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BGP Route Reflector Roles

BGP Route Reflector normally has three kinds of peers

  • EBGP Peer
  • Client Peer ( neighbor 1.2.3.4 route-reflect-client )
  • Non-client Peer ( no neighbor 1.2.3.4 route-reflect-client)

Route Reflector can learn from or advertise to all three kinds of peers. However, non-client peer can NOT learn from or advertise to non-client peer. As we can see from the following diagram, Since R3 is non-client, it can NOT advertise route to R4, which is also non-client peer.

BGP RR diagram 1There are two ways to solve this problem.(Of course, many ways can be done to implement this.)

One is to make R3 as Client Peer. Since R3 is Client Peer, it will advertise route to R4.

BGP RR Diagram 2The other is, we make R3 as another RR and make R4 and R2 as Client Peer. If we don’t make R2 as R3′s Client Peer, R2 is non-client peer with R3. Since R3 is RR, R3 is non-client peer with R2. If R3 has no neighbor relationship with R1, then two non-client peer can NOT advertise to each other. Therefore, we make R2 as Client Peer with R3.

BGP RR Diagram 3

Tags: BGP, Networks by hengdu
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OSPF Summarization

Briefly outline in here that OSPF has two ways to do summarization

For Internal Area: area <#> range <IP> <Mask>

The summarization has to be done on ABR. It actually said that, for area <#>, we are going to summary as this IP with this Mask and advertise it out by default. The summarization will be advertised as LSA Type 3 to the whole areas. The summarization will not suppress sub network, which is different with summary-address in OSPF and ip summary-address in RIP and EIGRP.

For External Area: summary-address <IP> <Mask>

The summarization has to be done on ASBR. (This command will do nothing if configured on ABR). It actually said that, all external route can be summrized as <IP> with <Mask>. It will be advertised to all OSPF area as LSA type 5. The advertising router is ASBR for the whole area, and most important, it will advertise back to IGP like RIP or EIGRP when doing mutual redistribution, since it will generate a discard route in the local routing table – “O    10.10.0.0/21 is a summary, 18:32:09, Null0″. The redistribution from OSPF to RIP, as an example, will do two steps.

  1. show ip route ospf to see all OSPF generated route.
  2. list all connected interface that advertised by OSPF

Therefore, we need route-map when advertise OSPF to RIP or EIGRP to filter out this summary route.

Tags: IGP, Networks by hengdu
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