Cisco BGP4 Case Studies

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We'll build the above configuration step by step and see what can go wrong along the way. Whenever you
have an AS that is connected to two ISPs via eBGP, it's always good to run iBGP within your AS in order to
have a better control of your routes. In this example we run iBGP inside AS100 between RTA and RTB, and
we run OSPF as an IGP. Assuming that we're connected to two ISPs, AS200 and AS300, the following is the
first run of the configurations for all the routers. These aren't the final configurations.
RTA#
hostname RTA
ip subnet-zero
interface Loopback0
ip address 203.250.13.41 255.255.255.0
Cisco - BGP Case Studies
interface Ethernet0
ip address 203.250.14.1 255.255.255.0
interface Serial0
ip address 128.213.63.1 255.255.255.252
router ospf 10
network 203.250.0.0 0.0.255.255 area 0
router bgp 100
network 203.250.13.0
network 203.250.14.0
neighbor 128.213.63.2 remote-as 200
neighbor 203.250.15.2 remote-as 100
neighbor 203.250.15.2 update-source Loopback0
RTF#
hostname RTF
ip subnet-zero
interface Ethernet0
ip address 203.250.14.2 255.255.255.0
interface Serial1
ip address 203.250.15.1 255.255.255.252
router ospf 10
network 203.250.0.0 0.0.255.255 area 0
RTB#
hostname RTB
ip subnet-zero
interface Serial0
ip address 203.250.15.2 255.255.255.252
interface Serial1
ip address 192.208.10.6 255.255.255.252
router ospf 10
network 203.250.0.0 0.0.255.255 area 0
router bgp 100
network 203.250.15.0
neighbor 192.208.10.5 remote-as 300
neighbor 203.250.13.41 remote-as 100
RTC#
hostname RTC
ip subnet-zero
interface Loopback0
ip address 128.213.63.130 255.255.255.192
interface Serial2/0
ip address 128.213.63.5 255.255.255.252
!
interface Serial2/1
ip address 128.213.63.2 255.255.255.252
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router bgp 200
network 128.213.0.0
neighbor 128.213.63.1 remote-as 100
neighbor 128.213.63.6 remote-as 400
RTD#
hostname RTD
ip subnet-zero
interface Loopback0
ip address 192.208.10.174 255.255.255.192
interface Serial0/0
ip address 192.208.10.5 255.255.255.252
!
interface Serial0/1
ip address 192.208.10.2 255.255.255.252
router bgp 300
network 192.208.10.0
neighbor 192.208.10.1 remote-as 500
neighbor 192.208.10.6 remote-as 100
RTE#
hostname RTE
ip subnet-zero
interface Loopback0
ip address 200.200.10.1 255.255.255.0
interface Serial0
ip address 195.211.10.2 255.255.255.252
interface Serial1
ip address 128.213.63.6 255.255.255.252
clockrate 1000000
router bgp 400
network 200.200.10.0
neighbor 128.213.63.5 remote-as 200
neighbor 195.211.10.1 remote-as 500
RTG#
hostname RTG
ip subnet-zero
interface Loopback0
ip address 195.211.10.174 255.255.255.192
interface Serial0
ip address 192.208.10.1 255.255.255.252
interface Serial1
ip address 195.211.10.1 255.255.255.252
router bgp 500
network 195.211.10.0
neighbor 192.208.10.2 remote-as 300
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neighbor 195.211.10.2 remote-as 400
It's always better to use the network command or redistribute static entries into BGP to advertise networks,
rather than redistributing IGP into BGP. This is why, throughout this example I use the network command to
inject networks into BGP.
Let's start with the s1 interface on RTB shutdown, as if the link between RTB and RTD doesn't exist. The
following is RTB's BGP table.
RTB#show ip bgp BGP
table version is 4, local router ID is 203.250.15.2 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
*i128.213.0.0 128.213.63.2 0 100 0 200 i
*i192.208.10.0 128.213.63.2 100 0 200 400 500
300 i
*i195.211.10.0 128.213.63.2 100 0 200 400 500 i
*i200.200.10.0 128.213.63.2 100 0 200 400 i
*>i203.250.13.0 203.250.13.41 0 100 0 i
*>i203.250.14.0 203.250.13.41 0 100 0 i
*>203.250.15.0 0.0.0.0 0 32768 i
Let me go over the basic notations of the above table. The "i" at the beginning means that the entry was
learned via an iBGP peer. The "i" at the end indicates the origin of the path information to be IGP. The path
info is intuitive. For example, network 128.213.0.0 is learned via path 200 with a next hop of 128.213.63.2.
Note that any locally generated entry, such as 203.250.15.0, has a next hop 0.0.0.0.
The > symbol indicates that BGP has chosen the best route based on the list of decision steps that I have gone
through earlier in this document under "How BGP selects a Path". BGP picks one best path to reach a
destination, installs it in the IP routing table and advertises it to other BGP peers. Notice the next hop
attribute. RTB knows about 128.213.0.0 via a next hop of 128.213.63.2, which is the eBGP next hop carried
into iBGP.
Let's look at the IP routing table:
RTB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
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
Gateway of last resort is not set
203.250.13.0 255.255.255.255 is subnetted, 1 subnets
O 203.250.13.41 [110/75] via 203.250.15.1, 02:50:45, Serial0
203.250.15.0 255.255.255.252 is subnetted, 1 subnets
C 203.250.15.0 is directly connected, Serial0
O 203.250.14.0 [110/74] via 203.250.15.1, 02:50:46, Serial0
It doesn't look like any of the BGP entries has made it to the routing table. There are two problems here,
which we'll examine in turn.
The first problem is the next hop for these entries, 128.213.63.2, is unreachable. This is true because we don't
have a way to reach that next hop via our IGP (OSPF). RTB hasn't learned about 128.213.63.0 via OSPF. We
Cisco - BGP Case Studies
can run OSPF on RTA's s0 interface and make it passive, and this way RTB would know how to reach the
next hop 128.213.63.2. We could also change the next hop by using the bgp nexthopself command between
RTA and RTB. Doing this, RTA's configuration would be:
RTA#
hostname RTA
ip subnet-zero
interface Loopback0
ip address 203.250.13.41 255.255.255.0
interface Ethernet0
ip address 203.250.14.1 255.255.255.0
interface Serial0
ip address 128.213.63.1 255.255.255.252
router ospf 10
passive-interface Serial0
network 203.250.0.0 0.0.255.255 area 0
network 128.213.0.0 0.0.255.255 area 0
router bgp 100
network 203.250.0.0 mask 255.255.0.0
neighbor 128.213.63.2 remote-as 200
neighbor 203.250.15.2 remote-as 100
neighbor 203.250.15.2 update-source Loopback0
The new BGP table on RTB now looks like this:
RTB#show ip bgp
BGP table version is 10, local router ID is 203.250.15.2
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
*>i128.213.0.0 128.213.63.2 0 100 0 200 i
*>i192.208.10.0 128.213.63.2 100 0 200 400 500
300 i
*>i195.211.10.0 128.213.63.2 100 0 200 400 500 i [ Pobierz całość w formacie PDF ]

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