BGP - Conditional Default Origination

[Joseph Nelson]

Hi,

We have a setup like the following:

e1------e2

|    \     /  |

|      \  /   |

|      / \    |

|    /     \  |

|   /       \ |

c1        c2

The scenario has the following features:

• iBGP full mesh between router loopbacks
• Collapsed core/distribution design
• Edge routers (e1, and e2) are dual multi-homed with 3 providers ( 2 providers are on e1 and another 1 provider is on e2). Both receive full+default announcements from the providers
• Edge routers are advertising only a default route to the core routers (c1 and c2)

Goal:

• Edge routers originate a default to core routers ( c1 and c2) if and only if they have a default route in their routing table

From the reading this "conditional announcement" can be achieved using the "neighbor x.x.x.x default-originate route-map rmap" command in router config mode. If at least one prefix in the route-map ( here: rmap) is matched in the local routing table, a default will be originated.

My question:

Can you specify the route-map to match on the default prefix as follows?:

config t

ip prefix-list IPV4-DEFAULT 10 permit 0.0.0.0/0

route-map IPV4-COND-DEFAULT seq 10 permit

match ip address IPV4-DEFAULT

end

config t

router bgp ASNX

neighbor x.x.x.x default-originate route-map IPV4-COND-DEFAULT

end

Reference:

http://www.cisco.com/en/US/docs/ios/iproute_bgp/command/reference/irg_bgp3.html#wp1105838

http://njetwork.wordpress.com/2013/01/28/bored-with-bgp-generating-a-default-route/

http://blog.ipspace.net/2011/09/responsible-generation-of-bgp-default.html

[Lei Tian]

Yes, that config looks right.

HTH,
Lei Tian

Sent from Cisco Technical Support iPhone App

[Joseph Nelson]

Lie,

Thanks for your reply. Have you every configured something like this? What was your experience configuration a conditional default in this way?

Other feedback welcomed.

Joe

[Lei Tian · Accepted solution]

Hi Joe,

Don't recall if I have any customer is doing it; just tested it in lab

R1#sh run | sec router bgp

router bgp 100

bgp log-neighbor-changes

neighbor 10.10.10.2 remote-as 200

neighbor 10.10.10.2 default-originate route-map DEFAULT

R1#sh route-map

route-map DEFAULT, permit, sequence 10

  Match clauses:

    ip address prefix-lists: DEFAULT

  Set clauses:

  Policy routing matches: 0 packets, 0 bytes

R1#sh ip prefix-list

ip prefix-list DEFAULT: 1 entries

   seq 5 permit 0.0.0.0/0

R1#sh ip route

S*    0.0.0.0/0 is directly connected, FastEthernet0/1

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

C        10.10.10.0/24 is directly connected, FastEthernet0/0

L        10.10.10.1/32 is directly connected, FastEthernet0/0

C        10.10.20.0/24 is directly connected, FastEthernet0/1

L        10.10.20.1/32 is directly connected, FastEthernet0/1

R2#sh ip route

B*    0.0.0.0/0 [20/0] via 10.10.10.1, 00:00:47

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C        10.10.10.0/24 is directly connected, FastEthernet0/0

L        10.10.10.2/32 is directly connected, FastEthernet0/0

remove static default from R1

R1#sh ip route

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C        10.10.10.0/24 is directly connected, FastEthernet0/0

L        10.10.10.1/32 is directly connected, FastEthernet0/0

R2#sh ip route

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C        10.10.10.0/24 is directly connected, FastEthernet0/0

L        10.10.10.2/32 is directly connected, FastEthernet0/0

HTH,

Lei Tian

[Joseph Nelson]

Excelent Lei!

Thanks I had some doubts but you've solidified it for me. Thanks again.

Joe