Cnse Study Injecting IGP Routes into BGP

Chapter 2 emphasizes that at an AS border, outgoing route advertisements affect incoming traffic, and incoming route advertisements affect outgoing traffic. As a result, outgoing and incoming advertisements should be considered separately. This section begins the discussion of BGP route advertisements by examining basic methods of injecting routes into BGP.

Figure 3-3 shows that AS 200 uses EIGRP as its IGP. Taos must advertise three addresses to its EBGP peer: 192.168.200.0/24 is learned via EIGRP, 192.168.100.0/24 is directly attached to Taos, and 192.168.1.216/30 is connecting Taos and AngelFire. Whereas the first two addresses are full class C addresses, the last is a subnet. Other subnets of 192.168.1.0 appear outside of AS 200, so the subnet only, not the major network address, must be advertised.

Figure 3-3 AS 200 Is Using EIGRP as Its IGP

Figure 3-3 AS 200 Is Using EIGRP as Its IGP

Picture Case Routing

Example 3-9 shows a "first-pass" configuration of Taos. Example 3-9 Taos ' Basic EIGRP and BGP Configuration router eigrp 200 passive-interface SerialO network 192.168.1.0 network 192.168.100.0

router bgp 200 redistribute eigrp 200 neighbor 192.168.1.226 remote-as 100

Example 3-10 shows the results in Vail's BGP table. All EIGRP networks have been advertised over the EBGP link. Notice in the configuration that no metric was specified with the redistribute command. As a result, the metric of each route defaults to the EIGRP metric, as shown in Taos' routing table in Example 3-11. The directly connected networks have a metric of 0, and 192.168.200.0/24 has a metric of 409600. You can change this default method of selecting a metric with the default-metric command.

NOTE The BGP metric is the MULTI_EXIT_DISC. The use and manipulation of this attribute is demonstrated in the section "Case Study: Using the MULTI_EXIT_DISC Attribute."

Example:! 10 '¡has Advertised 192.168.100.0/24 and 192.168.200.0/24 Correctly, but the Subnet 192.168.1.216/30 Was Summarized to the Major Network

Vail#show ip bgp

BGP table version is 15, local router ID is 192.168.255.254

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 192.168.200.0 192.168.1.225 409600 0 200 ? Vail#

Example:i-11 Taos'Routing Table Shows That the EIGRP Metrics Are the Same as the Metrics in Vail's BGP Table

Taos#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, 0 - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR T - traffic engineered route

Gateway of last resort is not set

D 192.168.200.0/24 [90/409600] via 192.168.1.217, 00:52:09, Ethernet©

192.168.1.0/24 is variably subnetted, 3 subnets, 2 masks D 192.168.1.0/24 is a summary, 00:52:11, Null0

C 192.168.1.224/30 is directly connected, Serial©

C 192.168.1.216/30 is directly connected, Ethernet©

C 192.168.100.0/24 is directly connected, Ethernetl Taos#

The two major networks in AS 200 are advertised correctly, but you can see in Example 3-9 that the subnet 192.168.1.216/30 has been summarized to the major network. The reason for this is that BGP-4, although it is classless, by default summarizes at network boundaries. In the internetwork of Figure 3-3, this summarization presents no problem. Vail is directly connected to the other two subnets of 192.168.1.0 and therefore knows the two more-specific routes. As the network grows and other subnets of that network are used on other routers, however, the summary can cause incorrect routing. To turn off BGP's automatic summarization, configure Vail as in Example 3-12.

Example 3-12 Vail Configuration to Turn Off BGP Automatic Summarization router eigrp 200 passive-interface Serial© network 192.168.1.0 network 192.168.100.0

i router bgp 200 redistribute eigrp 200 neighbor 192.168.1.226 remote-as 100 no auto-summary

Example 3-13 shows the results in Vail's BGP table. The subnets of 192.168.1.0 are now being advertised. However, the major network 192.168.1.0 is still being advertised in addition to the subnets. Another look at Taos' routing table in Example 3-12 shows why. EIGRP also performs automatic route summarization and has entered a summary route to NullO in the routing table. BGP is picking up this route in addition to the subnets and is advertising it to Vail.

Example 3-13 Vail's BGP Table, After BGP Auto-Summarization Is Turned Off at Taos

Vail#show ip bgp

BGP table version is 17, local router ID is 192.16 Status codes: s suppressed, d damped, h history, * Origin codes: i - IGP, e - EGP, ? - incomplete

3.255.254 valid, > best, internal

Network Next Hop

*> 192.168.1.216/30 192.168.1.225 *> 192.168.1.224/30 192.168.1.225 *> 192.168.100.0 192.168.1.225 *> 192.168.200.0 192.168.1.225 Vail#

Metric LocPrf Weight Path 281600 0 200 ? 0 0 200 ? 0 0 200 ? 0 0 200 ? 409600 0 200 ?

To turn off EIGRP auto-summarization for the Vail router, you use the same no auto-summary command as demonstrated in Example 3-14.

