State Less The Band Technology Site

Posts Tagged ‘cisco’

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.
Read the rest of this entry »

When you’re working on your BCMSN exam on your way to CCNP certification, you’ll read at length about how Cisco routers and multilayer switches can work to provide router redundancy – but there’s another helpful service, Server Load Balancing, that does the same for servers. While HSRP, VRRP, and CLBP all represent multiple physical routers to hosts as a single virtual router, SLB represents multiple physical servers to hosts as a single virtual server.

In the following example, three physical servers have been placed into the SRB group ServFarm. They’re represented to the hosts as the virtual server 210.1.1.14.

The hosts will seek to communicate with the server at 210.1.1.14, not knowing that they’re actually communicating with the routers in ServFarm. This allows quick cutover if one of the physical servers goes down, and also serves to hide the actual IP addresses of the servers in ServFarm.

The basic operations of SLB involves creating the server farm, followed by creating the virtual server. We’ll first add 210.1.1.11 to the server farm:

MLS(config)# ip slb serverfarm ServFarm

MLS(config-slb-sfarm)# real 210.1.1.11

MLS(config-slb-real)# inservice
Read the rest of this entry »

To pass the CCNP exams, you’ve got to master Quality of Service, and the first step in doing so is knowing the differences between the different QoS types.

Now this being Cisco, we can’t just have one kind of QoS! We’ve got best-effort delivery, Integrated Services, and Differentiated Services. Let’s take a quick look at all three.

Best-effort is just what it sounds like – routers and switches making their “best effort” to deliver data. This is considered QoS, but it’s kind of a “default QoS”. Best effort is strictly “first in, first out” (FIFO).

An entire path from Point A to Point B will be defined in advance when Integrated Services are in effect. Integrated Services is much like the High-Occupancy Vehicle lanes found in many larger cities. If your car has three or more people in it, you’re considered a “priority vehicle” and you can drive in a special lane with much less congestion than regular lanes. Integrated Services will create this lane in advance for “priority traffic”, and when that traffic comes along, the path already exists. Integrated Services uses the Resource Reservation Protocol (RSVP) to create these paths. RSVP guarantees a quality rate of service, since this “priority path” is created in advance. Read the rest of this entry »

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.
Read the rest of this entry »