We will build the above configuration step by step and see what can go wrong along the way. Whenver you have an AS that is connected to two ISPs via EBGP, it is always good to run IBGP within your AS in order to have a better control of your routes. In this example we will run IBGP inside AS100 between RTA and RTB and we will run OSPF as an IGP. Assuming that AS200 and AS300 are the two ISPs we are connected to, the following are the first run of configuration for all the routers. This is NOT the final configuration.
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 network 203.250.0.0 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 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 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 neighbor 195.211.10.2 remote-as 400
It is 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, throughtout this example I will only use the network command to inject networks into BGP.
Let us assume to start with that s1 on RTB is shutdown, as if the link between RTB and RTD does not exist. The following is RTB's BGP table.
RTB#sh 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 internal BGP 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 nexthop of 128.213.63.2. Note that any locally generated entry such as 203.250.15.0 has a nexthop 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 will only pick one best Path to reach a destination, will install this path in the ip routing table and will advertise it to other bgp peers. Notice the nexthop attribute. RTB knows about 128.213.0.0 via a nexthop of 128.213.63.2 which is the ebgp nexthop carried into IBGP.
Let us look at the IP routing table:
RTB#sh ip rou
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
Well, it doesn't look like any of the BGP entries has made it to the routing table. There are two problems here:
Problem 1:
The Nexthop for these entries 128.213.63.2 is unreachable. This is true
because we do not have a way to reach that nexthop via our IGP (OSPF).
RTB has not learned about 128.213.63.0 via OSPF. We can run OSPF on RTA
s0 and make it passive, and this way RTB would know how to reach the nexthop
128.213.63.2. We could also change the nexthop by using the bgp nexthopself
command between RTA and RTB.
RTA's configs 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#sh 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 *>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
Note that all the entries have >, which means that BGP is ok with next hop. Let us look at the routing table now:
RTB#sh ip rou
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, 00:04:46, 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, 00:04:46, Serial0
128.213.0.0 255.255.255.252 is subnetted, 1 subnets
O 128.213.63.0 [110/138] via 203.250.15.1, 00:04:47, Serial0
Problem 2:
We still do not see the BGP entries; the only difference is that 128.213.63.0 is now reachable via OSPF. This is the synchronization issue, BGP is not putting these entries in the routing table and will not send them in BGP updates because it is not synchronized with the IGP. Note that RTF has no notion of networks 192.208.10.0 or 195.211.10.0 because we have not redistributed BGP into OSPF yet.
In this scenario, if we turn synchronization off, we will have the entries in the routing table, but connectivity would still be broken.
If you turn off synchronization on RTB this is what will happen:
RTB#sh ip rou
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
B 200.200.10.0 [200/0] via 128.213.63.2, 00:01:07
B 195.211.10.0 [200/0] via 128.213.63.2, 00:01:07
B 192.208.10.0 [200/0] via 128.213.63.2, 00:01:07
203.250.13.0 is variably subnetted, 2 subnets, 2 masks
O 203.250.13.41 255.255.255.255
[110/75] via 203.250.15.1, 00:12:37, Serial0
B 203.250.13.0 255.255.255.0 [200/0] via 203.250.13.41, 00:01:08
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, 00:12:37, Serial0
128.213.0.0 is variably subnetted, 2 subnets, 2 masks
B 128.213.0.0 255.255.0.0 [200/0] via 128.213.63.2, 00:01:08
O 128.213.63.0 255.255.255.252
[110/138] via 203.250.15.1, 00:12:37, Serial0
The routing table looks fine, but there is no way we can reach those networks because RTF in the middle does not know how to reach them:
RTF#sh ip rou
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 [110/11] via 203.250.14.1, 00:14:15, Ethernet0
203.250.15.0 255.255.255.252 is subnetted, 1 subnets
C 203.250.15.0 is directly connected, Serial1
C 203.250.14.0 is directly connected, Ethernet0
128.213.0.0 255.255.255.252 is subnetted, 1 subnets
O 128.213.63.0 [110/74] via 203.250.14.1, 00:14:15, Ethernet0
So, turning off synchronization in this situation did not help this particular issue, but we will need it for other issues later on. Let's redistribute OSPF into BGP on RTA , with a metric of 2000.
