Virtual Links – Cisco 300-420 Certification Exams – Routing Protocol Characteristics, EIGRP, and IS-IS

11Sep, 2024

IS-IS Operation and Design – Routing Protocol Characteristics, EIGRP, and IS-IS

This section discusses IS-IS areas, designated routers, authentication, and the Network Entity Title (NET). IS-IS defines areas differently from OSPF; area boundaries are links and not routers. IS-IS has no BDRs. Because IS-IS is an OSI protocol, it uses a NET to identify each router.

IS-IS NET Addressing

Although you can configure IS-IS to route IP, the communication between routers uses OSI PDUs. The NET is the OSI address used for each router to communicate with OSI PDUs. A NET address ranges from 8 to 20 bytes in length and is hexadecimal. It consists of an authority and format identifier (AFI), area ID, system ID, and selector (SEL), as shown in Figure 3-10. The system ID must be unique within the network. An example of an IS-IS NET is 49.0001.1290.6600.1001.00, which consists of the following parts:

Figure 3-10 IS-IS NET

AFI: 49
Area ID: 0001
System ID: 1290.6600.1001
SEL: 00

Level 2 routers use the area ID. The system ID must be the same length for all routers in an area. For Cisco routers, it must be 6 bytes in length. Usually, a router MAC address identifies each unique router. The SEL is configured as 00. You configure the NET with the router isis command. In this example, the domain authority and format identifier (AFI) is 49, the area is 0001, the system ID is 00aa.0101.0001, and the SEL is 00:

router isis
net 49.0001.00aa.0101.0001.00

IS-IS DRs

As with OSPF, IS-IS selects DRs on multiaccess networks. It does not choose a backup DR, as OSPF does. By default, the priority value is 64. You can change the priority value to a value from 0 to 127. If you set the priority to 0, the router is not eligible to become a DR for that network. IS-IS uses the highest system ID to select the DR if there is a tie with the priorities. On point-to-point networks, the priority is 0 because no DR is elected. In IS-IS, all routers in a multiaccess network establish adjacencies with all others in the subnetwork, and IS-IS neighbors become adjacent upon the discovery of one another. Both of these characteristics are different from OSPF behavior.

IS-IS Interface Types

IS-IS has only two network types (or interface types): point-to-point and broadcast. Unlike OSPF, IS-IS does not have nonbroadcast and point-to-multipoint interface types.

IS-IS Area Design

IS-IS uses a two-level hierarchy. Routers are configured to route Level 1 (L1), Level 2 (L2), or both Level 1 and Level 2 (L1/L2). Level 1 routers are like OSPF internal routers in a Cisco totally stubby area. (OSPF is covered in Chapter 4.) An L2 router is similar to an OSPF backbone router. A router that has both Level 1 and 2 routes is similar to an OSPF area border router (ABR). IS-IS does not define a backbone area like OSPF’s area 0, but you can consider the IS-IS backbone a continuous path of adjacencies among Level 2 ISs. When you are creating a backbone, there should never be Level 1 routers between L2-only or L1/L2 routers.

L1/L2 routers maintain separate link-state databases for the L1 routes and for the L2 routes. Also, the L1/L2 routers do not advertise L2 routes to the L1 area. L1 routers do not have information about destinations outside the area and use L1 routes to the L1/L2 routers to reach outside destinations.

As shown in Figure 3-11, IS-IS areas are not bounded by the L1/L2 routers but by the links between L1/L2 routers and L2 backbone routers.

Figure 3-11 IS-IS Areas and Router Types

With IS-IS, you can create three types of topologies:

Flat
Hierarchical
Hybrid

The IS-IS flat design consists of a single area. As shown in Figure 3-12, all routers are on the same level. There is no summarization with this design. This design is recommended for a small network.

Figure 3-12 IS-IS Flat Topology Design

An IS-IS hierarchical topology is recommended for large networks. A backbone area can be created, much as with OSPF, and can have other areas connected via L1/L2 routers, as shown in Figure 3-13. This design allows for network summarization.

Figure 3-13 IS-IS Hierarchical Topology Design

A hybrid IS-IS design is recommended for networks that might not be large enough for a backbone area. As shown in Figure 3-14, the IS-IS areas are connected using L1/L2 routers. This design allows for summarization between areas.

