1. Introduction to Networking
Routing Optimization

2. Administrative Distance
When optimizing network routing, we may need a way to compare routes given to us by different routing protocols
Just like when we have redundant paths, we will assign a value and select the lowest one. This value is called the administrative distance. It can be conceptualized as the trustworthiness of the protocols
The administrative distance primarily uses the protocols metric to make its decision, though it is important to note that we can modify the default administrative distance values

3. Administrative Distance Table
The lowest value is a connected interface. In other words, if we are directly connected to a network, that always takes priority
The next priority is a static route. The router assumes that if we went in and set the route that we know this is the best route
After this the routes will be ordered by how well the protocol (typically) choses routes

4. Other Administrative Distance Info
The highest possible administrative distance value is 255, which the router doesn't like and will consider unusable
Be aware that it's really unlikely that a single organization is going to have multiple routing protocols running within the same network. Most of the time the organization will set all of the routers to one protocol. There are however extenuating circumstance - like if two companies want to merge their networks to exchange information easier

5. Route Re-distribution
Using multiple protocols managed by administrative distance is one way, but there is an alternative when merging two networks
We can install a border router between the two networks. This router would take all of the route information and convert it
For example, let’s say that we install a border router connecting an OSPF network and an RIP network. The router takes all the routes that are learned by OSPF on one network and advertises them as RIP routes on the other network and takes the routes learned by RIP on the other network and advertises them on the other side as OSPF routes
This process is called route re-distribution

6. Route Summarization / Aggregation
Route summarization is a technique for representing a collection of many routes with a single summary route in a router's routing table to make routing more efficient
In essence we would group together networks based on their subnets, and advertise the group as one subnet
For example, if I had networks /24, /24, /24, /24 on one router (router A), and /24, /24, /24, /24 on another router (router B) we can set up routing to say the /16 network routes to router A, and the /16 network routers to router B

7. Benefits of Route Summarization
Since we are consolidating routes, this makes the routing table shorter which will could show an improvement in message speed
However the biggest benefit will be in route sharing. Since the routing table is shorter, sharing it will be quicker and convergence will be achieved faster, no matter what sharing method is used
We must retain all necessary routing information, so all networks are still reachable after summarization. In other words, we can’t drop networks or misroute them

8. Automatic vs Manual Route Summarization
With automatic summarization, the router identifies adjacent networks and calculates the summarized route
RIP (version 1 and version 2) and EIGRP support auto-summarization; OSPF does not
Manual
With manual summarization, an administrator identifies the summarized route to advertise. The specified route includes the summarized subnet address with the subnet mask that includes all summarized subnets
Automatic summarization sends route summaries along class boundaries on a network of a different classful network only when advertising those routes. For example, if I were connecting the /24 and /24 to /24 and /24, we couldn’t automatically summarize to /16 and /16, since the classful address ( /8) is shared between the networks

9. FHRP
When connecting a private network to a public network we may want to implement some redundancy. If we only have one outbound default gateway, that’s a single point of failure. If that goes down we can’t connect
First Hop Redundancy Protocol (FHRP) will allow us to create redundant default gateways for a network segment
Without this protocol, if we set up redundant routers then every time our default gateway went down, we would have to go into each individual host and manually reconfigure a different IP address for the default gateway
To get around doing this we can use the Hot Standby Router Protocol (HSRP), the Virtual Router Redundancy Protocol (VRRP) or the Gateway Load Balancing Protocol (GLBP)
Each of these are examples of FHRP

10. How FHRP Works
FHRP configures our redundant default gateways to share a virtual IP address, and in some cases the same virtual MAC address. When you configure the default gateway address on your network host, you're going to specify the virtual IP address created by FHRP
FHRP can then dynamically determine which routers the traffic should actually go to. Routers will periodically send messages to see if the current active default router is up, and if it is not, they will negotiate which router will now be the active default gateway. The new active default gateway will then use the same virtual IP as the previous. The network hosts have no idea that anything has happened when a failover occurs

11. How HSRP and VRRP Work
HSRP and VRRP are very similar. Both use an active standby model for redundant routing
Using HSRP, multiple routers are configured as default gateways, but only one is going to function as such at any point in time. This is the active router
The other routers are in a dormant state. These are the standby routers. They monitor the network for changes to see if the active router stops responding
When you're using HSRP, each default gateway will be assigned the same virtual IP address and the same virtual MAC address. The virtual IP must be a unique and valid IP address for the connected subnet
The virtual MAC address, is automatically assigned for you

12. What Happens When the Active Router Fails
The standby routers will send HSRP messages to make sure that the active router is still up
If the active router doesn’t respond, the routers will use these messages to negotiate a new active router
While this works great, and the hosts won’t notice due to the virtual IP address, the switches will need to know that the default gateway address changed. The new router has the old virtual IP and MAC address, but we need to update the switch’s CAM table
The new active router will send an ARP frame to the switches it is connected to. This will change the port/MAC address connection, and now we can use the new router

13. GLBP
Instead of using an active standby model, GLBP actually balances the load between multiple redundant default gateways, using an active/active model
When we're dealing with GLBP, each of the redundant routers is called a forwarder. With GLBP, all redundant routers are active at the same time.
With GLBP, all of our network hosts will use the virtual IP address as their default gateway router address but each router has its own unique virtual MAC address.
GLBP designates one router as the active virtual gateway (AVG)

14. AVG
The AVG responds to all ARP requests for the shared virtual IP address. Whenever an ARP request for the default gateway is received, it will respond with one router's virtual MAC address. The next request will get a different router’s virtual MAC address
By doing this, some network hosts going to send frames to the MAC address of one redundant routers, while others are going to send frames to the MAC address of another router
By doing this, we balance the load between our redundant gateway routers, eliminating single points of failure
If an ARP request arrives at the active router that is not an active virtual gateway, the ARP request will be ignored. Only the active virtual gateway will respond to ARP requests for the virtual IP address