Welcome
First of all, may I welcome you to my site. My name is Chris and I'm from the UK and work as a Systems Engineer for Cisco. This blog was initially created to post up my subnetting technique but has now got more stuff to do with attaining Cisco certifications. Either way I really hope that the content is sufficent for your needs and I look forward to hearing your feedback. If you find that the content really helps you please feel free to donate using the PayPal link on the right.
To view the index of all my articles please click here.
PPP Authentication - Using AAA
Monday, 21 March 2011
Here we'll try to authenticate a session between two routers. One side of the router will use RADIUS while the other will use TACACS+. The side using RADIUS will be configured using a AAA server group. The TACACS+ server will be globally configured. Both RADIUS and TACACS+ use "cisco" as the password.
Router 1
1. Configure a new AAA model
aaa new-model
2. By default, the "aaa new-model" command will require local authentication on the console port. To override this, and save us from locking ourselves out, we must configure specific console authentication and the easiest way to do that is by using "none".
aaa authentication login CONSOLE none
!
line vty 0 4
login authentication CONSOLE
3. Configure a RADIUS server group called MY_RADIUS_GROUP and ensure that the RADIUS server at 192.168.1.1 only applies to this group.
aaa group server radius MY_RADIUS_GROUP
server-private 192.168.1.1 key cisco
4. Configure AAA to authenticate PPP sessions against the RADIUS server group and if that fails it should try the local database.
aaa authentication ppp PPP_AUTH group MY_RADIUS_GROUP local
5. Configure the phyiscal interface to use the AAA authentication session
interface s0/0
ppp authentication PPP_AUTH
Router 2
1. Configure a new AAA model
aaa new-model
2. By default, the "aaa new-model" command will require local authentication on the console port. To override this, and save us from locking ourselves out, we must configure specific console authentication and the easiest way to do that is by using "none".
aaa authentication login CONSOLE none
!
line vty 0 4
login authentication CONSOLE
3. Configure a TACACS+ server group globally at 192.168.1.2
tacacs-server host 192.168.1.2 key cisco
4. Configure AAA to authenticate PPP sessions against the TACACS+ server and if that fails it should try the local database.
aaa authentication ppp default group tacacs local
5. Configure the phyiscal interface to use the AAA authentication session
interface s0/0
ppp authentication PPP_AUTH
Posted byChris Bloomfield at 14:17 1 comments
When to use access-lists or prefix-lists
Sunday, 20 March 2011
Here's a great a bit of info that I pulled from INE's own Brian McGahan that is is worth remembering. Thanks Brian!
Some applications you can use both access-lists and prefix-lists. In general, anytime you are matching a route, like with a route-map for redistribution, a route-map for BGP, or a distribute-list, you should use a prefix-list. This is what they were designed for. An access-list should be used any time you’re trying to match traffic, or with other non-routing related applications.
Source: http://ieoc.com/forums/t/15006.aspx
Posted byChris Bloomfield at 08:31 0 comments
Labels: access-lists, ine, prefix-lists
PPPoE Server and Client
Saturday, 19 March 2011
I've done a load of Cisco 877 configurations in the past on ADSL lines and wondered how all of the virtual template stuff works so here's a lesson as to how to configure PPPoE Server and Client with the Server providing the Client with an IP address using DHCP.
We will also get the Server to authenticate the Client using CHAP and the Server will rate-limit the Client to a maximum of 10 sessions per minute over a period of 5 minutes.
Let's start with the Client as it is the least amount of work.
Client Tasks
1. Configure a Dialer interface
a. It should receive the IP address from the Server
b. Have PPP configured
c. Be part of a Dialer Pool
d. Set the CHAP hostname
e. Set the CHAP password
interface Dialer1
ip address dhcp
encapsulation ppp
dialer pool 1
ppp chap hostname Router1
ppp chap password cisco
2. Tie the Dialer to a physical interface
a. Remove any IP address from the interface
b. Enable PPPoE
c. Configure PPPoE to match the Dialer Pool
interface FastEthernet0/1
no ip address
pppoe enable
pppoe-client dial-pool-number 1
Server Tasks
1. Configure a Virtual Template interface
a. Apply an IP address
b. Apply PPP encapsulation
c. Enable CHAP authentication
interface Virtual-Template1
ip address 192.168.1.1 255.255.255.0
encapsulation ppp
ppp authentication chap
2. Create a Broadband Aggregation Group
a. Give the group a name
b. Tie the Virtual-Template to the group
c. Throttle the client to 10 sessions per minute over a period of 5 minutes.
bba-group pppoe MY_BBA_GROUP
virtual-template 1
sessions per-mac throttle 10 60 300
3. Configure the physical interface connected to the client
a. Tie the physical interface to the BBA group
interface FastEthernet0/1
pppoe enable group MY_BBA_GROUP
4. Create a DHCP pool for the client
a. Exclude the IP address assigned to the Virtual Template interface
ip dhcp excluded-address 192.168.1.1
ip dhcp pool MY_PPPoE_POOL
network 192.168.1.0
5. Create a username/password pair for Router1 for authentication
username Router1 password cisco
That's it. It takes a little while for it to kick in but worth trying to lab.
