Routing TCP IP Volume II CCIE Professional Development

Figure 7-14 depicts three autonomous systems. AS 200 is advertising unicast prefixes 172.16.226.0/24 and 172.16.227.0/24 to transit AS 100 and is used for normal inter-AS routing. AS 200 also has several multicast sources. These are hosts at 172.16.224.1 and 172.16.225.50. Additionally, several multicast sources are on subnet 172.16.227.0/24, and that prefix is advertised not only as a unicast prefix but also as a multicast source prefix.

Figure 7-14. AS 200 Is Advertising Several Prefixes and Addresses; Some Are Unicast, Some Are Multicast, and One Is Both

graphics/07fig14.gif

Example 7-7 shows the configurations of Gorgo and Rodan in Figure 7-14.

Example 7-7 The MBGP Configurations of Gorgo and Rodan in Figure 7-14



Gorgo


router bgp 200


 no synchronization


 network 172.16.226.0 mask 255.255.255.0


 network 172.16.227.0 mask 255.255.255.0


 neighbor 192.168.1.2 remote-as 100


 no auto-summary


 !


 address-family ipv4 multicast


 neighbor 192.168.1.2 activate


 network 172.16.224.1 mask 255.255.255.255


 network 172.16.225.50 mask 255.255.255.255


 network 172.16.227.0 mask 255.255.255.0


 exit-address-family


_______________________________________________________________________





Rodan


router bgp 100


 no synchronization


 neighbor 192.168.1.1 remote-as 200


 neighbor 192.168.254.2 remote-as 100


 neighbor 192.168.254.2 update-source Loopback0


 neighbor 192.168.254.2 next-hop-self


 !


 address-family ipv4 multicast


 neighbor 192.168.1.1 activate


 neighbor 192.168.254.2 activate


 neighbor 192.168.254.2 next-hop-self


 exit-address-family

The unicast portion of both routers' BGP configurations is no different from the configurations you observed in Chapter 3, "Configuring and Troubleshooting Border Gateway Protocol 4." Neighbors and their AS numbers are identified, as are the two unicast prefixes that Gorgo is to advertise into AS 100.

NOTE

This chapter assumes you are already familiar with unicast BGP configuration. If some of the IBGP tools such as next-hop-self and update-source are not clear to you, you are encouraged to review Chapter 3.

MBGP is activated with the address-family ipv4 multicast command. Recall from the section "Multiprotocol Extensions for BGP (MBGP)" that MBGP uses two new route attributes—MP_REACH_NLRI and MP_UNREACH_NLRI—and that the attributes' Address Family Indicator (AFI) code for IPv4 is 1. The multicast keyword sets the attributes' Sub-AFI to multicast. Following the address-family command, MBGP is configured very similarly to unicast BGP. MBGP neighbors are identified, and the prefixes to be advertised as multicast are identified. The activate keyword is used to show that MBGP is to be activated for that neighbor. The peer's AS number is specified only under BGP, not MBGP. Notice that IBGP configurations, such as next-hop-self, are used under MBGP just as they are with BGP. You also can configure policies separately for MBGP neighbors. The final command, exit-address-family, is entered automatically by Cisco IOS Software to mark the end of the MBGP configuration stanzas.

Enabling address-family ipv4 multicast implicitly enables the address-family ipv4 unicast command. Although the command is never displayed in the configuration, it is applied to the unicast BGP configuration. Its result is that the prefixes specified under that configuration section are given the MP_REACH_NLRI attribute and are assigned a unicast Sub-AFI. Notice that the prefix 172.16.227.0/24 appears in Gorgo's configuration under both BGP and MBGP. This prefix is then advertised as both unicast and multicast (Sub-AFI = 3).

In Example 7-8, the show ip bgp ipv4 command is used to show the results of the configurations. First the unicast keyword is used, and then the multicast keyword is used, and the prefixes whose Sub-AFI matches the keyword are displayed. Notice again that 172.16.227.0/24 is included in both displays because it has been configured as both a unicast and a multicast prefix.

NOTE

The output of show ip bgp ipv4 unicast is the same as the output of show ip bgp.

Example 7-8 The show ip bgp ipv4 Command Displays Prefixes According to Their Sub-AFI Values



Rodan#show ip bgp ipv4 unicast


BGP table version is 7, local router ID is 192.168.254.1


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


*> 172.16.226.0/24  192.168.1.1              0             0 200 i


*> 172.16.227.0/24  192.168.1.1              0             0 200 i





Rodan#show ip bgp ipv4 multicast


BGP table version is 10, local router ID is 192.168.254.1


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


*> 172.16.224.1/32  192.168.1.1              0             0 200 i


*> 172.16.225.50/32 192.168.1.1              0             0 200 i


*> 172.16.227.0/24  192.168.1.1              0             0 200 i


Rodan#

The configurations of Megalon and Kong from Figure 7-14 are a bit more complicated, because separate links are used for unicast BGP and for MBGP. Example 7-9 shows the configurations for these two routers.

