Primary and Backup Paths
The first example considered (shown in Figure 5-23) is one with two paths between the networks: One path is used as the primary link, and the other path is used exclusively for backup. This situation is used commonly when the primary path has a high bandwidth and the backup path is of low bandwidth or poor latency and is sufficient only when the main link has failed.
Figure 5-23. Primary and Backup Paths to the Same ISP
Figure 5-23. Primary and Backup Paths to the Same ISP
The primary path is between Router A and Router C, with the backup path between Router B and Router D. AS 65534 has an address block of its own—say, 221.10.0.0/19 for this example.
AS 65534 must announce its aggregate block to AS 100— it is very important that the aggregate always be announced because the smaller the prefix is, the more likely some ISPs are to filter it (thereby causing reachability problems for the network announcing subprefixes).
The configuration of the various routers follows:
Router A:
router bgp 65534 network 221.10.0.0 mask 255.255.224.0 neighbor 222.222.10.2 remote-as 100 neighbor 222.222.10.2 description RouterC neighbor 222.222.10.2 prefix-list aggregate out neighbor 222.222.10.2 prefix-list default in
ip prefix-list aggregate permit 221.10.0.0/19 ip prefix-list default permit 0.0.0.0/0 !
Router B:
router bgp 65534 network 221.10.0.0 mask 255.255.224.0 neighbor 222.222.10.6 remote-as 100 neighbor 222.222.10.6 description RouterD neighbor 222.222.10.6 prefix-list aggregate out neighbor 222.222.10.6 route-map routerD-out out neighbor 222.222.10.6 prefix-list default in neighbor 222.222.10.6 route-map routerD-in in
ip prefix-list aggregate permit 221.10.0.0/19 ip prefix-list default permit 0.0.0.0/0 !
route-map routerD-out permit 10
match ip address prefix-list aggregate set metric 10 route-map routerD-out permit 20
route-map routerD-in permit 10 set local-preference 90
Router C:
router bgp 100
neighbor 222.222.10.1 remote-as 65534
neighbor 222.222.10.1 default-originate neighbor 222.222.10.1 prefix-list Customer in neighbor 222.222.10.1 prefix-list default out
ip prefix-list Customer permit 221.10.0.0/19
ip prefix-list default permit 0.0.0.0/0
Router D:
router bgp 100 neighbor 222.222.10.5 remote-as 65534 neighbor 222.222.10.5 default-originate neighbor 222.222.10.5 prefix-list Customer in neighbor 222.222.10.5 prefix-list default out
ip prefix-list Customer permit 221.10.0.0/19 ip prefix-list default permit 0.0.0.0/0 !
The configuration on Router A is quite simple. The outbound announcement made is to send the /19 aggregate upstream; inbound, the router accepts only the default route from the upstream. For all intents and purposes, the default route is exactly the same as the full routing table; it says that the ISP announcing the default knows how to get to the whole Internet. One prefix quoting the default is much more preferable than 108,000 prefixes to do the same thing.
Router B is the chosen backup path between the two networks. As mentioned earlier, local preference is used to set a preference on inbound announcements to determine outbound traffic flow. MED is used to set a preference on outbound announcements to influence inbound traffic flow. So the configuration on Router B does just that:
• A local preference of 90 is set on the inbound announcements. As a result, any prefixes learned will have a lower priority than those learned from Router A.
• A MED of 10 is set on outbound announcements. AS 100 sees the prefixes from AS 65534 with an MED of 10, a higher value than heard through the other path and, therefore, less preferred.
Router C and Router D share very similar configurations: The upstream ISP has left all the multihoming options to its customer. The aggregate is allowed in, the default is sent out, and otherwise the configuration is quite simple. It also is easy to maintain—any changes required in the multihoming are left to the customer, reducing the support burden.
If the customer does not want to deal with the complexity of configuring the multihoming, the configuration can be swapped around (so that the MED and local prefs go on AS 100, with the configuration on AS 65534 routers made "simple").
Router E's configuration needs to be set so that it strips the private AS out of any announcements to the Internet. The configuration example follows:
Router E:
router bgp 100 neighbor 222.222.10.17 remote-as 200 neighbor 222.222.10.17 remove-private-AS neighbor 222.222.10.17 prefix-list Customer out
ip prefix-list Customer permit 221.10.0.0/19
Note the remove-private-as directive on the peering with AS 200. This strips the AS 65534 from the /19 aggregate announced by the customer of AS 100 and announces it as though it originated from AS 100.
Continue reading here: One Upstream ISP and One Local Peer
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