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 redistribute bgp 100 metric 2000 subnets 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 routing table will look like this:
RTB#sh ip rou
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
O E2 200.200.10.0 [110/2000] via 203.250.15.1, 00:00:14, Serial0
O E2 195.211.10.0 [110/2000] via 203.250.15.1, 00:00:14, Serial0
O E2 192.208.10.0 [110/2000] via 203.250.15.1, 00:00:14, Serial0
203.250.13.0 is variably subnetted, 2 subnets, 2 masks
O 203.250.13.41 255.255.255.255
[110/75] via 203.250.15.1, 00:00:15, Serial0
O E2 203.250.13.0 255.255.255.0
[110/2000] via 203.250.15.1, 00:00:15, Serial0
203.250.15.0 255.255.255.252 is subnetted, 2 subnets
C 203.250.15.8 is directly connected, Loopback1
C 203.250.15.0 is directly connected, Serial0
O 203.250.14.0 [110/74] via 203.250.15.1, 00:00:15, Serial0
128.213.0.0 is variably subnetted, 2 subnets, 2 masks
O E2 128.213.0.0 255.255.0.0 [110/2000] via 203.250.15.1,
00:00:15,Serial0
O 128.213.63.0 255.255.255.252
[110/138] via 203.250.15.1, 00:00:16, Serial0
The BGP entries have disappeared because OSPF has a better distance (110) than internal bgp (200).
I will also turn sync off on RTA in order for it to advertise 203.250.15.0, because it will not sync up with OSPF due to the difference in masks. I will also keep sync off on RTB in order for it to advertise 203.250.13.0 for the same reason.
Let us bring RTB's s1 up and see what all the routes will look like. I will also enable OSPF on serial 1 of RTB and make it passive in order for RTA to know about the nexthop 192.208.10.5 via IGP. Otherwise some looping will occur because in order to get to nexthop 192.208.10.5 we would have to go the other way via EBGP. The updated configs of RTA and RTB follow:
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 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 redistribute bgp 100 metric 2000 subnets 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 no synchronization 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 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 redistribute bgp 100 metric 1000 subnets passive-interface Serial1 network 203.250.0.0 0.0.255.255 area 0 network 192.208.0.0 0.0.255.255 area 0 router bgp 100 no synchronization network 203.250.15.0 neighbor 192.208.10.5 remote-as 300 neighbor 203.250.13.41 remote-as 100
And the BGP tables look like this:
RTA#sh ip bgp BGP table version is 117, local router ID is 203.250.13.41 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 *> 128.213.0.0 128.213.63.2 0 0 200 i *>i192.208.10.0 192.208.10.5 0 100 0 300 i *>i195.211.10.0 192.208.10.5 100 0 300 500 i * 128.213.63.2 0 200 400 500 i *> 200.200.10.0 128.213.63.2 0 200 400 i *> 203.250.13.0 0.0.0.0 0 32768 i *> 203.250.14.0 0.0.0.0 0 32768 i *>i203.250.15.0 203.250.15.2 0 100 0 i RTB#sh ip bgp BGP table version is 12, local router ID is 203.250.15.10 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 * 192.208.10.5 0 300 500 400 200 i *> 192.208.10.0 192.208.10.5 0 0 300 i *> 195.211.10.0 192.208.10.5 0 300 500 i *>i200.200.10.0 128.213.63.2 100 0 200 400 i * 192.208.10.5 0 300 500 400 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
There are multiple ways to design our network to talk to the two different ISPs AS200 and AS300. One way is to have a primary ISP and a backup ISP. We could learn partial routes from one of the ISP's and default routes to both ISPs. In this example, I have chosen to receive partial routes from AS200 and only local routes from AS300. Both RTA and RTB are generating default routes into OSPF with RTB being more preferred (lower metric). This way I could balance outgoing traffic between the two ISPs.
Potential asymmetry might occur if traffic going out from RTA comes back via RTB. This might occur if you are using the same pool of IP addresses (same major net) when talking to the two ISP's. Because of aggregation your whole AS might look as one whole entity to the outside world and entry points to your network could occur via RTA or RTB. You might find out that all incoming traffic to your AS is coming via one single point even though you have multiple points to the internet. In our example, I have chosen two different major nets when talking to the two ISPs.
One other potential reason for asymmetry is the different advertised path length to reach your AS. One service provider might be closer to a certain destination than another. In our example, traffic from AS400 destined to your network will always come in via RTA because of the shorter path. You might try to affect that decision by prepending path numbers to your updates to make the path length look longer (set as-path prepend). But, if AS400 has somehow set its exit point to be via AS200 based on attributes such as local preference or metric or weight then there is nothing you can do.
This is the final configuration for all of the routers:
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 redistribute bgp 100 metric 2000 subnets 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 default-information originate metric 2000 router bgp 100 no synchronization network 203.250.13.0 network 203.250.14.0 neighbor 128.213.63.2 remote-as 200 neighbor 128.213.63.2 route-map setlocalpref in neighbor 203.250.15.2 remote-as 100 neighbor 203.250.15.2 update-source Loopback0 ip classless ip default-network 200.200.0.0 route-map setlocalpref permit 10 set local-preference 200
On RTA, the local preference for routes coming from AS200 is set to 200. I have also picked network 200.200.0.0 to be the candidate default, using the "ip default-network command".
