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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
1.8 AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 C613-50032-01 REV D
Viewing a subset of the BGP Table
When accepting full BGP updates, the number of BGP table entries may be too large for
the show ip bgp (BGP) command listing thousands of prefixes. So instead use the show
ip bgp summary (BGP) command that only lists the number of prefixes received from
each neighbor.
Advertising eBGP routes to ISPs
Outbound routes let the Enterprise routers forward packets toward the Internet. At the
same time, the ISPs need to learn routes for the Enterprise’s public IP address space. This
section of this chapter examines the options for advertising these routes. In particular, this
section looks at two options:
■ BGP network (BGP and BGP4+) command
■ Redistribute from an IGP (RIP/RIPng/OSPF/OSPFv3)
Advertising eBGP routes with the network command
The BGP network (BGP and BGP4+) command installs the prefix defined in the BGP table
to be advertised to peers. The route does not have to exist in the routing table. BGP does
not use this command to enable BGP on interfaces. This command compares the
command’s parameters and the contents of that router’s IP routing table, as follows:
Look for a route in the router’s current IP routing table that exactly matches the
parameters of the network (BGP and BGP4+) command. If a route for that exact prefix/
length exists, then put the equivalent prefix/ length into the local BGP table.
This assumes a BGP default setting of no auto-summary. With auto-summary
configured, the router adds a route for that classful network to the BGP table:
■ If the exact classful route is in the IP routing table
■ If any subset routes of that classful network are in the routing table
Advertising eBGP routes with the redistribute command
Instead of using the network (BGP and BGP4+) command to add routes to the BGP table,
BGP routers can use a redistribute command for OSPF, OSPFv3, RIPng, and RIP to
redistribute routes from an IGP into BGP.
■ Advertise the public address range, not the private IP address range, to the BGP table.
■ Advertise one route for the public address range, not individual subnets of the range.
Routers that run BGP may already run an IGP and have learned routes for either the entire
public range either as one route or with subset routes. If a single route exists for the entire
public range, then you need to add a redistribute command for OSPF, OSPFv3, RIPng, and
RIP to the BGP configuration to redistribute only that single route into BGP.