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Sunday, February 28, 2010

Day 1: ICND1 Last Review (WAN basics, NAT/PAT terminology and RIP)

My ICND1 exam is booked on monday but I didn't have much time this week so I'll focus mainly on WAN connections, namings in NAT/PAT and RIP.

There are two major categories for WANs:
  1. Dedicated: point-point lines provide a preestablished WAN communications path from the customer premises through the provider network to a remote destination. Usually leased from a carrier and obviously called leased lines.
  2. Switched:
    1. Circuit-Switched: dynamically establishing a dedicated virtual connection. Before communication can start it's necessary to establish the connection through the network of the service provider. Examples are PSTN and DSL (I know it's also a bit packet-switched but for the exam I'll keep it to this).
    2. Packet-Switched: because data flow fluctuates (nobody uses 100% bandwith all the time) so in these networks the data is transmitted in labeled cells, frames or packets. There's no dedicated path between source and destination endpoints, allowing for the sharing of connection links and common carrier resources for data transmission. The carrier can create virtual circuits between customer sites. When the customer is not using the full bandwith on its virtual circuit, the carrier, through statistical multiplexing, can make the unused bandwith available to another customer. Examples are Cable, Frame Relay.
    3. Cell-Switched: an example is ATM and it uses small fixed-size cells of 53 bytes (48 bytes for data).
NAT connects two networks together and translates the private addresses (inside local) in the internal network into public addresses (inside global) before packets are forwarded to another network. The inside network is the set of networks that are subject to translation, the outside network refers to all other addresses.
  • inside local address: the IP address assigned to a host on the inside network, likely not assigned by the NIC or service provider.
  • inside global address: a legitimate IP address assigned by the NIC or service provider that represents one or more inside local IP addresses to the outside world.
  • outside local address: the IP address of an outside host as it appears to the inside network, not necessarily legitimate, it's allocated from an address space routable on the inside.
  • outside global address: the IP address assigned to a host on the outside network by the host owner. This address is allocated from a globally routable address or network space.
You can verify the NAT and PAT configuration by using
show ip nat translation to display active translations and
clear ip nat translation * to clear all dynamic address translation entries (they'll time out after 24 hours by default).

Static routes are commonly used when you're routing from a network to a stub network. This stub network or leaf node is a network accessed by a single route. They can also be useful for specifying a "gateway of last resort" to which all packets with an unknown destination address will be sent. Obviously the static route must be configured in both directions for end-to-end connectivity. An example:
ip route
Don't mix it up with default route, for this you also give the IP address of the next-hop router to be used as default for packet forwarding, an example:
ip route
The ip classless command tells the route not to drop packets that are destined for a network that's not in the routing table, but to use the default route instead. If you don't use the ip classless command, packets like that would be dropped before being sent to the default route. (ip classless is on by default in IOS 12.x.).

It's recommended to set the bandwith command on the serial interface, this provides a minimum bandwith guarantee during congestion (used by some routing protocols). It has no effect on the actual speed of the line. Bandwith refers to the rate at which data is transferred over the communication link.
1DS0 = 64 kb/s
24DS0s = DS1 or T1 = 1.544 Mb/s

The show controller command displays information about the physical interface itself. Very useful to termine the type of cable connected to the serial interface. Please note that this information is determined when the router initially starts!

High-Level Data Link Control (HDLC) protocol specifies an encapsulation method for data on synchronous serial data links using frame character and checksum. HDLC supports both point-to-point and multipoint configurations and includes a means for authentication. HDLC may not be compatible between vendors from different vendors. There's a Cisco implementation of HDLC which is the default encapsulation for serial lines, as you probably know it doesn't show up in running-config! Cisco HDLC has no windowing or flow control, and only point-to-point connections are allowed. The Cisco HDLC implementation includes proprietary extensions in the data field to allow multiprotocol support at a time before PPP was specified. Use ppp when interoperability is required.

Routing protocols (like RIP, EIGRP) are used between routers to determine paths and maintain routing tables. After the path is determined, a router can route a routed protocol (like IP).

An autonomous system is a collection of networks under a common administrative domain. IGPs operate within an autonomous system and EGPs connect different autonomous systems.

Classful routing protocols do not include the subnet mask with the route advertisements (most distance vector routing protocols). When a classful routing protocol is used, all subnetworks of the same major network must use the same subnet mask. Routers that are running a classful routing protocol perform automatic route summarization accross network boundaries. When receiving a routing update packet, the router will apply the default classful mask if the routing update information contains a major network that is different! If the routing update information contains the same major network number as is configured on the receiving interface, the router applies the subnet mask that is configured on the receiving interface.
Classless routing protocols include the subnet mask with the route advertisement, they support variable-length subnet mask (VLSM). The summarization process is controlled manually and can usually be invoked at any bit position with in the address. Manual summarization may be required to keep the size of the routing tables manageable.

RIP is capable of load-balancing over as many as 16 equal-cost paths (default = 4).
RIPv1: classful, no VLSM, subnet mask is not send, broadcast, no manual route summarization and no authentication support.
RIPv2: classless, VLSM, subnet mask is sent, multicast, manual route summarization, authentication support.

router rip command starts the RIP routing process.
version 2 enables RIP version 2
network selects the participating attached networks and requires a major classful network number!
Use the show ip protocols command to display values about routing protocols and the routing protocol timer information associated with the routers. Important values is the flushed field (240 seconds) which specifies the time after which the individual routing information will be thrown out. The hold down (180 seconds) field: an update to a route that was down and is now up will stay in the hold down (possibly down) state until 180 seconds have passed.

1 comment:

  1. I like your blog post. Keep on writing this type of great stuff. I'll make sure to follow up on your blog in the future.
    ISDN Configuration