hints:routing
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How do we inject the default route into IS-IS if it is heard from an EBGP neighbour? The following configuration example uses Cisco IOS syntax but can be easily adapted for your own favourite vendor (unless your favourite vendor has an easy mechanism to do this). | How do we inject the default route into IS-IS if it is heard from an EBGP neighbour? The following configuration example uses Cisco IOS syntax but can be easily adapted for your own favourite vendor (unless your favourite vendor has an easy mechanism to do this). | ||
+ | |||
+ | ==== Set EBGP distance ==== | ||
+ | |||
+ | Before we start, we need to make sure that the default distance for EBGP learned routes is set to something lower than those learned by IS-IS, otherwise the following will not work. Yes, Cisco' | ||
+ | |||
+ | ==== Default Originate in IS-IS ==== | ||
The way to make this work is to use the '' | The way to make this work is to use the '' | ||
- | To remedy this we need to use the '' | + | To remedy this we need to use the '' |
+ | |||
+ | < | ||
+ | ip access-list standard BGP-NH | ||
+ | | ||
+ | | ||
+ | ! | ||
+ | ip prefix-list DEFAULT permit 0.0.0.0/0 | ||
+ | ! | ||
+ | route-map DEFAULT-ORIG permit 5 | ||
+ | match ip address prefix-list DEFAULT | ||
+ | match ip next-hop BGP-NH | ||
+ | ! | ||
+ | </ | ||
+ | |||
+ | We can now apply this '' | ||
+ | |||
+ | < | ||
+ | router isis ISP | ||
+ | | ||
+ | | ||
+ | ! | ||
+ | </ | ||
+ | |||
+ | If we would like to prefer one border router over the other, then we would set a '' | ||
+ | |||
+ | < | ||
+ | route-map DEFAULT-ORIG permit 5 | ||
+ | match ip address prefix-list DEFAULT | ||
+ | match ip next-hop BGP-NH | ||
+ | ! | ||
+ | </ | ||
+ | |||
+ | and BR2's route-map would look like this: | ||
+ | |||
+ | < | ||
+ | route-map DEFAULT-ORIG permit 5 | ||
+ | match ip address prefix-list DEFAULT | ||
+ | match ip next-hop BGP-NH | ||
+ | set metric 10 | ||
+ | ! | ||
+ | </ | ||
+ | |||
+ | The other routers in the network will see the default being propagated by IS-IS from the BR1 router. If BR1 loses its externally learned default, the other routers will then see the default being propagated by IS-IS from the BR2 router, with a '' | ||
+ | |||
+ | ==== Removing default from IBGP ==== | ||
+ | |||
+ | Now that we are propagating the default from both routers using IS-IS, we no longer need to carry the default route in our IBGP. So we augment the existing inbound policy applied to the EBGP peers with a statement which looks for the default route and tags it with the '' | ||
+ | |||
+ | The following configuration example shows how to do this: | ||
+ | |||
+ | < | ||
+ | route-map EBGP-in permit 5 | ||
+ | | ||
+ | match ip address prefix-list DEFAULT | ||
+ | set community no-advertise | ||
+ | ! | ||
+ | route-map EBGP-in permit 10 | ||
+ | | ||
+ | | ||
+ | ! | ||
+ | router bgp 64512 | ||
+ | | ||
+ | neighbor 10.10.10.1 route-map EBGP-in in | ||
+ | distance bgp 20 200 200 | ||
+ | ! | ||
+ | </ | ||
+ | |||
+ | The '' | ||
+ | |||
+ | ==== Conclusion ==== | ||
+ | |||
+ | With the configuration applied to both Border Routers now, the rest of the routers will see a default route via IS-IS only. There will be no default route in BGP apart from at the Border Routers. Each Border Router will see the default route being learned from the attached external neighbour. And the best path to the default route will be determined by the IS-IS metric attached to the default being propagated by IS-IS. | ||
+ | |||
+ | ==== Complete Configuration ==== | ||
+ | |||
+ | For BR1 (making BR1 the //main// default gateway): | ||
+ | < | ||
+ | router isis ISP | ||
+ | ... | ||
+ | | ||
+ | ! | ||
+ | router bgp 64512 | ||
+ | | ||
+ | neighbor 10.10.10.1 route-map EBGP-in in | ||
+ | distance bgp 20 200 200 | ||
+ | ! | ||
+ | ip access-list standard BGP-NH | ||
+ | | ||
+ | | ||
+ | ! | ||
+ | ip prefix-list DEFAULT permit 0.0.0.0/0 | ||
+ | ! | ||
+ | route-map DEFAULT-ORIG permit 5 | ||
+ | match ip address prefix-list DEFAULT | ||
+ | match ip next-hop BGP-NH | ||
+ | ! | ||
+ | route-map EBGP-in permit 5 | ||
+ | | ||
+ | match ip address prefix-list DEFAULT | ||
+ | set community no-advertise | ||
+ | ! | ||
+ | route-map EBGP-in permit 10 | ||
+ | | ||
+ | ... | ||
+ | ! | ||
+ | </ | ||
+ | For BR2 (making BR2 the //backup// default gateway): | ||
+ | < | ||
+ | router isis ISP | ||
+ | ... | ||
+ | | ||
+ | ! | ||
+ | router bgp 64512 | ||
+ | | ||
+ | neighbor 10.10.20.1 route-map EBGP-in in | ||
+ | distance bgp 20 200 200 | ||
+ | ! | ||
+ | ip access-list standard BGP-NH | ||
+ | | ||
+ | | ||
+ | ! | ||
+ | ip prefix-list DEFAULT permit 0.0.0.0/0 | ||
+ | ! | ||
+ | route-map DEFAULT-ORIG permit 5 | ||
+ | match ip address prefix-list DEFAULT | ||
+ | match ip next-hop BGP-NH | ||
+ | set metric 10 | ||
+ | ! | ||
+ | route-map EBGP-in permit 5 | ||
+ | | ||
+ | match ip address prefix-list DEFAULT | ||
+ | set community no-advertise | ||
+ | ! | ||
+ | route-map EBGP-in permit 10 | ||
+ | | ||
+ | ... | ||
+ | ! | ||
+ | </ | ||
+ | [[start| Back to Home page]] | ||
hints/routing.1600771631.txt.gz · Last modified: 2020/09/22 20:47 by philip