Whenever someone speaks of BGP, The only thing we suddenly recall is dual homing on internet or Internet link failover or load sharing because BGP is always believed to be the protocol for the internet and another thing we recall is to be very very careful with the BGP configurations. To set up BGP to support two redundant Internet
connections is easy to configure, The following config could be used (Please do not use this config in the real scenario as this config is missing several key components, but we would try to put relevant config in later posts) -
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#interface Serial0 Router1(config-if)#description connection to ISP #1, ASN 65510 Router1(config-if)#ip address 192.168.1.6 255.255.255.252 Router1(config-if)#exit Router1(config)#interface Serial1 Router1(config-if)#description connection to ISP #2, ASN 65520 Router1(config-if)#ip address 192.168.2.6 255.255.255.252 Router1(config-if)#exit Router1(config)#interface Ethernet0 Router1(config-if)#description connection to internal network, ASN 65501 Router1(config-if)#ip address 172.18.5.2 255.255.255.0 Router1(config-if)#exit Router1(config)#router bgp 65501 Router1(config-router)#network 172.18.5.0 mask 255.255.255.0 Router1(config-router)#neighbor 192.168.1.5 remote-as 65510 Router1(config-router)#neighbor 192.168.2.5 remote-as 65520 Router1(config-router)#no synchronization Router1(config-router)#end Router1#
This example shows the configuration for the router at the
customer site. The customer network uses ASN 65501, while the two ISPs use 65510
and 65520, respectively. Both of these connections are made through serial
connections. This router is configured to distribute routing information for its
172.18.5.0/24 segment with both ISPs, and to receive their routing
tables.
There are two critical problems with this simple configuration.
First, the full Internet routing table is extremely large and consumes a vast
amount of memory, so we will probably want to do some filtering. The second
problem is that this configuration allows your network to act as a transit path
between the two ISPs.
The full Internet routing table has well over 400,000 prefixes.
Each BGP route entry consumes somewhere between 100 and 200 bytes of memory on
the router, and you wouldn't use BGP unless there were at least two ISPs. Each
ISP will likely supply a similar sized routing table, doubling the memory
requirement. Then, if the router puts all of these prefixes into its main
routing table (as well as in the CEF table), you can wind up consuming as much
as 1KB of router memory per route prefix. So we don't recommend using a router
with less than 512MB of memory when connecting to the Internet without
significant filtering. In fact, Internet backbone routers frequently have thousands of megabytes of memory.
Here is a typical routing summary taken from a BGP route
server:
route-server.x>show ip route summary Route Source Networks Subnets Overhead Memory (bytes) connected 0 23 56 3680 static 1 4 280 2840 bgp xxx 87075 37990 7003640 20206240 External: 0 Internal: 125065 Local: 0 internal 2078 2452040 Total 89154 38017 7003976 22664800 route-server.x>
As you can see here, this router's routing table consumes most
of the over 22MB of system memory. The same device uses roughly 20MB of memory
just for its BGP table, as you can see from the following output:
route-server.x>show ip bgp summary
BGP router identifier xxx.xxx.xxx.xxx, local AS number xxx
BGP table version is 205557022, main routing table version 205557022
126231 network entries and 252452 paths using 20827755 bytes of memory
40380 BGP path attribute entries using 2099760 bytes of memory
173 BGP rrinfo entries using 4824 bytes of memory
24797 BGP AS-PATH entries using 606020 bytes of memory
336 BGP community entries using 16266 bytes of memory
Dampening enabled. 0 history paths, 0 dampened paths
BGP activity 2543115/2416883 prefixes, 11282422/11029970 paths
<information deleted for brevity>
route-server.x>
Before you can solve the problem of the large size of the Internet routing
tables, you have to make some decisions about how you want your Internet
connections to work. Specifically, you might want one of these ISPs to be the
primary and the other the backup for all traffic. Alternatively, you might want
to just use the first ISP to handle traffic for its directly connected
customers, while the second ISP handles everything else. Or you could opt to have load sharing between the two ISPs.
You should also think about whether you want to control which path inbound
traffic uses to reach you. If one of your ISP links has a large usage charge,
you might prefer to force all of the inbound traffic through the other link.
This can be slightly tricky, because you don't directly control the ISP routers.
But you can control how your routing information looks to the ISP.