The "default-information originate" command is used with OSPF to inject the default route inside the OSPF domain. This command is also used with ISIS and BGP. For RIP, 0.0.0.0 is automatically redistributed into RIP without additional configuration. For IGRP and EIGRP, the default information is injected into the IGP domain after redistributing BGP into IGRP/EIGRP. Also with IGRP/EIGRP we can redistribute a static route to 0.0.0.0 into the IGP domain.
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 ip classless RTB# hostname RTB ip subnet-zero interface Loopback1 ip address 203.250.15.10 255.255.255.252 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 redistribute bgp 100 metric 1000 subnets passive-interface Serial1 network 203.250.0.0 0.0.255.255 area 0 network 192.208.10.6 0.0.0.0 area 0 default-information originate metric 1000 ! router bgp 100 no synchronization network 203.250.15.0 neighbor 192.208.10.5 remote-as 300 neighbor 192.208.10.5 route-map localonly in neighbor 203.250.13.41 remote-as 100 ! ip classless ip default-network 192.208.10.0 ip as-path access-list 1 permit ^300$ route-map localonly permit 10 match as-path 1 set local-preference 300
For RTB, the local preference for updates coming in from AS300 is set
to 300 which is higher than the IBGP updates coming in from RTA. This way
AS100 will pick RTB for AS300's local routes. Any other routes on RTB (if
they exist) will be sent internally with a local preference of 100 which
is lower than 200 coming in from RTA and this way RTA will be preferred.
Note that I have only advertised AS300's local routes. Any path info that
does not match ^300$ will be dropped. If you wanted to advertise the local
routes and the neighbor routes (customers of the ISP) you can use the following:
^300_[0-9]*
This is the output of the regular expression indicating AS300's local routes:
RTB#sh ip bgp regexp ^300$ BGP table version is 14, local router ID is 203.250.15.10 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 *> 192.208.10.0 192.208.10.5 0 300 0 300 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 router bgp 200 network 128.213.0.0 aggregate-address 128.213.0.0 255.255.0.0 summary-only neighbor 128.213.63.1 remote-as 100 neighbor 128.213.63.1 distribute-list 1 out neighbor 128.213.63.6 remote-as 400 ip classless access-list 1 deny 195.211.0.0 0.0.255.255 access-list 1 permit any
On RTC, I have aggregated 128.213.0.0/16 and indicated the specific routes to be injected into AS100. If the ISP refuses to do this task then you have to filter on the incoming end of AS100.
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 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 aggregate-address 195.211.0.0 255.255.0.0 summary-only neighbor 192.208.10.2 remote-as 300 neighbor 192.208.10.2 send-community neighbor 192.208.10.2 route-map setcommunity out neighbor 195.211.10.2 remote-as 400 ! ip classless access-list 1 permit 195.211.0.0 0.0.255.255 access-list 2 permit any access-list 101 permit ip 195.211.0.0 0.0.255.255 host 255.255.0.0 route-map setcommunity permit 20 match ip address 2 ! route-map setcommunity permit 10 match ip address 1 set community no-export
On RTG, I have demonstrated the use of community filtering by adding a no-export community to 195.211.0.0 updates towards RTD. This way RTD will not export that route to RTB. It doesn't matter in our case because RTB is not accepting these routes anyway.
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 router bgp 400 network 200.200.10.0 aggregate-address 200.200.0.0 255.255.0.0 summary-only neighbor 128.213.63.5 remote-as 200 neighbor 195.211.10.1 remote-as 500 ip classless
RTE is aggregating 200.200.0.0/16.
