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.
CCIE - Warm Up Phase - Multicast
Tuesday, 3 May 2011
Some general notes from the ATC -
(*,G) = I know what group I want to join but I don't know who is sourcing that traffic
Shared Tree uses shortest path to the RP and all traffic passes through the RP
Source tree uses shortest path to the source and traffic may or may not pass through the RP
Bi-directional PIM uses Shared Trees only
Multicast Source Discovery Protocol (MSDP) is a logical peering between RPs in order to exchange Source Active (SA) information. RPF checks are used to prevent loops. Additionally, able to limit or filter SAs. Specifiy the MSDP peer using "ip msdp peer
Anycast RP - RPs use same IP address. The IGP determines which RP is "closest". If one RP fails, the other RP will be used once IGP reconverges. Used in conjunction with MSDP for exchanging information.
Useful commands:
show ip pim neighbor
show ip pim interface
show ip pim rp mapping
show ip mroute
debug ip mpacket (may need to use "no ip mroute-cache" to see thru-traffic)
debug ip pim
Auto-RP, by default, will not work in a pure Sparse Mode domain as router will not join 224.0.1.39 and 224.0.1.40 as it doesn't know of an RP. However, without joining those groups it won't know what the RP is. There are three workarounds:
a. Default RP assignment where you assign a static RP for 224.0.1.39 and 224.0.1.40 but this defeats the purpose of Auto-RP
b. Configure sparse-dense-mode. This acts as dense mode for those groups with no RP (e.g. 224.0.1.39 and 224.0.1.40).
c. Configure ip pim autorp listener which automatically goes in dense mode for 224.0.1.39 and 224.0.1.40.
Sample Network
--------------
R1--R2--R3--R4--R5--R6
R3 is RP on Lo0 interface. All devices configured with "ip pim rp-address 150.1.3.3" including R3 itself.
All interfaces connected to another router confogured for PIM Sparse Mode.
On R1 configure "ip igmp join-group 224.101.101.101".
R3 should now have a (*,224.101.101.101) as will all routers in the path between R1 and R3.
On R6 try pinging 224.101.101.101 which effectively makes R6 a source for the 224.101.101.101 group.
R3 should now have a (S,G) entry (assume R6 interface is 155.1.146.6) the entry will be (155,1,146,6, 224.101.101.101).
You could use the "ip igmp static-group" command but this will not respond t pings, however, traffic is still processed the normal way and you ought to be able to see the (*,G) and (S,G) entries associated with the static group.
8.1 - A 3560 requires "ip multicast-routing distributed" to be configured.
To test a configuration make an interface join a group and try to ping that group from another router. that interface must also be configured for PIM. What happens here is the interface acts as a host which is listening to a multicast group and the router that is pinging that group address is acting as the multicast source. Example:
int lo0
ip pim dense-mode
ip igmp join-group 224.10.10.10
8.2 If the RPF check fails (i.e. the IGP prefers a path to a source via a different interface, maybe because PIM is not enabled on that interface) then you could always add a static route to the mroute table.
ip mroute 155.1.146.0 255.255.255.0 155.1.0.4
In this example on R5, the IGP preferred a route in EIGRP via the point-to-point serial link to R4. However, PIM was not enabled on that interface but instead on the frame Relay cloud between R4 and R5. Hence, multicast traffic was attempting to go over the Frame Relay cloud but was failing the RPF check (e.g. 155.1.146.6 is meant to be reachable via the point-to-point link and not the Frame Realy cloud).
To change how often a router checks RPF use the "ip multicast rpf interval" command.
To configure a router to perform triggered RPF checks after a topology change and specify a maximum delay use "ip multicast rpf backoff
8.3 - There is a nother RPF failure but that is because R5's Lo0 interface was being advertised as a /32 by OSPF and therefore was the longest match when compared to the /24 advertised by EIGRP. By changing the OSPF network type to point-to-point under the Lo0 interface of R5, the RPF check passed and traffic flowed.