Cheers,
Chris
Posted byChris Bloomfield at 14:34 1 comments
Labels: ppp, pppoe, pppoe client, pppoe server
PPP Authentication - PAP and CHAP
Friday, 18 March 2011
I don't really use serial interfaces in my day-to-day job so when it comes to lab questions regarding PPP, HDLC, and Frame Relay I am immediately horrified.
Here's the question given:
1. Enable PPP encapsulation for the Serial link connecting R4 and R5 and use the IP subnet 155.1.45.0/24 for this link.
2. R4 should attempt to authenticate R5 using PAP and then CHAP. R5 should refuse PAP authentication and use CHAP.
3. Make sure R4 uses an alternate CHAP hostname R4CHAP.
4. Use the name R5CHAP and the password of CISCO to accomplish this.
5. R5 should authenticate R4 using PAP only. R4 should use the name R4PPP and the password of CISCO.
Let's say that s0/0 is the interface at either end and R4 is the DCE.
Step 1:
Apply PPP, clock rate on R4, and IP address.
R4:
interface s0/0
encapsulation ppp
clock rate 64000
ip address 155.1.45.4 255.255.255.0
R5:
interface s0/0
encapsulation ppp
ip address 155.1.45.5 255.255.255.0
Step 2:
R4 needs to authenticate R5 using PAP, and if it is refused, should use CHAP. R5 will be configured to refuse PAP authentication from R4.
R4:
interface s0/0
encapsulation ppp
clock rate 64000
ip address 155.1.45.4 255.255.255.0
ppp authentication pap chap
R5:
interface s0/0
encapsulation ppp
ip address 155.1.45.5 255.255.255.0
ppp pap refuse
Step 3:
R4 needs to specify a CHAP hostname of R4CHAP. If this wasn't specified then the CHAP hostname would be set as the hostname of the router (in this case, R4).
R4:
interface s0/0
encapsulation ppp
clock rate 64000
ip address 155.1.45.4 255.255.255.0
ppp authentication pap chap
ppp chap hostname R4CHAP
R5:
interface s0/0
encapsulation ppp
ip address 155.1.45.5 255.255.255.0
ppp pap refuse
Step 4:
R5 should respond with a CHAP hostname of R5CHAP and a CHAP password of CISCO. Therefore on R4 we must configure a username/password pair for R5's details. What isn't obvious is that R5 needs a username/password pair for R4's details. In this case the R4 CHAP hostname is R4CHAP and the password must match R5's CHAP password which is CISCO.
R4:
username R5CHAP password CISCO
!
interface s0/0
encapsulation ppp
clock rate 64000
ip address 155.1.45.4 255.255.255.0
ppp authentication pap chap
ppp chap hostname R4CHAP
R5:
username R4CHAP password CISCO
!
interface s0/0
encapsulation ppp
ip address 155.1.45.5 255.255.255.0
ppp pap refuse
ppp chap hostname R5CHAP
Step 5:
R5 wants to authenticate R4 using PAP and R4 responds with a PAP username of R4PPP and a PAP password of CISCO.
R4:
username R5CHAP password CISCO
!
interface s0/0
encapsulation ppp
clock rate 64000
ip address 155.1.45.4 255.255.255.0
ppp authentication pap chap
ppp chap hostname R4CHAP
ppp pap sent-username R4PPP password CISCO
R5:
username R4CHAP password CISCO
!
interface s0/0
encapsulation ppp
ip address 155.1.45.5 255.255.255.0
ppp pap refuse
ppp chap hostname R5CHAP
ppp authentication pap
That's it. Not too bad but worth working through to see how it all fits together.
Posted byChris Bloomfield at 13:16 0 comments
Labels: authentication, chap, pap, ppp
Multicast RPF
Wednesday, 16 March 2011
Right then, I'm happy with RPF as a principle and that the router checks the interface on which it receives multicast traffic and consults its routing table to see if that interface would be used to reach the multicast source.
What I didn't know (or at least I hadn't remembered) until now is that when there are equal-cost paths to the multicast source (e.g. OSPF, EIGRP etc) the router must pick one of them for Multicast RPF. Which one does it pick? It picks the one with highest neighbouring router ID.
For example, let's say that the multicast RP is located on 192.168.1.0/24 network. You downstream router receives two equal-cost routes for that subnet, one from R1 with a router-id of 1.1.1.1 and the other from R2 with a router-id of 2.2.2.2. The router will pick the interface connected to R2 as it has the highest router-id.
You can frig this by using tunnelling but that is a whole new ball game and one I'm not going into right now.
Posted byChris Bloomfield at 11:06 0 comments