And following are the final BGP and routing tables for RTA, RTF and RTB:
RTA#sh ip bgp
BGP table version is 21, local router ID is 203.250.13.41
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
*> 128.213.0.0 128.213.63.2 0 200 0 200 i
*>i192.208.10.0 192.208.10.5 0 300 0 300 i
*> 200.200.0.0/16 128.213.63.2 200 0 200 400 i
*> 203.250.13.0 0.0.0.0 0 32768 i
*> 203.250.14.0 0.0.0.0 0 32768 i
*>i203.250.15.0 203.250.15.2 0 100 0 i
RTA#sh ip rou
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 128.213.63.2 to network 200.200.0.0
192.208.10.0 is variably subnetted, 2 subnets, 2 masks
O E2 192.208.10.0 255.255.255.0
[110/1000] via 203.250.14.2, 00:41:25, Ethernet0
O 192.208.10.4 255.255.255.252
[110/138] via 203.250.14.2, 00:41:25, Ethernet0
C 203.250.13.0 is directly connected, Loopback0
203.250.15.0 is variably subnetted, 3 subnets, 3 masks
O 203.250.15.10 255.255.255.255
[110/75] via 203.250.14.2, 00:41:25, Ethernet0
O 203.250.15.0 255.255.255.252
[110/74] via 203.250.14.2, 00:41:25, Ethernet0
B 203.250.15.0 255.255.255.0 [200/0] via 203.250.15.2, 00:41:25
C 203.250.14.0 is directly connected, Ethernet0
128.213.0.0 is variably subnetted, 2 subnets, 2 masks
B 128.213.0.0 255.255.0.0 [20/0] via 128.213.63.2, 00:41:26
C 128.213.63.0 255.255.255.252 is directly connected, Serial0
B* 200.200.0.0 255.255.0.0 [20/0] via 128.213.63.2, 00:02:38
RTF#sh ip rou
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 203.250.15.2 to network 0.0.0.0
192.208.10.0 is variably subnetted, 2 subnets, 2 masks
O E2 192.208.10.0 255.255.255.0
[110/1000] via 203.250.15.2, 00:48:50, Serial1
O 192.208.10.4 255.255.255.252
[110/128] via 203.250.15.2, 01:12:09, Serial1
203.250.13.0 is variably subnetted, 2 subnets, 2 masks
O 203.250.13.41 255.255.255.255
[110/11] via 203.250.14.1, 01:12:09, Ethernet0
O E2 203.250.13.0 255.255.255.0
[110/2000] via 203.250.14.1, 01:12:09, Ethernet0
203.250.15.0 is variably subnetted, 2 subnets, 2 masks
O 203.250.15.10 255.255.255.255
[110/65] via 203.250.15.2, 01:12:09, Serial1
C 203.250.15.0 255.255.255.252 is directly connected, Serial1
C 203.250.14.0 is directly connected, Ethernet0
128.213.0.0 is variably subnetted, 2 subnets, 2 masks
O E2 128.213.0.0 255.255.0.0
[110/2000] via 203.250.14.1, 00:45:01, Ethernet0
O 128.213.63.0 255.255.255.252
[110/74] via 203.250.14.1, 01:12:11, Ethernet0
O E2 200.200.0.0 255.255.0.0 [110/2000] via 203.250.14.1, 00:03:47,
Ethernet0
O*E2 0.0.0.0 0.0.0.0 [110/1000] via 203.250.15.2, 00:03:33, Serial1
Note RTF's routing table which indicates that networks local to AS300 such as 192.208.10.0 are to be reached via RTB. Other known networks such as 200.200.0.0 are to be reached via RTA. The gateway of last resort is set to RTB. In case something happens to the connection between RTB and RTD, then the default advertised by RTA will kick in with a metric of 2000.
RTB#sh ip bgp
BGP table version is 14, local router ID is 203.250.15.10
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 0 200 0 200 i
*> 192.208.10.0 192.208.10.5 0 300 0 300 i
*>i200.200.0.0/16 128.213.63.2 200 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
RTB#sh ip rou
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 192.208.10.5 to network 192.208.10.0
* 192.208.10.0 is variably subnetted, 2 subnets, 2 masks
B* 192.208.10.0 255.255.255.0 [20/0] via 192.208.10.5, 00:50:46
C 192.208.10.4 255.255.255.252 is directly connected, Serial1
203.250.13.0 is variably subnetted, 2 subnets, 2 masks
O 203.250.13.41 255.255.255.255
[110/75] via 203.250.15.1, 01:20:33, Serial0
O E2 203.250.13.0 255.255.255.0
[110/2000] via 203.250.15.1, 01:15:40, Serial0
203.250.15.0 255.255.255.252 is subnetted, 2 subnets
C 203.250.15.8 is directly connected, Loopback1
C 203.250.15.0 is directly connected, Serial0
O 203.250.14.0 [110/74] via 203.250.15.1, 01:20:33, Serial0
128.213.0.0 is variably subnetted, 2 subnets, 2 masks
O E2 128.213.0.0 255.255.0.0 [110/2000] via 203.250.15.1, 00:46:55,
Serial0
O 128.213.63.0 255.255.255.252
[110/138] via 203.250.15.1, 01:20:34, Serial0
O E2 200.200.0.0 255.255.0.0 [110/2000] via 203.250.15.1, 00:05:42,
Serial0
(End of document)