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Posts Tagged ‘bsci’

Like TCP, BGP is connection-oriented. An underlying connection between two BGP speakers is established before any routing information is exchanged. This connection takes place on TCP port 179. As with EIGRP and OSPF, keepalive messages are sent out by the BGP speakers in order to keep this relationship alive.

Once the connection is established, the BGP speakers exchange routes and synchronize their tables. After this initial exchange, a BGP speaker will only send further updates upon a change in the network topology.

The IGP protocols that use Autonomous Systems, IGRP and EIGRP, require prospective neighbors to be in the same AS. This is not true with BGP. Routers can be in different Autonomous Systems and still exchange routes. The BGP neighbors do not have to be directly connected, and often are not, but do need to be able to reach the IP addresses they use in their neighbor statements.

A BGP peer that is in the same AS is referred to as an Internal BGP (iBGP) Peer, where a BGP peer in another AS is an External BGP (eBGP) Peer.

A sample iBGP configuration:

Router bgp 100

Neighbor 10.1.1.2 remote-as 100

A sample eBGP configuration:

Router bgp 100

Neighbor 10.1.1.2 remote-as 200

Cisco recommends that eBGP peers be directly connected, where iBGP peers generally will not be.

Before we get too much farther into BGP theory, let’s get a configuration started. You’ll use the router bgp command to configure a router as a BGP speaker. Right after that, the neighbor command will be used to identify this BGP speaker’s potential neighbors. (The terms “peer” and “neighbor” are interchangeable in BGP, but it’s the neighbor statement that is used to statically define neighbors. BGP is not capable of discovering neighbors dynamically.)

R1(config-router)#neighbor 172.12.123.3 remote-as 200

While almost all of the neighbor options are just that — optional — you do have to specify the BGP AS of the remote router. BGP has no mechanism to dynamically discover neighbors. Remember, BGP speakers do not have to be in the same AS to become peers. To verify that the remote BGP speaker has become a peer, run show ip bgp neighbor.
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Passing the BSCI exam and earning your CCNP is all about knowing the details, and when it comes to EIGRP SIA routes, there are plenty of details to know. A quick check in a search engine for “troubleshoot SIA” will bring up quite a few matches. Troubleshooting SIA routes is very challengin in that there’s no one reason they occur.

View the EIGRP topology table with the show ip eigrp topology command, and you’ll see a code next to every successor and feasible successor. A popular misconception is that we want these routes to have an “A” next to them – so they’re active. That’s what we want, right? Active routes sound good, right?

Well, they sound good, but they’re not. If a route shows as Active in the EIGRP topology table, that means that DUAL is currently calculating that route, and it’s currently unusable. When a route is Passive (“P), that means it’s not being recalculated and it’s a usable route.

Generally, a route shown as Active is going to be there for a very short period of time by the time you repeat the command, hopefully that Active route has gone Passive. Sometimes that doesn’t happen, though, and the route becomes SIA – Stuck In Active.
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To earn your CCNP certification and pass the BCMSN exam, you’ve got to know what HSRP does and the many configurable options. While the operation of HSRP is quite simple (and covered in a previous tutorial), you also need to know how HSRP arrives at the MAC address for the virtual router – as well as how to configure a new MAC for this virtual router. This puts us in the unusual position of creating a physical address for a router that doesn’t exist!

The output of show standby for a two-router HSRP configuration is shown below.

R2#show standby

Ethernet0 – Group 5

Local state is Standby, priority 100

Hellotime 3 sec, holdtime 10 sec

Next hello sent in 0.776

Virtual IP address is 172.12.23.10 configured

Active router is 172.12.23.3, priority 100 expires in 9.568

Standby router is local

1 state changes, last state change 00:00:22

R3#show standby

Ethernet0 – Group 5

Local state is Active, priority 100

Hellotime 3 sec, holdtime 10 sec

Next hello sent in 2.592

Virtual IP address is 172.12.23.10 configured

Active router is local

Standby router is 172.12.23.2 expires in 8.020

Virtual mac address is 0000.0c07.ac05

2 state changes, last state change 00:02:08

R3 is in Active state, while R2 is in Standby. The hosts are using the 172.12.123.10 address as their gateway, but R3 is actually handling the workload. R2 will take over if R3 becomes unavailable.
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CCNP exam success, particularly on the BSCI exam, demands you understand the details of route summarization. This skill not only requires that you have a comfort level with binary conversions, but you have to know how and where to apply route summarization with each individual protocol.

You also have to know the “side effects” of route summarization. With OSPF, there will actually be an extra interface created at the point of summarization, and this catches a lot of CCNP candidates by surprise. Let’s take a look at the null0 interface and how it relates to OSPF summarization.

On R1, the following networks are redistributed into OSPF, and then summarized.

interface Loopback16

ip address 16.16.16.16 255.0.0.0

interface Loopback17

ip address 17.17.17.17 255.0.0.0

interface Loopback18

ip address 18.18.18.18 255.0.0.0

interface Loopback19

ip address 19.19.19.19 255.0.0.0

R1(config)#router ospf 1 Read the rest of this entry »