Figure 3-14 IS-IS Hybrid Topology Design

20Mar, 2024

OSPFv2 Areas – OSPF, BGP, and Route Manipulation

As a network grows, the initial flooding and database maintenance of LSAs can burden a router’s CPU. OSPF uses areas to reduce these effects. An area is a logical grouping of routers and links that divides the network. Routers share link-state information with only the routers in their areas. This setup reduces the size of the database and the cost of computing the SPF tree at each router.

Using a topology with multiple areas provides the following benefits:

The segmentation of the network reduces the number of SFP tree calculations.
The segmentation of the network reduces the amount of LSA flooding.
Multi-area design allows for summarization at the area border routers (ABRs).
One OSPF area hides the topology from another area.

Each area is assigned a 32-bit integer number. Area 0 (or 0.0.0.0) is reserved for the backbone area. Every OSPF network should have a backbone area. The backbone area must exist in any internetwork using OSPF as a routing protocol over multiple areas. As you can see in Figure 4-2, communication between Area 1 and Area 2 must flow through Area 0. This communication can be internal to a single router that has interfaces directly connected to Areas 0, 1, and 2.

Figure 4-2 OSPF Areas

Intra-area traffic is packets passed between routers in a single area.

OSPF Area Design Considerations

A CCNP enterprise designer should be aware of a few considerations in the design of OSPF areas. First, in a hub-and-spoke design, you have a remote branch keep the OSPF boundary at the hub side, as shown in Figure 4-3. This allows the branch router to calculate SPF only within its own area and limits the LSA flooding. If the OSPF Area 0 boundary were extended to the branch, the branch router would have to do OSPF calculations for Area 0 and its own area, and LSAs would flood over the WAN link.

Figure 4-3 OSPF Area Design

The second design consideration is to avoid grouping remote branches into a single area. Having all remote branches in the same area is not scalable. Instead, place each remote branch in its own area to limit LSA flooding and SPF recalculations.

20Aug, 2023

Virtual Links – OSPF, BGP, and Route Manipulation

OSPF requires that all areas be connected to a backbone router. Sometimes, WAN link provisioning or failures can prevent an OSPF area from being directly connected to a backbone router. You can use virtual links to temporarily connect (virtually) an area to the backbone.

In Figure 4-7, Area 4 is not directly connected to the backbone. A virtual link is configured between Router A and Router B. The flow of the virtual link is unidirectional and must be configured in each router of the link. Area 2 becomes the transit area through which the virtual link is configured. Traffic between Areas 2 and 4 does not flow directly to Router B. Instead, the traffic must flow to Router A to reach Area 0 and then pass through the virtual link.

Figure 4-7 OSPF Virtual Link

OSPFv2 Router Authentication

OSPFv2 supports the authentication of routes using 64-bit plaintext, cryptographic Message Digest 5 (MD5), and Secure Hash (SHA) Standard authentication. Authentication can be performed on a per-area or per-interface basis. Plaintext authentication passwords do not need to be the same for the routers throughout the area, but they must be the same between neighbors.

MD5 authentication provides higher security than plaintext authentication. As with plaintext authentication, passwords do not have to be the same throughout an area, but they do need to be the same between neighbors. SHA-1 authentication is recommended because it is the most secure.

OSPF supports the National Institute of Standards and Technology (NIST) Secure Hash Standard family of algorithms for authentication.

OSPFv2 Summary

OSPFv2 is used in large enterprise IPv4 networks. The network topology must be hierarchical. OSPF is used in the enterprise campus building access, distribution, and core layers. OSPF is also used in the enterprise data center, WAN/MAN, and branch offices.

The characteristics of OSPFv2 follow:

Is a link-state routing protocol
Uses IP protocol 89
Is a classless protocol (supports VLSM and CIDR)
Uses cost as the metric (based on interface bandwidth, by default)
Provides fast convergence, using link-state updates and SPF calculation
Uses reduced bandwidth thanks to sending LSA updates only when changes occur
Labels routes as intra-area, interarea, external Type 1, or external Type 2
Supports authentication
Uses the Dijkstra algorithm to calculate the SPF tree
Has a default administrative distance of 110
Uses multicast address 224.0.0.5 (ALLSPFRouters) and multicast address 224.0.0.6 (ALLDRouters)
Provides good scalability and recommended for large networks

2Feb, 2023

BGP – OSPF, BGP, and Route Manipulation

This section covers Border Gateway Protocol (BGP) theory and design concepts. The current version of BGP, version 4, is defined in RFC 4271. BGP is an interdomain routing protocol, which means it is used to exchange routing information between autonomous systems. (It is used for inter-autonomous system routing.) The primary function of BGP is to provide and exchange network-reachability information between domains or autonomous systems. BGP is a path vector protocol. BGP is best suited for setting routing policies between autonomous systems. In the enterprise campus architecture, BGP is used for Internet connectivity.