To specify a traffic limit when the Shortest Path Tree (SPT) is used (switched over to) use the "ip pim spt-threshold" command.
8.4 - note that Dense Mode uses (*,G) tuples and Sparse Mode uses (S,G) tuples. to specify which groups an RP can be an RP for, create an access-list and apply that to the rp-address command:
ip access-list standard ACL_ALLOWED_GROUPS
permit 224.0.0.0 0.0.0.255
!
ip pim rp-address 150.1.5.5 ACL_ALLOWED_GROUPS
8.5 - the PIM Assert message tells a shared segment which router should be forwarding on that segment for that particular group. The tiebreak on this is the administrative distance used to reach the source. So, for example, you had a device using EIGRP and one using OSPF, the one using EIGRP would win the assertion due to the lower AD (90 versus 110). To change this you would need to alter the AD of the routing protocol.
8.6 - The Accept RP tells a router to only accept specific groups from trying to join an RP.
For example, if R5 Lo0 is the RP (150.1.5.5) and you only wanted groups 224.10.10.10 and 224.110.110.110 to be able to reach that RP use the following:
ip access-list standard ACL_ACCEPT_RP
permit 224.10.10.10
permit 224.110.110.110
!
ip pim accept-rp 150.1.5.5 ACL_ACCEPT_RP
8.7 - The PIM DR Election process is pre-emptive, unlike OSPF DR election. The winner is the router with the highest priority (default of 1) and if they are tied then the router with the highest IP address will be the PIM DR.
ip pim dr-priority
The "show ip pim interface
8.8. The PIM Accept Register is configured on a RP and tells the RP which sources can register with it. If a source tries to register with the RP that is not on the register you will see Register-Stop messages (assuming "debug ip pim" is configured).
8.9 Multicast tunnelling is straightforward. Try using ip unnumbered on the tunnel as its IP address. Must ensure PIM is configured on tunnel interface and make the tunnel interface passive under the routing process. May also need a static mroute to ensure that the RPF checks do not fail.
Posted byChris Bloomfield at 16:59 0 comments
Labels: CCIE, ine, multicast, pim, rpf, shared tree, source tree, spt
CCIE - Warm-Up Phase - BGP
Tuesday, 26 April 2011
Hi,
I am pretty happy with BGP but still there are a few things that I need to get into my head.
7.5 - Disable Connected Check - this allows eBGP peers to use an IP address to peer that is not directly connected to the neighbor (e.g. a loopback interface). The difference between this and ebgp-multihop is that the TTL is not adjusted for disable-connected-check and that eBGP sessions will not form over transit routers.
7.8 - The bgp cluster-id is applied to the route reflector only.
7.9 - The confederation AS is used in the "router bgp" command and the bgp-confederation identifier refers to the proper AS. You must also specify bgp confederation peers otherwise the adjacency won't come up. In addition, confederation peers are treated as eBGP neighbors and should use directly connected interfaces, otherwise need to use things like update-source and ebgp-multihop.
7.11 - Instead of using next-hop-self you can create a route-map to "set ip next-hop" and apply that to the neighbor (e.g. neighbor 1.2.3.4 route-map ROUTE_MAP_NAME in/out"
7.12 - iBGP synchronization - Do not advertise a BGP route unless it is also learned by the IGP running in the network. In essence this requires the edge router to redistribute the eBGP routes into the IGP in order for iBGP to advertise the routes. Be careful when redistributing routes into IGP. Use an AS PATH access-list:
ip as-path access-list 1 permit ^_54
!
route-map MATCH_AS_PATH permit 10
match as-path 1
!
router eigrp 100
redistribute bgp 100 metric 100000 1000 255 1 1500 route-map MATCH_AS_PATH
Posted byChris Bloomfield at 08:52 0 comments
CCIE - Warm-Up Phase - OSPF
Wednesday, 20 April 2011
This is in relation to INE Workbook 1 and their 48 week program to get up to speed with passing the CCIE exam.
