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hints:routing [2020/09/22 21:05] – [Propagating default using IS-IS rather than IBGP] philiphints:routing [2020/09/22 21:12] (current) – [Complete Configuration] philip
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 The way to make this work is to use the ''default-information originate'' command in IS-IS. However, without any options applied to it, this command will unconditionally originate the default route. Even if the router has no path to the default. Which can be somewhat problematic if the default route learned from the EBGP speaker disappears. The way to make this work is to use the ''default-information originate'' command in IS-IS. However, without any options applied to it, this command will unconditionally originate the default route. Even if the router has no path to the default. Which can be somewhat problematic if the default route learned from the EBGP speaker disappears.
  
-To remedy this we need to use the ''route-map'' option for the ''default-information originate'' command. The route-map we are going to add needs to look for the default route appearing in the router's global RIB and, if it exists, inject it into the IS-IS RIB so that it is then propagated to IS-IS speaking routers in the network. But left like this, the default route seen in the RIB could be created by the IS-IS process on a neighbouring internal router. So we need to add another condition to make sure that the default we are seeing in the RIB is learned from the EBGP neighbour. Our resulting configuration is below:+To remedy this we need to use the ''route-map'' option for the ''default-information originate'' command. The ''route-map'' we are going to add needs to look for the default route appearing in the router's global RIB and, if it exists, inject it into the IS-IS RIB so that it is then propagated to IS-IS speaking routers in the network. But left like this, the default route seen in the RIB could be created by the IS-IS process on a neighbouring internal router. So we need to add another condition to make sure that the default we are seeing in the RIB is learned from the EBGP neighbour. Our resulting configuration is below:
  
 <code> <code>
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 </code> </code>
  
-We can now apply this route-map to the Border Router IS-IS process:+We can now apply this ''route-map'' to the Border Router IS-IS process:
  
 <code> <code>
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 </code> </code>
  
-If we would like to prefer one border router over the other, then we would set a metric in the route-map. For example, if we want to prefer BR1 over BR2 as the exit point, then BR1 route-map would look like this:+If we would like to prefer one border router over the other, then we would set a ''metric'' in the ''route-map''. For example, if we want to prefer BR1 over BR2 as the exit point, then BR1 ''route-map'' would look like this:
  
 <code> <code>
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 </code> </code>
  
-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 metric increased by 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 ''metric'' increased by 10.
  
 ==== Removing default from IBGP ==== ==== Removing default from IBGP ====
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 route-map EBGP-in permit 5 route-map EBGP-in permit 5
  description Do not propagate default route  description Do not propagate default route
- match ip address prefix-list default+ match ip address prefix-list DEFAULT
  set community no-advertise  set community no-advertise
 ! !
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  address-family ipv4  address-family ipv4
   neighbor 10.10.10.1 route-map EBGP-in in   neighbor 10.10.10.1 route-map EBGP-in in
 +  distance bgp 20 200 200
 ! !
 </code> </code>
  
-The no-advertise community will make sure that the matched route will not be announced to any other BGP speaker.+The ''no-advertise'' community will make sure that the matched route will not be announced to any other BGP speaker.
  
 ==== Conclusion ==== ==== Conclusion ====
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 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. 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):
 +
 +<code>
 +router isis ISP
 + ...
 + default-information originate route-map DEFAULT-ORIG
 +!
 +router bgp 64512
 + address-family ipv4
 +  neighbor 10.10.10.1 route-map EBGP-in in
 +  distance bgp 20 200 200
 +!
 +ip access-list standard BGP-NH
 + remark External BGP speaker
 + permit 10.10.10.1
 +!
 +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
 + description Do not propagate default route
 + match ip address prefix-list DEFAULT
 + set community no-advertise
 +!
 +route-map EBGP-in permit 10
 + description Other policy
 + ...
 +!
 +</code>
 +
 +For BR2 (making BR2 the //backup// default gateway):
 +
 +<code>
 +router isis ISP
 + ...
 + default-information originate route-map DEFAULT-ORIG
 +!
 +router bgp 64512
 + address-family ipv4
 +  neighbor 10.10.20.1 route-map EBGP-in in
 +  distance bgp 20 200 200
 +!
 +ip access-list standard BGP-NH
 + remark External BGP speaker
 + permit 10.10.20.1
 +!
 +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
 + description Do not propagate default route
 + match ip address prefix-list DEFAULT
 + set community no-advertise
 +!
 +route-map EBGP-in permit 10
 + description Other policy
 + ...
 +!
 +</code>
  
  
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hints/routing.1600772705.txt.gz · Last modified: 2020/09/22 21:05 by philip