BGP is the de facto standard for routing between service providers on the Internet because of its rich features. You can also use it to exchange routes in large internal networks. The Internet Assigned Numbers Authority (IANA) reserved TCP port 179 to identify BGP. BGPv4 was created to provide CIDR, a feature that was not present in the earlier versions of BGP. BGP is a path vector routing protocol; it is neither a distance vector nor link-state routing protocol.

Note

RFC 1519 describes CIDR, which provides the capability to forward packets based on IP prefixes only, with no concern for IP address class boundaries. CIDR was created as a means to constrain the growth of the routing tables in the Internet core through the summarization of IP addresses across network class boundaries. The early 1990s saw an increase in the growth of Internet routing tables and a reduction in Class B address space. CIDR provides a way for service providers to assign address blocks smaller than a Class B network but larger than a Class C network.

BGP Neighbors

BGP is usually configured between two directly connected routers that belong to different autonomous systems. Each autonomous system is under different technical administration. BGP is frequently used to connect the enterprise to service providers and to interconnect service providers, as shown in Figure 4-9. The routing protocol within the enterprise could be any interior gateway protocol (IGP). Common IGP choices include RIPv2, EIGRP, OSPF, and IS-IS. BGPv4 is the only deployed exterior gateway protocol (EGP).

Figure 4-9 BGP Neighbors

BGP is an interdomain routing protocol that allows BGP speakers residing in different autonomous systems to exchange routing network layer reachability information. An autonomous system is a collection of devices under common administration. BGP autonomous systems range from 1 through 65,535. Autonomous system numbers (ASNs) 1 through 64,511 are considered public ASNs. These are allocated by IANA to Regional Internet Registries (RIR). Entities wanting to receive an ASN must complete the application process with the local RIR and be approved before being assigned an ASN. ASNs 64,512 through 65,535 are considered private ASNs. These ASNs can be used by any organization, but, like RFC 1918 addresses, they cannot be used on the Internet.

Before two BGP routers can exchange routing updates, they must become established neighbors. After BGP routers establish a TCP connection, exchange information, and accept the information, they become established neighbors and start exchanging routing updates. If the neighbors do not reach an established state, they do not exchange BGP updates. The information exchanged before the neighbors are established includes the BGP version number, ASN, BGP router ID, and BGP capabilities.

15Oct, 2022

Route Reflectors – OSPF, BGP, and Route Manipulation

iBGP requires that all routers be configured to establish a logical connection with all other iBGP routers. The logical connection is a TCP link between all iBGP-speaking routers. The routers in each TCP link become BGP peers. In large networks, the number of iBGP-meshed peers can become very large. Network administrators can use route reflectors to reduce the number of required mesh links between iBGP peers. Some routers are selected to become the route reflectors to serve several other routers that act as route-reflector clients. Route reflectors allow a router to advertise or reflect routes to clients. The route reflector and its clients form a cluster. All client routers in the cluster peer with the route reflectors within the cluster. The route reflectors also peer with all other route reflectors in the internetwork. A cluster can have more than one route reflector.

In Figure 4-12, without route reflectors, all iBGP routers are configured in an iBGP mesh, as required by the protocol. When Routers A and G become route reflectors, they peer with Routers C and D; Router B becomes a route reflector for Routers E and F. Routers A, B, and G peer among each other.

Figure 4-12 Route Reflectors

Routers A and G are configured to peer with each other and with Routers B, C, and D. The configuration of Routers C and D is different from the configuration of the rest of the routers; they are configured to peer with Routers A and G only. All route reflectors in the same cluster must have the same cluster ID number.

Note

The combination of the route reflector and its clients is called a cluster. In Figure 4-12, Routers A, G, C, and D form a cluster. Routers B, E, and F form another cluster.

Router B is the route reflector for the second cluster. Router B peers with Routers A and G and with Routers E and F in its cluster. Routers E and F are route-reflector clients and peer only with Router B. If Router B goes down, the cluster on the right goes down because no second route reflector is configured.

