Type field

  • The IEEE 802.2 Ethernet DSAP field
  • The SNAP header Protocol type field

Private Address Space 10/8 172.16/12 192.168/16

What Is Administrative Distance?
Where to Place the Access Lists

TCP / IP Layers

TCP/IP protocols map to a four-layered conceptual model: Application, Transport, Internet, and Network Interface. This model is officially known as the TCP/IP Internet Protocol Suite but is often referred to as the TCP/IP protocol family. As shown in Figure 2-1, each layer in the TCP/IP model corresponds to one or more layers of the International Standards Organization (ISO) seven-layer Open Systems Interconnection (OSI) model.

Configuration Register Setting

Router Behavior


  • Boots into bootstrap

  • Ignores break

  • Boots into ROM if initial boot fails

  • 9600 console baud rate


  • Ignores break

  • Boots into ROM if initial boot fails

  • 9600 console baud rate default value for most platforms


  • Ignores break

  • Boots into ROM if initial boot fails

  • 9600 console baud rate

  • Ignores the contents of Non-Volatile RAM (NVRAM) (ignores configuration)

PPP's physical interfaces: Async, sync HSSI , ISDN

Link Control Protocol (LCP) forms part of the PPP. In setting up PPP communications, both the sending and receiving devices send out LCP packets to determine specific information that the prospective data transmission will require. The LCP protocol:

  • checks the identity of the linked device and either accepts or rejects the peer device
  • determines the acceptable packet size for transmission
  • searches for errors in configuration
  • can terminate the link if requirements exceed the parameters

Devices cannot use PPP to transmit data over a network until the LCP packet determines the acceptability of the link, but LCP packets are embedded into PPP packets and therefore a basic PPP connection has to be established before LCP can reconfigure it. The LCP over PPP packets have control code 0xC021 and their info field contains the LCP packet, which has four fields. (Code, Id, Length, Data)

  • Code: Operation requested: configure link, terminate link, ... and acknowledge and deny codes.
  • data: Parameters for the operation

Route Summarization

When you summarize routes in RIP, IGRP, EIGRP, or OSPF, you're replacing a series of routes with a summary route and mask. With RIP, IGRP, and EIGRP, this actually lessens the size of the routing update packet itself - multiple routes are replaced with the summary route. For instance, the routes,,, and can be summarized as Only the summary address will be found in the update packet, making it concise yet complete.

Summarizing routes can also make the routing table smaller, yet still allow for complete IP connectivity when done correctly. Using the above example, the four more-specific routes will be replaced by a single summary route. Since the entire routing table is parsed before the routing process is complete, keeping the routing table as small as possible does help speed the routing process as a whole.

With RIP version 2 and EIGRP, manual route summarization is configured on the interface that will be advertising the summary.
Router (config-if)#ip summary-address rip <ip_address> <ip_network_mask>
Router (config-if)#ip summary-address eigrp <autonomous-system-number> <address mask>

RIP version 2 and EIGRP also both perform autosummarization on routes that are advertised across classful network boundaries. This is disabled with the protocol-level command "no auto-summary".

For OSPF, the commands differ. If youre configuring inter-area route summarization, use the "area range" command. The number following "area" is the area containing the routes being summarized, not the area receiving the summary.
R1(config)#router ospf 1
R1(config-router)#area 1 range

If you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR.
R1(config)#router ospf 1

Extended access list close to the source
Standard access list close to the destination

CISCO 'ip route' command:
ip route <network> <mask> <next hop|exit interface> [admin distance]
A loopback interface is a virtual interface that can be configured with the interface loopback interface-number command, where interface-number is an integer. Loopback interfaces are always in an “up and up” state unless administratively placed into a shutdown state. For instance, a simple configuration of the command interface loopback 0, followed by ip address would create a loopback interface, and assign it an IP address. Assuming the subnet on the loopback interface is advertised into the internetwork, an engineer can ping, trace, and telnet to the loopback IP address.
Distance Vector Protocols: Bellman-Ford.
Distance Vector Protocols advertise routing information by sending messages, called routing updates, out the interfaces on a router. These updates contain a series of entries,
with each entry representing a subnet and a metric. With distance vector routing, each node has information only
about the next hop
Link-State Protocols: Dijkstra.
Send partial updates when link status changes and floods full routing table updates every 30 minutes. The flooding, however, does not happen all at once, so the overhead is minimal. OSPF and IS-IS are true link-state routing protocols. In link state routing, each node has a complete map of the topology.

Routing Protocols
  1. Dynamic Route Updates
  2. Prevent Routing Loops
  3. Prioritize Multiple Routes to Same Destination
  1. IEEE 802.1w is a Rapid Spanning Tree Protocol (RSTP)
  2. -
  3. -
Protocol IP Protocol
Field Values
UDP 17



Well Known Ports
20 FTP TCP FTP - data port
21 FTP TCP FTP - control (command) port
22 SSH TCP,UDP SSH (Secure Shell) - used for secure logins, file transfers (scp, sftp) and port forwarding
23 TELNET TCP Telnet protocol - unencrypted text communications
25 SMTP TCP SMTP - used for e-mail routing between mailservers E-mails
53 DNS TCP,UDP DNS (Domain Name System)
67, 68 DHCP UDP BOOTP (BootStrap Protocol) server (also used by Dynamic Host Configuration Protocol)
69 TFTP UDP TFTP (Trivial File Transfer Protocol)
80 HTTP TCP HTTP (HyperText Transfer Protocol) - used for transferring web pages
110 POP3 TCP POP3 (Post Office Protocol version 3) - used for retrieving E-mails
161 SNMP UDP SNMP (Simple Network Management Protocol)
443 HTTPS TCP HTTPS - HTTP Protocol over TLS/SSL (encrypted transmission)

Access-lists configuration
access-list <acl#> {permit|deny}  1  2  3  [4]
Extended access-list range :100 - 199  og  2000-2699

              OSPF   (RP)
              EIGRP   (RP)
          +   ICMP
   IP   +   TCP       (IP gir ICMP, TCP, UDP)
          +   UDP
  1. Protocol
  2. <sc ip> <src wildcard>
    host <src ip>
  3. <dst ip> <dst wildcard>
    host <src ip>
  4. [operator port]

Cisco documentation

Well-known multicast IP addresses
Address Usage All Hosts All Routers OSPF Routers OSPF Designated Rrouters ST Routers ST Hosts RIP2 Routers IGRP Routers Mobile-Agents DHCP Server/Relay Agent All PIM Routers

show ip route

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

1       2                 3  4           5        6      7
D [90/183736] via, 2d16h, ATM1/0.222
  1. Where the route came from, in this case, “D” is EIGRP
  2. The network and netmask (in CIDR notation)
  3. The administrative distance, this case 90 is the default for EIGRP
  4. The metric assigned by the routing protocol
  5. The IP next hop
  6. The last time this route changed in the ip routing table
  7. The outgoing interface

#2, 3, 4, and 5 are what’s needed to route, BTW.