope multicast group containing all nodes.
Introduction to 802.11 Wireless -
802.11 devices have no method of detecting a collision beyond the receiving device failing to send an acknowledgment. Instead, 802.11 devices attempt to avoid collisions using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Because 802.11a operates in the regulated 5.0 GHz band, it is generally free from interference from other RF devices.
802.11a is generally not compatible with other 802.11 extensions, as most of the other modifications operate in the 2.4 GHz band. Because 802.11b operates in the unregulated 2.4-GHz band, it is susceptible to interference from other RF devices in the household. A pure 802.11n environment should operate in the 5.0-GHz band to maximize throughput and limit interference.
MAC address filtering is an additional form of authentication, although it is not defined in the 802.11 standard.
Router Components -
If the IOS is not found in Flash, the bootstrap loads the base IOS stored in ROM into RAM. If the boot configuration is not found in NVRAM, iOS will attempt to load a configuration file from TFTP. Interfaces are identified by both the interface type and the interface number (which always starts with “0”).
Thus, the third Fast Ethernet interface on the first modular slot would be identified as FastEthernet0/2. Like interfaces, lines are identified by both line type and line number (again, always starting at "0"). From a PC, software such as HyperTerminal is required to establish a connection from the local serial port to the router's console port.
The auxiliary port can function in the same way as a console port and can be accessed using a roller cable.
Introduction to the Cisco IOS -
There are literally dozens of arguments to the show command, and each provides information on a specific aspect of the router. The first part of this status (Serial0 is up) refers to the physical layer status of the interface. The second part (line protocol is up) refers to the data link layer status of the interface.
The most likely cause of the above status is a faulty (or disconnected) cable or interface. So we need to save the contents of the running-config in the startup-config file. In newer versions of Cisco IOS, the output of show commands can be filtered to remove irrelevant lines by using the pipe “ | " character.
The # symbol is used as a delimiter to indicate the beginning and end of the banner.
Advanced IOS Functions -
CDP is a proprietary Cisco protocol used to collect information about locally connected Cisco switches and routers.
The Routing Table -
If your router runs multiple routing protocols, Administrative Distance is used to determine which routing protocol you trust the most. If a router receives a RIP and an OSPF route to the same network, it will use Administrative Distance to determine which routing path to choose. This indicates both the administrative distance and the metric (the 120 is the AD and the 1 is the hop counter).
A static route has the smallest administrative distance (1) of all routes; however, its prefix length is less specific. NOTE: The calculation of the lowest metric path within a routing protocol is performed before the administrative distance selects a path that it "trusts". Therefore, the order of the above "measures" is prefix length, metric, and then administrative distance.
However, the route with the lowest administrative distance is always preferred regardless of the metric (assuming the prefix length is the same).
Static vs. Dynamic Routing -
Link state routing protocols were developed to alleviate the convergence and looping problems of distance vector protocols. Topology table - Also known as the "link state" table, it contains a map of all the links within the area, including the status of each link. Link state protocols do not route via rumor. Instead, routers send updates that advertise the state of their links (a link is a directly connected network).
By maintaining a consistent topology table among all routers within an area, link-state protocols can converge very quickly and are immune to routing loops. Additionally, because updates are sent only during a link-state change and contain only the change (and not the full table), link-state protocols are less bandwidth-intensive than distance-vector protocols. However, the three link-state tables use more RAM and CPU on the router itself.
Link-state protocols use some form of cost, usually based on bandwidth, to calculate a route's metric.
Classful vs. Classless Routing -
When a "class" router has an interface connected to a core network, it believes it knows all routes connected to that core network. However, if the classful router receives a packet destined for a 10.x.x.x subnet that is not in the routing table, it will drop that packet even if there is a default route. This particular scenario will work when using RIPv1, despite the fact that we have subnetted the large 10.0.0.0 network.
When Router A sends a RIPv1 update to Router B via Serial0, it will not include the subnet mask for the 10.1.0.0 network. Again, when Router A sends a RIPv1 update to Router B via Serial, it will not include the subnet mask for the 10.1.0.0 network. Router B will accept this routing update and realize that it does not have a directly connected interface in the 10.x.x.x scheme.
This is because it already has the summary network 10.0.0.0 in its routing table and it is directly connected.
Configuring Static Routes -
The Administrative Distance of a static route can be changed to form a floating static route, which is only used if there are no other routes with a lower AD in the routing table. Static routes remain in the routing table only as long as the interface connecting to the next-hop router is active. Normally, if a specific route to a particular network does not exist, a router will drop all traffic destined to that network.
A default route, or gateway of last resort, allows traffic to be forwarded even without a specific route to a particular network. It is the least specific route possible, and will therefore only be used if a more specific route does not exist (hence "gateway of last resort").
Routing Information Protocol -
RIPv1 (RFC 1058) is stylish and therefore does not include the subnet mask with the routing table updates. RIPv2 (RFC 2543) is classless and thus includes the subnet mask with the routing table updates. Update Timer (default 30 seconds) – specifies how often the router sends a routing table update.
Invalid timer (default 180 seconds) – indicates how long a route will stay in the route table before being marked as invalid if no new updates are heard for this route. An update has been received from another router, marking that route with a higher metric than what is currently in the routing table. Flush Timer (default 240 seconds) – indicates how long a route can stay in a routing table before being flushed if no new updates are heard for this route.
If no update is heard at all, the route will be deleted from the routing table once the flush timer expires, which is 60 seconds after the route is marked as invalid.