1May, 2022

Next-Hop Attribute – OSPF, BGP, and Route Manipulation

The next-hop attribute is the IP address of the next IP hop that will be used to reach the destination. The next-hop attribute is a well-known mandatory attribute.

Local Preference Attribute

The local preference attribute indicates which path to use to exit the autonomous system. It is a well-known discretionary attribute used between iBGP peers and is not passed on to external BGP peers. In Cisco IOS software, the default local preference is 100. The higher local preference is preferred.

The default local preference is configured on the BGP router with an external path; it then advertises its local preference to internal iBGP peers. Figure 4-14 shows an example of the local preference attribute where Routers B and C are configured with different local preference values. Router A and other iBGP routers then receive routes from both Router B and Router C. Between the two possible paths (shown with arrows), Router A prefers using Router C to route Internet packets because it has a higher local preference (400) than Router B (300).

Figure 4-14 BGP Local Preference

Origin Attribute

Origin is a well-known mandatory attribute that defines the source of the path information. Do not confuse the origin with comparing whether a route is external (eBGP) or internal (iBGP). The origin attribute is received from the source BGP router. There are three types:

IGP: Indicated by an i in the BGP table. Present when the route is learned by way of the network statement.
EGP: Indicated by an e in the BGP table. Learned from an EGP peer.
Incomplete: Indicated by a question mark (?) in the BGP table. Learned from redistribution of the route.

In terms of choosing a route based on origin, BGP prefers routes that have been verified by an IGP over routes that have been learned from EGP peers, and BGP prefers routes learned from eBGP peers over incomplete paths.

Autonomous System Path (AS_Path) Attribute

The autonomous system path is a well-known mandatory attribute that contains a list of ASNs in the path to the destination. Each autonomous system prepends its own ASN to the autonomous system path. The autonomous system path describes all the autonomous systems a packet would have to travel to reach the destination IP network. It is used to ensure that the path is loop free. When the AS_Path attribute is used to select a path, the route with the fewest autonomous system hops is preferred. In the case of a tie, other attributes, such as MED, break the tie. Example 4-1 shows the autonomous system path for network 200.50.32.0/19. To reach the destination, a packet must pass autonomous systems 3561, 7004, and 7418. The command show ip bgp 200.50.32.0 displays the autonomous system path information.

Example 4-1 Autonomous System Path Attribute

Click here to view code image Router#
show ip bgp 200.50.32.0

BGP routing table entry for 200.50.32.0/19, version 93313535
Paths: (1 available, best #1)
Not advertised to any peer
3561 7004 7418
   206.24.241.181 (metric 490201) from 165.117.1.219 (165.117.1.219)
    Origin IGP, metric 4294967294, localpref 100, valid, internal, best
    Community: 2548:182 2548:337 2548:666 3706:153

10Jan, 2022

Weight Attribute – OSPF, BGP, and Route Manipulation

Weight is assigned locally on a router to specify a preferred path if multiple paths exist out of a router for a destination. Weights can be applied to individual routes or to all routes received from a peer. Weight is specific to Cisco routers and is not propagated to other routers. The weight value ranges from 0 to 65,535. Routes with a higher weight are preferred when multiple routes to a destination exist. Routes that are originated by the local router have a default weight of 32,768. The default weight for learned routes is 0.

You can use weight rather than local preference to influence the selected path to external BGP peers. The difference is that weight is configured locally and is not exchanged in BGP updates. On the other hand, the local preference attribute is exchanged between iBGP peers and is configured at the gateway router.

When the same destinations are advertised from both Router B and Router C, as shown in Figure 4-16, Router A prefers the routes from Router C over those from Router B because the routes received from Router C have a larger weight (600) locally assigned.

Figure 4-16 BGP Weight

Table 4-7 lists and describes the BGP attributes and their categories.

Table 4-7 BGP Attributes

BGP Attribute	Description	Category
Origin	Indicates the source of the path information: IGP, EGP, or incomplete.	Well-known mandatory
AS_Path	Lists the ASNs in the path to the destination.	Well-known mandatory
Next hop	Specifies the IP address of the router as the next hop to the destination.	Well-known mandatory
Local preference	Specifies the path to use to exit the AS.	Well-known discretionary
MED	Tells an external BGP peer the preferred path into the AS.	Optional non-transitive
Community	Groups routes and applies policies or decisions (accept, prefer) to those routes. (Not an attribute used in the routing-decision process.)	Optional transitive
Atomic aggregate	Informs BGP peers that the local router used a less specific (aggregated) route to a destination instead of using a more specific route.	Well-known discretionary
Weight	Specifies a preferred path if multiple paths exist out of a router for a destination. Assigned locally on a router.	Optional (Cisco specific)

11Sep, 2021

BGP Route Manipulation and Load Balancing – OSPF, BGP, and Route Manipulation

By default, BGP chooses a single path to a destination. With thousands of routes, this choice can cause uneven traffic patterns. When you are working with BGP, there are different methods to control traffic paths and handle load balancing of traffic. Furthermore, outbound and inbound traffic flows are independent of each other.

