OSPF and MPGBP

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May 24th, 2010
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I have three routers A (PE), B (PE) and C (CE). A and B has MPBGP between them and B and C has OSPF (vrf CCIE) between the. When trying to redistribute OSPF in to MPGBP on B, I saw a book configured


router bgp 100

address-family ipvpn4 vrf CCIE

redistribute connected                          ...................................................1

redistribute OSPF 5 vrf CCIE match internal external 2 external 1................2


My questions are below.

1.  Why is "redistrubuted connected" , wont the connected route between B and C be redistributed if its not configured ?

2.  If  only "redistribute OSPF 5 vrf CCIE" is configured, wont external (1 and 2) and internal route be redistributed along ?

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Giuseppe Larosa Tue, 05/25/2010 - 00:14
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Hello Just,


1) the VRF access link would be present in router LSA or both CE and PE node, as a result of this it should be redistributed from OSPF 5 vrf CCIE into BGP address-faimly ipv4 vrf CCIE. your understanding is correct


2) again without any matching conditions the PE should accept all LSAs received from CE node unless they have the down bit set that would  mean they were originated by another PE node.


Hope to help

Giuseppe

Nagendra Kumar ... Tue, 05/25/2010 - 03:52
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Hi Just,


1.  Why is "redistrubuted connected" , wont the connected route between B  and C be redistributed if its not configured ?

- As mentioned by Giuseppe, redistributing OSPF will adverise connected interfaces whcih are part of OSPF process. Basically when any routing protocol is redistributed into other, below 2 steps will be followed. For simplicity assume protocol A is redistributed to protocol B,


a. All prefixes learnt via protocol A and installed in routing table "show ip route " will be candidates for redistribution

b. All connected interface whcih are part of protocol A will be candidates for redistribution.


Skip step b if you have "redistribute connected" enabled under protocol B.


2.  If  only "redistribute OSPF 5 vrf CCIE" is configured, wont  external (1 and 2) and internal route be redistributed along ?

- When OSPF is redistributed into BGP, by default it will not advertise any external prefixes. You need to match external inorder to get them redistributed.


HTH,

Nagendra

Giuseppe Larosa Tue, 05/25/2010 - 08:00
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Hello Nagendra,

thanks for your correction


Best Regards

Giuseppe

Just Kennie Tue, 05/25/2010 - 22:36
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Thanks both,

Just Lab it, my finding is...


1. "redistributed connected" need not be specified as the OSPF connected route (and internal routes) is redistributed in BGP by default.

2. " redistribute ospf 1 vrf CCIE  match external 1 external 2" needs to be explicitly configured before external (1 and 2) can be redistributed.

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