< Free Open Study > |
7-1 maximum-paths number-of-pathsSyntax Description:
Purpose: By default, BGP installs only the best path to a destination in the IP routing table. The maximum-paths command allows up to six paths to the same destination to be installed in the IP routing table. IOS Release: 11.2 Configuration ExampleIn Figure 7-1, Router A is learning two paths to network 172.17.1.x via EBGP. By default, BGP will install only one of these paths in the IP routing table. If all the attributes of the paths are equal, such as MED, Local Preference, and Weight, the route that will be installed is the one learned from the router with the lowest router ID. Initially, the routers will be configured without using the maximum-paths command, as shown in the following listing. This is done to demonstrate that only one route to 172.17.1.0 will be installed. Figure 7-1. Configuration Used to Demonstrate the maximum-paths Command
Router A ip subnet-zero ! interface Serial0 ip address 10.1.1.1 255.255.255.252 ! interface Serial1 ip address 10.1.2.1 255.255.255.252 ! router bgp 2 neighbor 10.1.1.2 remote-as 1 neighbor 10.1.2.2 remote-as 1 ___________________________________________________________________________ Router B ip subnet-zero ! interface Ethernet0 ip address 172.17.1.1 255.255.255.0 ! interface Serial0 ip address 10.1.1.2 255.255.255.252 clockrate 64000 ! router bgp 1 network 172.17.1.0 mask 255.255.255.0 network 10.1.1.0 mask 255.255.255.252 neighbor 10.1.1.1 remote-as 2 neighbor 172.17.1.2 remote-as 1 no synchronization ___________________________________________________________________________ Router C ip subnet-zero ! interface Ethernet0 ip address 172.17.1.2 255.255.255.0 ! interface Serial0 ip address 10.1.2.2 255.255.255.252 clockrate 64000 ! router bgp 1 network 172.17.1.0 mask 255.255.255.0 network 10.1.2.0 mask 255.255.255.252 neighbor 10.1.2.1 remote-as 2 neighbor 172.17.1.1 remote-as 1 no synchronization The BGP table on Router A should contain two paths to network 172.17.1.0: rtrA# show ip bgp BGP table version is 4, local router ID is 10.1.1.1 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 * 172.17.1.0/24 10.1.2.2 0 0 1 i *> 10.1.1.2 0 0 1 I Notice that the path to 172.17.1.0/24 learned from 10.1.1.2 is considered the best path, as denoted by the > symbol. This is the best path because the BGP neighbor advertising this path has a lower router ID than the neighbor advertising the other path, as seen in the show ip bgp neighbors command on Router A: rtrA# show ip bgp neighbors BGP neighbor is 10.1.1.2, remote AS 1, external link Index 1, Offset 0, Mask 0x2 BGP version 4, remote router ID 172.17.1.1 BGP state = Established, table version = 6, up for 00:01:38 Last read 00:00:37, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 24 messages, 0 notifications, 0 in queue Sent 20 messages, 0 notifications, 0 in queue Prefix advertised 0, suppressed 0, withdrawn 0 Connections established 3; dropped 2 Last reset 00:02:00, due to Peer closed the session 1 accepted prefixes consume 32 bytes 0 history paths consume 0 bytes Connection state is ESTAB, I/O status: 1, unread input bytes: 0 Local host: 10.1.1.1, Local port: 179 Foreign host: 10.1.1.2, Foreign port: 11006 BGP neighbor is 10.1.2.2, remote AS 1, external link Index 2, Offset 0, Mask 0x4 BGP version 4, remote router ID 172.17.1.2 BGP state = Established, table version = 6, up for 00:02:54 Last read 00:00:55, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 23 messages, 0 notifications, 0 in queue Sent 20 messages, 0 notifications, 0 in queue Prefix advertised 0, suppressed 0, withdrawn 0 Connections established 3; dropped 2 Last reset 00:03:19, due to Peer closed the session 1 accepted prefixes consume 32 bytes 0 history paths consume 0 bytes Only one of the paths to 172.17.1.0/24 will be installed in the IP routing table on Router A: rtrA# 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 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, P - periodic downloaded static route T - traffic engineered route Gateway of last resort is not set 172.17.0.0/24 is subnetted, 1 subnets B 172.17.1.0 [20/0] via 10.1.1.2 10.0.0.0/30 is subnetted, 2 subnets C 10.1.2.0 is directly connected, Serial1 C 10.1.1.0 is directly connected, Serial0 Now add the maximum-paths 2 command to the configuration on Router A: Router A router bgp 2 neighbor 10.1.1.2 remote-as 1 neighbor 10.1.2.2 remote-as 1 maximim-paths 2 VerificationVerify that both routes to 172.17.1.0/24 learned via EBGP are being installed in the IP routing table on Router A: rtrA# 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 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, P - periodic downloaded static route T - traffic engineered route Gateway of last resort is not set 172.17.0.0/24 is subnetted, 1 subnets B 172.17.1.0 [20/0] via 10.1.1.2 [20/0] via 10.1.2.2 10.0.0.0/30 is subnetted, 2 subnets C 10.1.2.0 is directly connected, Serial1 C 10.1.1.0 is directly connected, Serial0 Troubleshooting
|
< Free Open Study > |