For incoming traffic, the following methods can be used to manipulate traffic:

You can advertise a subset of the network via ISP 1 and another via ISP 2. For example, if you have a /22 network, you can advertise subnet /23 on your side and the second /23 on the other side and the /22 on both sides.
With AS_Path prepending, you can add one or more AS numbers to the left of the AS_Path to make it longer.
You can use the BGP multi-exit discriminator (MED) attribute. When there are multiple entry points, MED tells the other AS how to route traffic into the AS.
You can use BGP communities with local preference to set up flags in order to mark a set of routes. Then the service provider can use those flags to apply local preference within the network.
You can use the BGP allow-as and as-override subcommands to allow eBGP routers to accept routes from the same AS.

For outbound traffic, the following methods can be used to manipulate traffic:

You can use default routes to provide outbound traffic load balancing.
You can use provider-advertised partial routes from each provider or request full Internet tables and perform AS_Path filtering.
You can use the local preference attribute to indicate which path to use to exit the autonomous system. The higher local preference is used.

eBGP Multihop

eBGP Multihop can be used to connect to a BGP neighbor across multiple equal-cost links, as shown in Figure 4-17. The BGP peering is established between the loopback addresses and not the point-to-point links. This means the BGP peers do not have to be directly connected.

Figure 4-17 eBGP Multihop and Multipath

BGP Multipath

BGP Multipath Load Sharing for eBGP and iBGP allows you to configure multipath load balancing with both eBGP and iBGP paths in BGP networks that are configured to use MPLS. This is accomplished using the maximum-paths command. For example, in Figure 4-17, the eBGP peering can be established by using the interface address of each link and not the loopback address. As a result, the routers receive multiple paths—one for each link—and install all paths up to the maximum-paths value that was configured.

BGP Summary

The characteristics of BGP follow:

BGP is an EGP used in routing on the Internet. It is an interdomain routing protocol.
BGP is a path vector routing protocol suited for strategic routing policies.
It uses TCP port 179 to establish connections with neighbors.
BGPv4 implements CIDR.
eBGP is used for external neighbors. It is used between different autonomous systems.
iBGP is used for internal neighbors. It is used within an autonomous system.
BGP uses several attributes in the routing-decision algorithm.
It uses confederations and route reflectors to reduce BGP peering overhead.
The MED attribute is used between autonomous systems to influence inbound traffic.
Weight is used to influence the path of outbound traffic from a single router, configured locally.
eBGP routes have an administrative distance of 20, and iBGP routes have an administrative distance of 200.

10Apr, 2021

Route Redistribution – OSPF, BGP, and Route Manipulation

Route redistribution is an exchange of routes between routing protocols (for example, between EIGRP and OSPF). You configure the redistribution of routing protocols on routers that reside at the service provider edge of the network or an autonomous system boundary within the internal network. These routers exchange routes with other autonomous systems. Redistribution is also done on routers that run more than one routing protocol. Here are some reasons to do redistribution:

Migration from an older routing protocol to a new routing protocol
Mixed-vendor environment in which Cisco routers might be using EIGRP and other vendor routers might be using OSPF
Different administrative domain between company departments using different routing protocols
Mergers and acquisitions in which the networks initially need to communicate, when two different EIGRP processes might exist

Routes can be learned from different sources. One source is a static route that is configured when not peering with the AS external router. Another source is a different routing protocol where you might be running EIGRP and the other network uses OSPF. Another common example is when peering with an ISP, the enterprise is commonly using OSPF, and the Internet routers peer with the ISP router using BGP.

Figure 4-21 shows an example of the exchange of routes between two autonomous systems. Routes from AS 100 are redistributed into BGP on Router A. Routes from AS 200 are redistributed into BGP on Router B. Then Routers A and B exchange BGP routes. Router A and Router B also implement filters to redistribute only the desired networks.

