Internet Routing Architectures (CISCO):Configurinbg Effective Internet Routing Policies

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Note how RTA only sees networks that belong to AS1 and its customer AS6 (except for the default route). For network 193.78.0.0/16, which is the default, RTA follows the NY link because of the local preference 250. For traffic toward AS1 and AS6, RTA follows the RTA-RTC link (local preference 300). The following IP routing table of RTA shows that RTA has set its default to 193.78.0.0/16, which is reachable via 172.16.1.2.

    RTA#sh 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
    Gateway of last resort is 193.78.0.0 to network 0.0.0.0
    B  192.68.10.0/24 [20/0] via 172.16.20.1, 00:07:34
    B  192.68.11.0/24 [20/0] via 172.16.20.1, 00:07:34
    B  192.68.40.0/24 [20/0] via 172.16.20.1, 00:07:34
      172.16.0.0/16 is variably subnetted, 6 subnets, 3 masks
    C  172.16.2.254/32 is directly connected, Loopback0
    C  172.16.220.0/24 is directly connected, Ethernet0
    C  172.16.20.0/24 is directly connected, Serial0
    O  172.16.10.0/24 [110/20] via 172.16.1.2, 01:39:52, Ethernet1
    C  172.16.1.0/24 is directly connected, Ethernet1
    O  172.16.65.0/26 [110/20] via 172.16.1.2, 01:39:52, Ethernet1
    S*  0.0.0.0/0 [1/0] via 193.78.0.0
    B  193.78.0.0/16 [200/0] via 172.16.1.2, 00:03:07

The following is an output of RTD's BGP table:

    RTD#sh ip bgp
    BGP table version is 14, local router ID is 192.68.10.1
    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
    *> 172.16.1.0/24   192.68.5.1      250       0 3 i
    *> 172.16.10.0/24  192.68.5.1      250       0 3 i
    *> 172.16.65.0/26  192.68.5.1      250       0 3 i
    *>i172.16.220.0/24 192.68.6.2      200  100    0 3 i
    *                  192.68.5.1   250       0 3 i
    *> 192.68.10.0     0.0.0.0       0       32768 i
    *>i192.68.11.0     192.68.6.2      0  100     0 i
    *>i192.68.40.0     192.68.6.2      0  100    0 6 i
    *> 193.78.0.0/16   192.68.10.2             0 7 8 i

RTD can reach all networks in AS3 via the RTD-RTF direct link except for prefix 172.16.220.0/24, which is reachable via the RTC-RTA link because of the better metric 200.

Automatic Load Balancing

Considering figure 11-5, we will show how the Cisco BGP implementation can do dynamic load balancing for identical updates received by the same router from the same autonomous system. RTA is EBGP peering with routers RTC and RTD in AS1. RTA is receiving identical updates about prefixes 192.68.11.0/24 and 192.68.40.0/24 from two links. We can configure RTA with the maximum-paths router subcommand to enable IP routing to load balance among up to six paths. In our example, the maximum-paths number is set to 2.

Figure 11-5 Automatic load balancing.

RTA configuration:

    router bgp 3
    no synchronization
    neighbor 172.16.1.2 remote-as 3
    neighbor 172.16.1.2 update-source Loopback0
    neighbor 172.16.20.1 remote-as 1
    neighbor 172.16.20.1 filter-list 10 out
    neighbor 172.16.60.1 remote-as 1
    neighbor 172.16.60.1 filter-list 10 out
    maximum-paths 2
    no auto-summary

    ip as-path access-list 10 permit ^$

Looking at RTA's BGP table, we see that RTA has identical path information regarding 192.68.11.0/24 and 192.68.40.0/24. Normally, BGP will pick one of the entries as the "best" path and give it to the IP routing table.

    RTA#show ip bgp
    BGP table version is 8, local router ID is 172.16.2.254
    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
    *>i172.16.10.0/24  172.16.1.2    0    100   0 i
    *> 192.68.11.0     172.16.20.1  0        0 1 i
    *                  172.16.60.1      0 1 i
    *> 192.68.40.0     172.16.20.1       0 1 6 i
    *                  172.16.60.1      0 1 6 i

Using the maximum-paths command will instruct BGP to give all the identical paths (up to six) to the IP routing table. Note that the requirement for these paths is that they come from the same AS.

The following shows how RTA will keep multiple entries from the same destination in its IP routing table. Note how prefixes 192.68.11.0/24 and 192.68.40.0/24 are learned from both links.

    RTA#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

    Gateway of last resort is not set

    B  192.68.11.0/24 [20/0] via 172.16.60.1, 00:03:20
                  [20/0] via 172.16.20.1, 00:03:18
    B  192.68.40.0/24 [20/0] via 172.16.60.1, 00:03:20
                  [20/0] via 172.16.20.1, 00:03:18
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
    C    172.16.2.254/32 is directly connected, Loopback0
    C    172.16.60.0/24 is directly connected, Ethernet0
    C    172.16.20.0/24 is directly connected, Serial0
    O    172.16.10.0/24 [110/20] via 172.16.1.2, 00:20:23, Ethernet1
    C    172.16.1.0/24 is directly connected, Ethernet1

When dealing with IBGP peers, RTA will only advertise a single BGP entry out of the multiple identical entries with a next-hop-self. Because RTA is IBGP peered with RTF, RTA will advertise only one BGP update about 192.68.11.0/24 and 192.68.40.0/24 with a next-hop 172.16.2.254 rather than the external next hop; this is illustrated in the following BGP table. For external peers, BGP will still pass on the best path as usual.

    RTF#show ip bgp
    BGP table version is 56, local router ID is 172.16.10.1
    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
    *> 172.16.10.0/24  0.0.0.0     0         32768 i
    *>i192.68.11.0     172.16.2.254   0    100      0 1 i
    *>i192.68.40.0     172.16.2.254        100      0 1 6 i

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