What have I learned?
6.1 - A network statement of "network 0.0.0.0 0.0.0.0 area 0" gets changed to "network 0.0.0.0 255.255.255.255 area 0" which is basically a single network statement that covers all interfaces.
6.2 - By default Frame Relay defaults to an OSPF network type of non-broadcast. This means that a DR/BDR is elected, however, hellos are sent as UNICASTS therefore you must configure static neighbor commands. This needs to be done only on one end of the link but it may be best practice to configure it at both ends for clarity. In addition, as a DR is elected it does not change the next-hop value when it sends out Type 2 LSAs hence spoke routers need a path to the next-hop value via the PVC to the hub (e.g. frame-relay map ip 155.1.0.4 105 -- this will send traffic to 155.1.0.4 via DLCI 105 which is the PVC to R5 which is the hub for this network).
6.4 - Point-to-point uses multicast hellos and no need for DR/BDR election or neighbor statements.
6.5 - Point-to-multipoint. As this is over Frame Relay and the initial Frame Relay maps did not include the "broadcast" keyword no adjacencies formed. Point-to-multipoint uses multicast hellos so you have to specify the "broadcast" keyword. There are also no DR/BDR elections so the hub (R5 in this case) changes the next-hop to itself (155.1.0.5). As each router (R1 to R4) has a Frame Relay map to R5 full-reachability is made.
6.6 - Point-to-multipoint non-broadcast. The only difference here is that it sends hellos as unicast and requires a neighbor statement.
6.7 - Changing the network type on Loopback interfaces to point-to-point advertises the network with its correct mask (in this example /24) rather than the default Loopback type which advertises them as a stub with a /32 even if they are not configured as /32.
6.8 - The reference bandwidth is configured in Mbps.
6.10 - OSPF Path selection with Bandwidth - Change the bandwidth on an interface that RECEIVES the LSA and that is downstream from the device you wish to be affected (e.g. this example requires R6 to route via R1 so change the bandwidth value on R1 Se0/0 to a greater value to affect the overall cost. Note that R1, R4, and R6 share an Ethernet segment)
6.11 - Changing the cost per neighbor. To find out what cost OSPF would assign a bandwidth value change the bandwidth on an interface and use "show ip ospf interface
6.12 - If a router connects two areas together and neither of those areas are Area 0 then the ABR will not act as an ordinary ABR and will not forward Inter-Area routes as all Inter_Area routes must be advertised via Area 0. Therefore you need to use a Virtual Link.
6.13 - OSPF Path Selection with Non-Backbone Transit Areas. After the virtual link was formed and I adjusted some costs on R4, routes to SW2 were being routed via Area 1 (specifically, via R4) and not to R1 even though that link is effectively Area 0 as it is a virtual link. To allow the use of the virtual link you must use "no capability transit" command at BOTH ends of the virtual link.
6.14 - OSPF virtual links will not come up if the cost used to reach the other end of the virtual link is 65535 which is the maximum cost associated with an interface. You must therefore change the OSPF cost at BOTH ENDS of the virtual link.
Posted byChris Bloomfield at 16:58 0 comments
Labels: CCIE, ine, ine workbook 1, ospf, virtual link
RIP - Adjusting default timers
Saturday, 9 April 2011
In my last post about RIP I mentioned quite a few different bits of RIP. Here I just want to focus on changing the timers and how to override the timers.
You can adjust your timers using the timers basic update invalid holddown flush under the routing process:
router rip
version 2
timers basic 30 90 120 120
This will set the update to 30 secs, invalid to 90 secs, holddown to 120 secs, and flush to 120 secs.
What would happen if you need to change that behaviour on just one link? Well, we can configure the update timer under the interface:
interface fa0/0
ip rip advertise 30
This then changes the advertise time to 30 seconds but note that this doesn't change the other timers, only the advertise time!
Posted byChris Bloomfield at 07:56 0 comments
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
