BGP Configuration Considerations and Limitations

Use the information in this section to help you configure BGP on your switch, which supports BGPv4 as described in RFC 1771.

BGP Implementation Guidelines

The following list provides guidelines to successfully implement BGP:

Minimum Requirements

You must configure the following minimum parameters:

The router ID must be a valid IP address of an IP interface on the router or a CLIP address. BGP update messages use this IP address. By default, the BGP router ID automatically uses the OSPF router ID.

You cannot configure the BGP router ID if you configure BGP before you configured the OSPF router ID. You must first disable BGP, configure the OSPF route ID, and then enable BGP globally.

You can add BGP policies to the BGP peer configuration to influence route decisions. BGP policies apply to the peer through the soft-reconfiguration commands.

After you configure the switch for BGP, some parameter changes can require you to enable or disable the BGP global state or the neighbor admin-state.

You can dynamically modify BGP policies. On the global level, the BGP redistribution command has an apply parameter that causes the policy to take effect after you issue the command.

BGP Neighbor Maximum Prefix Configuration

By default, the maximum prefix parameter limits 12 000 NLRI messages for each neighbor. The maximum prefix parameter limits the number of routes that the switch can accept.

The maximum prefix parameter prevents large numbers of BGP routes from flooding the network if you implement an incorrect configuration. You can assign a value to the maximum prefix limit, including 0 (0 means unlimited routes). When you configure the maximum prefix value, consider the maximum number of active routes that your equipment configuration can support.

BGP and OSPF Interaction

RFC1745 defines the interaction between BGP and OSPF when OSPF is the IGP within an autonomous system. For routers that use both protocols, the OSPF router ID and the BGP ID must be the same IP address. You must configure a BGP route policy to allow BGP advertisement of OSPF routes.

Interaction between BGPv4 and OSPF can advertise supernets to support CIDR. BGPv4 supports interdomain supernet advertisements; OSPF can carry supernet advertisements within a routing domain.

BGP and Internet Peering

By using BGP, you can perform Internet peering directly between the switch and another edge router. In such a scenario, you can use each switch for aggregation and link it with a Layer 3 edge router, as shown in the following figure.

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BGP and Internet peering

In cases where the Internet connection is single-homed, to reduce the size of the routing table, as a best practice, advertise Internet routes as the default route to the IGP.

For route scaling information, see Fabric Engine Release Notes.

Routing Domain Interconnection with BGP

You can implement BGP so that autonomous routing domains, such as OSPF routing domains, connect. This connection allows the two different networks to begin communicating quickly over a common infrastructure, thus providing additional time to plan the IGP merger. Such a scenario is particularly effective when you need to merge two OSPF area 0.0.0.0s, as shown in the following figure.

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Routing Domain Interconnection with BGP

BGP and Edge Aggregation

You can perform edge aggregation with multiple point of presence or edge concentrations. The switch supports 12 pairs (peering services). You can use BGP to inject dynamic routes rather than using static routes or RIP (see the following figure).

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BGP and Edge Aggregation

BGP and ISP Segmentation

You can use the platform as a peering point between different regions or autonomous systems (AS) that belong to the same ISP. In such cases, you can define a region as an OSPF area, an AS, or a part of an AS.

You can divide the AS into multiple regions that each run different IGPs. Interconnect regions logically by using a full iBGP mesh. Each region then injects its IGP routes into iBGP and also injects a default route inside the region. For destinations that do not belong to the region, each region defaults to the BGP border router.

Use the community parameter to differentiate between regions. To provide Internet connectivity, this scenario requires you to make your Internet connections part of the central iBGP mesh (see the following figure).

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Multiple Regions Separated by iBGP

In the preceding figure, consider the following:

To configure multiple policies between regions, represent each region as a separate AS. Implement eBGP between autonomous systems, and implement iBGP within each AS. In such instances, each AS injects its IGP routes into BGP, where they are propagated to all other regions and the Internet.

The following figure shows the use of eBGP to join several autonomous systems.

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Multiple regions Separated by eBGP

You can obtain AS numbers from the Inter-Network Information Center (NIC) or use private AS numbers. If you use private AS numbers, be sure to design your Internet connectivity carefully. For example, you can introduce a central, well-known AS to provide interconnections between all private autonomous systems and the Internet. Before it propagates the BGP updates, this central AS strips the private AS numbers to prevent them from leaking to providers.

The following figure illustrates a design scenario in which you use multiple OSPF regions to enable peering with the Internet.

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Multiple OSPF Regions Peering with the Internet

BGP Peers

The following list provides rules related to BGP peers:

BGP and Route Aggregation

When you configure the attribute-map with the aggregate command, community, metric, AS Path, and next-hop attributes are set, while the origin attribute is not set.

BGP Session Flapping when IPv6 Forwarding is Enabled or Disabled

In a BGP session that is established with IPv4 and IPv6 capability, disabling or enabling IPv6 forwarding results in BGP session flapping due to capability negotiation. The flapping session in turn affects the IPv4 routing through BGP and the BGP session gets terminated. Ultimately, a capability negotiation takes place to re-establish the IPv4 and IPv6 capable session.