What’s Route Aggregation | How Route Matric Works?
Hello, I’m Sourav Khanna and welcome to our article on the introduction to routing concepts part two. Today I’m going to be talking about routing metrics and routing aggregation, and then I’m going to conclude with a brief discussion on high availability, we have a fair amount of ground to cover, but not a whole lot of time.
So let’s go ahead and begin the session. Of course, I’m going to begin by talking about routing metrics. It is quite common for there to be more than one route available to a remote network. Routing protocols use metrics to determine which route is the best route to reach those remote networks.
Each routing protocol will use its own set of metrics in determining which routes to which networks are placed in its routing table. The same basic metric may be used by different routing protocols. But when this occurs, the metric is usually implemented in a different manner through the use of different algorithms.
The first metric that we’re going to discuss is the hop count. The hop count is the number of routers between two endpoints. This is determined from the sending routers perspective, the maximum transmission unit, or MTU is another metric that is used by routing protocols.
Maximum Transmission Unit (MTU)
The MTU is the maximum allowed size of a packet measured in bytes that’s allowed through an interface. The standard MTU for Ethernet is 1500 bytes. packets that exceed the MTU must be fragmented into smaller pieces, leading to more packets leading to a slower connection.
Bandwidth is another common routing metric bandwidth is a measure of the speed of the network connection, the speed is commonly measured in either kilobit per second, megabits per second, or gigabits per second. Another common metric is latency.
Latency is a measure of time that a packet takes to traverse a link. When latency is implemented by routing protocols. The total amount of latency or delay to go between two points is what is used in the metric the administrative distance, or ad is probably the most important metric that’s used on routers.
The administrative distance is the believability of routing protocols advertised routes, different routing protocols are considered to be more believable, or trustworthy than others. routers use the ad to help determine which routing protocol to use when more than one protocol is installed on the router. The lowest ad of an advertised route will determine the protocol that’s used.
There are some common standard administrative distances. First up is the directly connected route. That’s a direct link between two routers that has an ad of zero in it is the most believable or trustworthy route.
The Statically Configured Route
It has an ad for one external. Border Gateway Protocol has an ad of 20. It’s still fairly trustworthy. Internal II II GRP has an ad of 90 it’s not as trustworthy as BGP, but it is more trustworthy than OSPF’s open shortest path first, which has an ad of 110. an ad of 115. So not quite as believable as OSPF but more believable than rip, which has an ad of 120.
External AIG RP has an ad of 170 in internal BGP, and I’ve never seen internal BGP use have an ad of 200. Now if you see an administrative distance of 255 that means that that route is not believable at all. As a side note, the ad can be set by an administrator.
So if you are running both OSPF and it is on a router But you want to be used you could actually set the OSPF ad to a higher number than it is and then it would always be used before OSPF. Now let’s move on to route aggregation. Without some mechanism put in place, routing tables would soon become very large and highly inefficient.
Through careful planning networks, administrators use a process called route aggregation to condense the size of routing tables, they do so through the use of classless inter-domain routing cider.
To summarize routes to different networks, route aggregation is common in networking. Let’s take a look at an example of Route aggregation.
Suppose we have a router that has the following networks on its serial zero slashes one interface. It has 10.1.1.0/24 known on that interface 10.1.17.0/24
10.1.32.0/24 and 10.1.1 28.0/24.
All of those networks are known to that interface that s/0/1 interface. These routes are what are known as contiguous routes, they’re all in line, and they can be summarized are aggregated by a common sider entry in the routing table. They could all be summarized by the following entry 10.1.0.0/16.
Now there is a warning about route aggregation. Route aggregation takes careful planning during the network design phase.
The above example would not work if the serial interface one slash one on that same router was connected to network 10.1.2.0/24. Because that new network makes those networks on the zero-slash one interface, non-contiguous networks, all the known networks are no longer all in a row.
This leads to the fact that the routes could no longer be aggregated or summarised. Let’s conclude with a discussion on high availability. part of a network administrator’s job is to ensure that networks remain up and active for the maximum amount of time. In an effort to ensure that networks don’t go down.
Administrators often remove single points of failure. A single point of failure in a network is the point where a single failure will cause the network to cease functioning. A network administrator often uses high availability techniques in order to remove those single points of failure.
An example of a high availability technique is the use of redundant links to outside networks. Hot standby router protocol HSRP is a specific example of a high-availability technique. HSRP is a proprietary Cisco method of creating a fault-tolerant link using two or more routers, with connections outside of the local subnet.
The two routers are connected together as well as having connections outside of the local network. A virtual IP address is created and shared between the two routers. Devices on the network are configured to use that virtual IP address as their default gateway for packets leaving the network. If a single router goes down, the link outside of the network is still available.
Another high-availability technique is the virtual router Redundancy Protocol VRRP. It is an IETF Internet Engineering Task Force standard that is similar in operation to HSRP.
That concludes this article on the introduction to routing concepts. In part Two, I discussed some routing metrics. Then I moved on to route aggregation. And I concluded with a brief discussion on high availability.