Figure 4-21 IDS and IPS Operational Differences

Say that a company acquires another company that is running another routing protocol. Figure 4-22 shows a network using both OSPF and EIGRP routing protocols. Routers A and B perform redistribution between OSPF and EIGRP. Both routers must filter routes from OSPF before redistributing them into EIGRP and must filter routes from EIGRP before redistributing them into OSPF. This setup prevents route feedback.

Figure 4-22 Redistribution Between IGPs

Route feedback occurs when a routing protocol learns routes from another routing protocol and then announces the routes to the other routing protocol. In Figure 4-21, OSPF should not advertise the routes it learned from EIGRP on Router A back to EIGRP on Router B. And EIGRP should not announce the routes it learned from OSPF on Router B back to OSPF on Router A.

You can use access lists, distribution lists, and route maps when redistributing routes. You can use these methods to specify (select) routes for redistribution, to set metrics, or to set other policies for the routes. These methods are used to prevent loops in the redistribution. They are also used to control routes’ redistribution direction. Redistribution can be accomplished by two methods:

Two-way redistribution: In two-way redistribution, routing information is exchanged between the two routing protocols. No static routes are used in this exchange. Route filters are used to prevent routing loops. Routing loops can be caused by one route protocol redistributing routes that were learned from a second route protocol back to that second routing protocol.
One-way redistribution: One-way redistribution allows redistribution from only one routing protocol to another. Normally, it is used in conjunction with a default or static route at the edge of a network. Figure 4-23 shows an example of one-way redistribution. The routing information from the WAN routes is redistributed into the campus, but campus routes are not redistributed out to the WAN. The WAN routers use a default route to get back to the campus.

Figure 4-23 One-Way Route Redistribution

Other locations for one-way redistribution are from building access networks, from BGP routes or static routes into the IGP, and from VPN static routes into the IGP.

5Feb, 2021

Default Metric – OSPF, BGP, and Route Manipulation

You should configure the metric of the redistributed routes to a metric other than 0. You can configure the metric in the redistribution command or configure a default seed metric. You can also use the command default-metric with OSPF. IS-IS does not use the default-metric command. The default-metric command is used to specify the seed metric that is used if a metric is not specified during redistribution. The default-metric command has the following syntax for EIGRP:

Click here to view code image

default-metric
bandwidth delay reliability load mtu

OSPF Redistribution

This section reviews a few things you need to remember when designing a network that will redistribute with OSPF.

When redistributing routes into OSPF, use the subnets keyword to permit subnetted routes to be received. If you do not use it, only the major network route is redistributed, without subnetworks. In other words, OSPF performs automatic summarization to IP classful network values. Also, unlike EIGRP and RIPv2, OSPF does not need a metric to be specified during redistribution; it also does not need a seed metric to be specified because it uses a default metric for redistributed routes.

By default, redistributed routes are classified as external Type 2 (E2) in OSPF. You can use the metric-type keyword to change the external route to an external Type 1 (E1). The network design can take into account the after-redistribution cost (Type 2) or the after-redistribution cost plus the path’s cost (Type 1).

In Figure 4-24, Router B is configured to perform mutual redistribution between EIGRP 100 and OSPF process ID 50. In this example, you can use route maps and access lists to prevent routing loops. The route maps permit or deny the networks that are listed in the access lists. The subnets keyword redistributes every subnet in EIGRP into OSPF. (This book does not cover exact configurations.)

Figure 4-24 OSPF and EIGRP Redistribution

Route Filtering

Filtering of routes can occur either on a redistribution point or in the routing domain to prevent some parts of the network from accessing other sections of the network. Route filtering could be used to filter routes when redistributing between routing protocols.

Filtering at a redistribution point provides the following benefits:

Avoids routing loops
Avoids suboptimal routing
Prevents certain routes from entering the domain

Transit Traffic

With BGP, you should be sure not to configure a network as a transit network between autonomous systems. A transit network can occur when connecting your enterprise network to two ISPs in a multihomed BGP configuration. When BGP routes get exchanged with multiple Internet service providers (ISPs), route filtering is used to prevent advertisement of private addresses and addresses that are out of the scope of the domain. The recommendation is to filter routes so that only the enterprise prefixes are advertised to the ISPs, as illustrated in Figure 4-25.

Figure 4-25 Avoiding BGP Transit Traffic