Tag: path

Cisco: How to configure HSRP for load-balancing traffic

I believe many of you are already familiar with the Hot Standby Router Protocol (HSRP), but just for the one that are not I will make a short review of this protocol.
Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway, and has been described in detail in RFC 2281. The Virtual Router Redundancy Protocol (VRRP) is a standards-based alternative to HSRP defined in IETF standard RFC 3768. The two technologies are similar in concept, but not compatible.

The protocol establishes a framework between network routers in order to achieve default gateway failover if the primary gateway should become inaccessible, in close association with a rapid-converging routing protocol like EIGRP or OSPF. HSRP sends its hello messages to the multicast address 224.0.0.2 (all routers) using UDP port 1985, to other HSRP-enabled routers, defining priority between the routers. The primary router with the highest configured priority will act as a virtual router with a pre-defined gateway IP and will respond to the ARP request from machines connected to the LAN with the mac address 0000.0c07.acXX where XX is the group ID. By sharing an IP address and a MAC (Layer 2) address, two or more routers can act as a single “virtual” router. The members of the virtual router group continually exchange status messages. This way, one router can assume the routing responsibility of another, should it go out of commission for either planned or unplanned reasons. Hosts continue to forward IP packets to a consistent IP and MAC address, and the changeover of devices doing the routing is transparent. If the primary router should fail, the router with the next-highest priority would take over the gateway IP and answer ARP requests with the same mac address, thus achieving transparent default gateway fail-over.

HSRP and VRRP on some routers have the ability to trigger a failover if one or more interfaces on the router go down. This can be useful for dual branch routers each with a single serial link back to the head end. If the serial link of the primary router goes down, you would want the backup router to take over the primary functionality and thus retain connectivity to the head end.

Now, as you probably know already, HSRP is not supporting by default load-balancing, meaning that only one router can be active in the virtual router group, and only that path is used for traffic leaving the other paths unused. In this way there is a waste on bandwidth, as only one router is used to forward traffic. In normal cases, I would recommend to use another protocol named Gateway Load Balancing Protocol (GLBP), that perform the same operation as HSRP with the additional load balance feature. Anyway since we are not talking about GLBP here, and load balance with HSRP can be a subject for some Cisco exams, read below how you can achieve this feature.

First please have a look at the topology used for this example. This will make things more clear for you. As you can see R1 and R2 are connected to the same network segment, so they can share the same subnet. Let configure R1 and R2 for a basic HSRP (without load balancing):

R1
interface FastEthernet0/0
ip address 10.10.12.1 255.255.255.0
standby 1 preempt
standby 1 ip 10.10.12.3
standby 1 priority 110

R2
interface FastEthernet0/0
ip address 10.10.12.2 255.255.255.0
standby 1 preempt
standby 1 ip 10.10.12.3

R1 is the active router for group 1 (priority 110, default 100), so all the traffic will flow through R1’s path. Following I will apply the configuration to migrate this default HSRP to Multigroup HSRP (MHSRP) which is load balance aware:

R1
interface FastEthernet0/0
ip address 10.10.12.1 255.255.255.0
standby 1 preempt
standby 1 ip 10.10.12.3
standby 1 priority 110
standby 2 preempt
standby 2 ip 10.10.12.4

R2
interface FastEthernet0/0
ip address 10.10.12.2 255.255.255.0
standby 1 preempt
standby 1 ip 10.10.12.3
standby 2 preempt
standby 2 ip 10.10.12.4
standby 2 priority 110

Now we have group 1 with R1 active (10.10.12.3) and group 2 with R2 active (10.10.12.4). Of course you will have to find a way to push to the clients the 2 gateways (10.10.12.3 and 10.10.12.4) or to configure them manually on your users machines, to really achieve the load balance feature with HSRP.

To see the live presentation of how MHSRP works please click on the image below:

Cisco HSRP

Files needed for this tutorial: The topology

Cisco: BGP path selection for inbound traffic

In some previous post we saw how we can manipulate BGP paths using attributes for outgoing traffic. Today we will see how to use another BGP attribute, but this time for manipulating inbound traffic. First I would like to ask you to have a look into the topology file and also to check the config files (if you have a basic idea about how BGP is configured that you don’t need the config files).

From the table above:
we see that the attributes needed for inbound traffic manipulation are AS-Path and MED. Today we will use AS-Path for traffic manipulation (MED sometime in closer future).

Beside using BGP attributes, there are other ways to manipulate traffic and paths in a BGP environment, but usually this need that the provider will support your actions. As an example to understand, you can do route tagging in your network (for example in MPLS on your CE) and your ISP will apply rules based on different tags (on PE side), but this is out of the scope of this tutorial and will be discussed maybe in another tutorial.

For our test environment, that you see in the topology, we advertise everything into BGP domain, exactly as draw, so there will be no problems of reachability.

Please see the tutorial and explanations below: