DHCP

configuracion DHCP SERVER EN EL ROUTER1. Exclude addresses from being assigned by DHCP: ip dhcp excluded-address first last 2. Create a DHCP pool and go to pool configuration mode: ip dhcp pool name A. Define subnet that the DHCP server should support: network subnet-ID mask or network subnet-ID prefix-length B. Define default router IP address(es) in that subnet: default-router address1 address2... C. Define list of DNS server IP addresses: dns-server address1 address2... D. Define length of lease, in days, hours, and minutes: lease days hours minutes E. Define the DNS domain name: domain-name name

VERIFICACION show ip dhcp binding: enumera estado de informacion sobre cada dir. IP  actualmente arredendada a un cliente show ip dhcp pool [poolname]: enumera el rando de dir. IP configurados, mas estadisticas para el numero de direcciones actualmente arrendadas y the high-water mark for leases from each pool.  show ip dhcp server statistics: Lists DHCP server statistics

DETECTANDO CONFLICTOS DHCP servers detect conflicts by using pings. Before offering a new IP address to a client, the DHCP server first pings the address. If the server receives a response to the ping, some other host must already be using the address, which lets the server know a conflict exists. The server notes that particular address as being in conflict, and the server does not offer the address, moving on to the next address in the pool.The DHCP client can also detect conflicts, but instead of using ping, it uses ARP. In the client case, when the DHCP client receives from the DHCP server an offer to use a particular IP address, the client sends an ARP for that address. If another host replies, the DHCP client has found a conflict.

The show ip dhcp conflict command lists the method through which the server added each address to the conflict list: either gratuitous ARP, as detected by the client, or ping, as detected by the server. The server avoids offering these conflicted addresses to any future clients, until the engineer uses the clear ip dhcp conflict command to clear the list.

ipconfig OS windowsifconfig  OS linux y mac

nslookup hostname

En el router, las config pueden ser verifcadas con los comandos de CLI:  show interfaces  show ip interface brief,  show protocols, and show running-config. En el switch, verificar el asignamiento de vlans: show interfaces status,  show vlan,  show interfaces switchport.

Para verificar el default router en el host:netstat -rnotra forma de verificar el default router es ver si ARP trabaja, enviando un paquete a un host en otra subredarp -a: enumera los host de la tabla de arpen el router:show arpnota:la tabla ARP lista dir. IP que se encuentran en la misma subred y no esas que estan en rubredes remotas.

Testing Connectivity with ping, traceroute, and telnetping:usa ICMP: ICMP Echo Request and ICMP Echo Reply messages

clear ip arp 172.16.2.101, BORRA la  ip de la cache arp del router

Figure 18-10 shows the default behavior of the router ping command. It has to pick a source IP address to use for the Echo Request, and routers choose the IP address of the outgoing interface. The Echo Request, from R1 to host B, flows with source IP address 172.16.4.1 (R1’s S0/0/0 IP address), and of course destination IP address 172.16.2.101.

ping extendido the extended ping command allows the user to type all the parameters on a potentially long command, it also allows users to simply issue the ping command, press Enter, with IOS then asking the user to answer questions to complete the commandnota:The extended ping command does allow you to refer to the interface, but it must be the full spelled-out interface name, for example, gigabitethernet0/0.the extended ping tests the likely route back to the user host (host A), while the standard ping does not.

traceroute that fully completes identifies the routers in the path from source host to destination host. Specifically, it lists the next-hop IP address of each router that would be in each of the individual routes.

When a router decrements the TTL to 0, the router perceives the packet is looping around, and the router discards the packet. The router also notifies the host that sent the discarded packet by sending an ICMP TTL Exceeded message

traceroute sends messages to make the routers send back a TTL exceeded message, but without a routing loop existing in the network. As a result, the traceroute command can identify the router based on the source IP address of the packet holding the ICMP TTL exceeded message.

a traceroute command begins by sending several packets (usually 3), each with the header TTL field equal to 1. When that packet arrives at the next router—host A’s default Router R1 in this example—the router decrements TTL to 0, and discards the packet. The router then sends host A the TTL Exceeded message, which identifies the router’s IP address to the traceroute command.sends packets with TTL=2, then 3, 4, and so on, until the destination host actually replies.

For example, if the earlier traceroute 172.16.2.101 command listed R1 (172.16.1.1), but no other routers, the next troubleshooting steps should happen at R1 and R2. In a large network, narrowing down a problem to a couple of routers can be a great early step to troubleshoot a routing problem

en windows: tracert and pathping, y no traceroute. Linux y MAC suportan el traceroute..

varias sesiones de telnet o sshterminar una sesion ctrl+shitf 6 y xshow sessions o whereThe asterisk marks the most recently suspended session.resume num_conexion : resume 1disconnect num_conexion: disconnect 1If the resume command is used without a connection ID, the command reconnects the user to the most recently suspended connectionshow sshshow users command lists all users logged in to the router on which the command is used.

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