Example 7-9 Configuring Megalon and Kong to Use Separate Data Links for Multicast and Unicast



Megalon


router bgp 100


 no synchronization


 no bgp default ipv4-unicast


 neighbor 192.168.1.5 remote-as 300


 neighbor 192.168.1.5 activate


 neighbor 192.168.1.9 remote-as 300


 neighbor 192.168.254.1 remote-as 100


 neighbor 192.168.254.1 update-source Loopback0


 neighbor 192.168.254.1 activate


 neighbor 192.168.254.1 next-hop-self


 no auto-summary


 !


 address-family ipv4 multicast


 neighbor 192.168.1.9 activate


 neighbor 192.168.254.1 activate


 exit-address-family


_______________________________________________________________________





Kong


router bgp 300


 no synchronization


 no bgp default ipv4-unicast


 neighbor 192.168.1.6 remote-as 100


 neighbor 192.168.1.6 activate


 neighbor 192.168.1.10 remote-as 100


 no auto-summary


 !


 address-family ipv4 multicast


 neighbor 192.168.1.10 activate


 exit-address-family

The MBGP configurations show that only the 192.168.1.8/30 subnet is used for MBGP peering, and there are some new commands under the unicast BGP section. Remember that when the address-family ipv4 multicast command is invoked, the address-family ipv4 unicast command is invoked automatically and implicitly. In the case of subnet 192.168.1.8/30, unicast BGP traffic is unwanted. Therefore, the command no ip default ipv4-unicast is used to prevent this automatic behavior. Then, the neighbor activate command is used to explicitly enable unicast BGP on the desired links. Notice that the 192.168.2.1/30 and 192.168.1.4/30 subnets are activated for unicast, but the 192.168.1.8/30 subnet is not. This link has only the AS number specified under BGP so that peering can occur.

Example 7-10 shows the results of the configurations in Example 7-9. The output here looks similar to that in Example 7-8, in that the unicast and multicast prefixes are correctly classified. In this case, however, the next-hop address of the unicast prefixes is 192.168.1.6, and the next-hop address (RPF neighbor) of the multicast prefixes is 192.168.1.10.

Example 7-10 AS 300 Has Received the Prefixes Advertised by AS 200, Using the Correct Next-Hop Addresses for the Unicast-Only and Multicast-Only Links Between Kong and Megalon



Kong#show ip bgp ipv4 unicast


BGP table version is 7, local router ID is 10.254.254.1


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


*> 172.16.226.0/24  192.168.1.6                            0 100 200 i


*> 172.16.227.0/24  192.168.1.6                            0 100 200 i





Kong#show ip bgp ipv4 multicast


BGP table version is 10, local router ID is 10.254.254.1


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


*> 172.16.224.1/32  192.168.1.10                           0 100 200 i


*> 172.16.225.50/32 192.168.1.10                           0 100 200 i


*> 172.16.227.0/24  192.168.1.10                           0 100 200 i


Kong#

Example 7-11 shows the practical application of BGP versus MBGP advertisements. Using the 172.16.227.0/24 prefix, which is advertised both as unicast and multicast, a route lookup is performed for 172.16.227.1. The display shows that the route carries a next-hop address of 192.168.1.6, which is the unicast-only link in Figure 7-14. Next, an RPF lookup is performed on the same address. That lookup returns a next-hop address of 192.168.1.10, the multicast-only link. So the same address references two different links, depending on the function for which the address is being used.

Example 7-11 An IP Route Lookup for 172.16.227.1 Shows the Next Hop to Be 192.168.1.6, but an RPF Lookup of the Same Address Shows a Next Hop of 192.168.1.10



Kong#show ip route 172.16.227.1


Routing entry for 172.16.227.0/24


  Known via "bgp 300", distance 20, metric 0


  Tag 100, type external


  Last update from 192.168.1.6 04:10:21 ago


  Routing Descriptor Blocks:


  * 192.168.1.6, from 192.168.1.6, 04:10:21 ago


      Route metric is 0, traffic share count is 1


      AS Hops 2





Kong#show ip rpf 172.16.227.1


RPF information for ? (172.16.227.1)


  RPF interface: Serial1


  RPF neighbor: ? (192.168.1.10)


  RPF route/mask: 172.16.227.0/24


  RPF type: mbgp


  RPF recursion count: 0


  Doing distance-preferred lookups across tables


Kong#

It is worth emphasizing one last time that MBGP does not affect the forwarding of multicast traffic. Further configuration is needed in a situation such as the parallel links depicted in Figure 7-14 to force multicast traffic over the multicast-only link. MBGP just allows the dissemination of RPF information across AS boundaries.