Example 3-14 Vail Configuration to Turn Off EIGRP Automatic Summarization router eigrp 200 passive-interface Serial© network 192.168.1.0 network 192.168.100.0 no auto-summary i router bgp 200 redistribute eigrp 200 neighbor 192.168.1.226 remote-as 100 no auto-summary

Example 3-15 shows the resulting BGP table at Vail.

Example1 15 VaiVs BGP Table After EIGRP Auto-Summarization Is Turned Off at Taos

Vail#show ip bgp

BGP table version is 20, local router ID is 192.168.255.254

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete

Vail#show ip bgp

BGP table version is 20, local router ID is 192.168.255.254

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete

Network

Next Hop

Metric

LocPrf Weight

Path

*> 192.168.

.1 .216/30

192.168.1

.225

0

0

200 ?

*> 192.168.

.1.224/30

192.168.1

.225

0

0

200 ?

*> 192.168.

,100.0

192.168.1

.225

0

0

200 ?

*> 192.168.

.200.0

192.168.1

.225

409600

0

200 ?

Vail#

The advantage of using redistribution to inject routes into BGP is that internal changes can be advertised with few or no changes to the BGP configuration. If a network is added or removed within the EIGRP domain of AS 200, the change is automatically advertised to Vail. However, advertising every IGP route is also the major disadvantage of IGP-to-BGP redistribution. For example, the administrators of autonomous systems 100 and 200 might or might not want subnet 192.168.1.224/30 advertised from Taos to Vail, as it is in Example 3-15. If the subnet should not be advertised, a route filter must be used. Later in this chapter, the section "Routing Policies" demonstrates, through several case studies, various options for configuring route filters.

Route filters are almost always necessary when redistributing an IGP's routes into BGP. By default, every route known by the IGP is redistributed. The administrator of the AS might want to advertise only a subset of the IGP routes, and so must filter the others. Or, perhaps a multihomed AS should not be a transit for any of its neighboring autonomous systems. Route filters must be used to prevent external routes learned from one AS from being advertised to other autonomous systems. Then there is the problem of route feedback, in which external routes received from EBGP are advertised into an IGP and then are redistributed from that IGP back into EBGP. At a minimum, best practice dictates that route filters should be used to ensure that only the correct routes are redistributed. In actual practice, redistribution of IGP prefixes into BGP is rarely used because of this lack of precise control.

An alternative to redistributing IGP routes into BGP is to use the network command. As discussed in Chapter 1, this command functions differently under EGP and BGP than it does under an IGP. When used with an IGP, the network command specifies the address of an interface or group of interfaces on which the routing protocol will be enabled. When used with EGP and BGP, network specifies an IP prefix to be advertised. For each prefix specified with the command, BGP looks into the routing table. If an entry in the table exactly matches the network prefix, that prefix is entered into the BGP table and advertised.

Example 3-16 shows the configuration for Taos using the network command rather than redistribution.

Example 3-16 Configuring Taos with the network Command router eigrp 200 passive-interface Serial© network 192.168.1.0 network 192.168.100.0

i router bgp 200 network 192.168.1.216 mask 255.255.255.252 network 192.168.100.0 network 192.168.200.0 neighbor 192.168.1.226 remote-as 100

The major networks 192.168.100.0 and 192.168.200.0 are specified alone. For the subnet 192.168.1.216, a mask is also specified. Subnets and masks can be specified only under BGP-4; under EGP or earlier versions of BGP, all of which are classful, only major networks can be specified.

Notice that the no auto-summary command is not used under either EIGRP or BGP in this configuration. Because no redistribution is taking place, turning off auto-summarization is unnecessary. Example 3-17 shows the result of the configuration.

Example 3-17 Vail's BGP Table After Taos Is Reconfigured Using the BGP network Command

Vail#show ip bgp

BGP table version is 36, local router ID is 192.168.255.254

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop

*> 192.168.1.216/30 192.168.1.225 *> 192.168.100.0 192.168.1.225 *> 192.168.200.0 192.168.1.225

Vail#

Metric LocPrf Weight Path 0 0 200 i 0 0 200 i 409600 0 200 i

Unlike in Example 3-15, subnet 192.168.1.224/30 is not advertised, because it was not specified with a network command. The administrator has more control than with redistribution, and no filtering is necessary. Comparing Example 3-15 and Example 3-17, notice that the ORIGIN codes differ. Whereas the redistributed routes in Example 3-15 are tagged with a ?, indicating an ORIGIN of "incomplete," the routes in Example 3-17 are tagged with an i, indicating an ORIGIN of IGP. This tagging can make a difference in some circumstances because the BGP decision process, discussed in Chapter 2, gives a higher preference to ORIGIN code 1 (IGP) than to code 3 (incomplete).

A limit of 200 on the number of addresses that can be specified applies with the network command. If you must advertise more addresses across a BGP connection, you must use redistribution.

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