Routing Summarization is a major factor in the success of designing your network. To ensure that your network can scale properly, route summarization is the biggest factor against which to measure your success. Without summarization, you have a flat address design with specific route information for every subnet being transmitted across the network—a bad thing in large networks.
The six time-proven steps to designing a network are as follows:
A dynamic routing protocol supports a routed protocol and maintains routing tables.
The most common use of static routes is in stub networks.
In Link-State routing protocols, each router sends only that portion of the routing table that describes the state of its own links.
Link-state protocols are based on the distributed map concept, which means that every router has a copy of the network map that is regularly updated.
The principle of link-state routing is that all the routers within an area maintain an identical copy of the network topology.
Link-state protocols such as OSPF flood all the routing information when they first become active in link-state packets. After the network converges, they send only small updates via link-state packets.
In OSPF, because each router knows the complete topology of the network, the use of the SPF algorithm creates an extremely fast convergence.
Sends updates to tables only, instead of entire tables, to routers.
Is a more economical routing protocol than RIP over time because it involves less network traffic.
During an external convergence event, OSPF could flood more traffic than RIP. Consider that RIP carries 25 routes per update; on the other hand, OSPF floods a single LSA per external route that is affected by the convergence event.
Distance vector means that information sent from router to router is based on an entry in a routing table that consists of the distance and vector to destination—distance being what it “costs” to get there and vector being the “direction” to get to the destination.
Call for each router to send its entire routing table, but only to its neighbors. The neighbor then forwards its entire routing table to its neighbors, and so.
The application layer essentially acts as the end-user interface. This is the layer where inter-action between the mail application (cc:Mail, MS Outlook, and so on) or communications package (SecureCRT for Telnet or FTP Voyager for FTP) and the user occurs.
The presentation layer is responsible for the agreement and translation of the communication format (syntax) between applications.
The session layer responsibilities range from managing the application layer’s transfer of information to the data transport portion of the OSI reference model. An example is Sun’s or Novell’s Remote Procedure Call (RPC), which uses Layer 5.
The transport layer is responsible for the logical transport mechanism, which includes functions conforming to the mechanisms characteristics. Provides a level of error checking and reliability (through sequence numbers) to the transmission of user data to the lower layers of the OSI reference model. This is the only layer that provides true source-to-destination, end-to-end connectivity through the use of routing protocols such as open shortest path first (OSPF) or the file transfer protocol (FTP) application as examples of TCP.
The most common usage of UDP is streaming media solutions, such as Real Audio.
The data link layer provides framing, error, and flow control across the network media being used. An important characteristic of this layer is that the information that is applied to it is used by devices to determine if the packet needs to be acted upon by this layer (that is, proceed to Layer 3 or discard). Serial interfaces do not normally require unique Layer 2 station addresses, such as MAC addresses, unless it is necessary to identify the receiving end in a multipoint network.
24 bits are dedicated for Organization Unique Identification (OUI) and 24 bits are for unique identification.
First 3 bytes of an Ethernet address are the company ID, and the last 3 bytes are assigned by the manufacturer.
Physical layer is responsible for defining information regarding the physical media, such as electrical, mechanical, and functional specifications to connect two systems.
The physical layer is composed of three main areas: wires, connectors, and encoding.
OSPF external routes are automatically blocked from being redistributed in BGP by default.
A solution to minimize Internet route instability is using Aggregation. Fluctuation of any single route in an Aggregation does not cause fluctuation in the Aggregate itself.
Backdoor routes offer an alternative IGP path instead of external BGP path. Using Backdoor for specific routes, cause the administrative distance to be equal to BGP Local (200), so the IGP with the lower AD will be preferred.
By default, MED is not compared when routes are learned from different ASs. This behavior could be changed using the bgp always-compare-med command. When the bgp deterministic-med command is enabled, routes from the same autonomous system are grouped together, and the best entries of each group are compared. (useful link) An example of BGP table looks like this:
entry1: AS(PATH) 100, med 200, external, rid 1.1.1.1
entry2: AS(PATH) 500, med 100, internal, rid 172.16.8.4
entry3: AS(PATH) 500, med 150, external, rid 172.16.13.1
+ bgp deterministic-med Enabled, bgp always-compare-med Disabled: There is a group for AS 100 and a group for AS 500. The best entries for each group are compared. Entry1 is the best of its group because it is the only route from AS 100. Entry2 is the best for AS 500 because it has the lowest MED. Next, entry1 is compared to entry2. Since the two entries are not from the same neighbor autonomous system, the MED is not considered in the comparison. The external BGP route wins over the internal BGP route, making entry1 the best route.
When passing an EBGP route to an IBGP neighbor, the EBGP neighbor is set as the next-hop.
In NMBA partial-mesh networks, sometimes the next-hop-self command is required.
When connecting to an Internet eBGP neighbor, AS_PATH list that contains Private ASs, should be stripped.
An AS that advertises an Aggregate, considers itself the originator of that route, irrespective of where that route came from. This issue can cause a loop, so the solution is to use the AS_SET parameter before an aggregate-address.
Sometimes, customers prepend fake ASes, to prevent becoming a transit AS for providers.
Combine route injection in BGP with static Routes (with distance 254, for example) to Null0 if you want to prevent route fluctuation even if your IGP routing is not stable.
In most of the situations, METRIC is used for inbound traffic management and LOCAL_PREFERENCE is used for outbound traffic administration.
BGP multipath can be used to install multiple paths in the IP routing table if the paths are learned via the same neighboring AS. The maximum-paths command can be used to install up to six paths to a single destination. The following attributes of parallel paths have to match with the best path:
Service Provider should filter some IP prefixes in incoming updates, such as RFC1918. Because a customer should only advertise its global networks to the Service Provider.
Multihomed Customers should avoid becoming a Transit-AS. As by default in most of the cases the tie breaker for BGP is the Shortest AS-Path, so the providers connected to the to customer will use the customer link as a Transit-AS to reach each other.
Service Providers should filter Private addresses in incoming updates of Customers.
In a scenario where a customer has two border routers without IBGP, and IGP inside the AS, there will be no loops, but if running IBGP between the border routers, special care should be taken or a direct link between the two border routers is required.
Policy Routing only affects the Next-Hop. The destination is unchanged!
Policy Routing is CPU intensive, because it is based on the source unlike Dynamic and static routing. So, when routing based on the destination there is no need of Policy Routing.
Customers can only affect their outgoing traffic, and can’t directly affect incoming traffic.
(config)# ip as-path access-list# [permit/deny] regexp
(config-router)# neighborip-addressfilter-listas-path-filter# [in/out]
