I have been with the Open University now for a few years studying Computing & IT and Business and thought I would give my views on some good IT tips students can take advantage of to ensure their work is secure and backed up.
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PPP stands for Point-to-Point and is a protocol operating at Layer 2 of the OSI model.
HDLC is Cisco proprietary and is the default encapsulation on serial links great if you have all Cisco devices, otherwise you will need PPP configured.
There are two types of authentication that can be used with PPP. These are PAP and CHAP.
PAP (Password Authentication Protocol) – Passwords are sent in plain text. No periodic checks.
CHAP (Challenge Handshake Authentication Protocol) – Password is encrypted. Sends periodic checks to ensure the router is talking to the same router.
Configuring PPP is rather simple:
configure terminal
int s2/0
encapsulation ppp
Configure PPP with PAP authentication
RouterA:
configure terminal
hostname RouterA
username RouterB password Cisco
int s2/0
ip address 10.0.0.1 255.255.255.0
encapsulation ppp
ppp authentication pap
ppp pap sent-username RouterA password Cisco
RouterB:
configure terminal
hostname RouterB
username RouterA password Cisco
int s2/0
ip address 10.0.0.2 255.255.255.0
encapsulation ppp
ppp pap sent-username RouterB password Cisco
ppp authentication pap
Configure PPP with CHAP authentication
RouterA:
configure terminal
hostname RouterA
username RouterB password Cisco
int s2/0
ip address 10.0.0.1 255.255.255.0
encapsulation ppp
ppp authentication chap
RouterB:
configure terminal
hostname RouterB
username RouterA password Cisco
int s2/0
ip address 10.0.0.2 255.255.255.0
encapsulation ppp
ppp authentication chap
Show commands:
show interface s0/0 – Look for encapsulation ppp and LCP is open. Check interface is up and line protocol is up. Password/Username or encapsulation mismatch will bring the line protocol down.
Debug:
debug ppp authentication – Look for any failures of the three-way handshake.
DCLI’s are local – layer 2 addressing. Not advertised to other routers
Permanent virtual circuit (PVC) – Dedicated circuit always up
Switched virtual circuit (SVC) – Temporary circuit that is only up for example during a data transfer session
Three LMI types cisoc, ansi and q933a
Local Management Interface (LMI) – passes information about the status of the circuit. Must match on the DCE (Provider) and DTE end
non-broadcast multi-access (NBMA) – broadcasts are not sent over frame-relay by default
inverse ARP – Used to allow dynamic mapping. On by default can be turned off using no frame-relay inverse-arp
Split horizon – Stops rouging updates such as RIP from being sent out of the same interface it was received on. Turning off split horizon may cause routing loops
Sub-interfaces can overcome the split horizon by creating sub interfaces out of a serial interface. This gets around the problem of a routing update being sent and received out of the same interface. Sub-interfaces can either be point-to-point or multipoint
Show commands:
show frame-relay map – Table of frame maps, shows dlci and lmi information
show frame-relay lmi – Shows configured LMI type, if timeouts occur and increase could indiciate a lmi mis-match
show frame-relay pvc – Shows BECN and FECN information
Showing examples of Dynamic, Static and Nat overload.
Configuring NAT Overload on Router1:
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.224
ip nat inside
interface Serial0/1/0
ip address 62.0.0.2 255.255.255.0
ip nat outside
ip nat pool natpool 62.0.0.2 62.0.0.2 netmask 255.255.255.0
ip nat inside source list 1 pool natpool overload
access-list 1 permit 10.0.0.0 0.0.0.31
Configuring Static NAT on Router1:
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.224
ip nat inside
interface Serial0/1/0
ip address 62.0.0.2 255.255.255.0
ip nat outside
ip nat inside source static 10.0.0.2 62.0.0.2
ip nat inside source static 10.0.0.3 62.0.0.4
Configuring Dynamic NAT on Router1:
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.224
ip nat inside
interface Serial0/1/0
ip address 62.0.0.2 255.255.255.0
ip nat outside
ip nat pool natpool 62.0.0.3 62.0.0.4 netmask 255.255.255.0
ip nat inside source list 1 pool natpool
access-list 1 permit 10.0.0.0 0.0.0.31
