Networking

At BGP best path selection algorithm the weight attribute is first. The default weight for learned routes is 0 and the default weight for a locally originated route is 32768. Weight attribute is preferred large number weight. Only effect on outgoing traffic.   Figure 1.1 In this scenario by default R1 will choose AS200 to reach AS500. We want to use AS300 path to reach to AS500. So let play with weight. We have  two options using to Weight Set the weight attribute on the neighbor (That will affect all routes learn from that neighbor) Using Route map can specific routes to have the weight set. Configuration Weight attribute on neighbor R1 router bgp 140  no synchronization  bgp log-neighbor-changes  network 1.1.1.0 mask 255.255.255.0  neighbor 10.1.12.2 remote-as 200  neighbor 10.1.14.4 remote-as 140  neighbor 10.1.14.4 next-hop-self  neighbor 10.1.14.4 weight 100  no auto-summary R2  router bgp 200 ...

Never redistribute BGP to IGP on internet facing router. It look like you try to put space ship in your house.

The LOCAL_PREF attribute affects only traffic leaving the AS. To influence incoming traffic, the MULTI_EXIT_DISC attribute, known as the MED for short, is used.

Figure 1.1 In this scenario I use OSPF for TCP reachability. We can see the difference 'disable connected' check and 'EBGP multihop'. We can use both for EBGP neighbor connection with loopback address. Disable connected check This command used to disable the connection verification process for ebgp. Use for ebgp neighbor are directly connected and they using the loopback address for peering. It not increment TTL but neighbor must reachable at single hop. EBGP multihop This command can use for ebgp neighbor are not directly connected. It increment TTL. R1 router bgp 100  neighbor 2.2.2.2 remote-as 200  neighbor 2.2.2.2 disable-connected-check  neighbor 2.2.2.2 update-source Loopback0  neighbor 3.3.3.3 remote-as 300  neighbor 3.3.3.3 ebgp-multihop 2  neighbor 3.3.3.3 update-source Loopback0 R2 router bgp 200  neighbor 1.1.1.1 remote-as 100  neighbor 1.1.1.1 disable-connected-check  neighbor 1.1.1.1 update...

Figure 1.1 R1 router bgp 65000  bgp cluster-id 10  neighbor 2.2.2.2 remote-as 65000  neighbor 2.2.2.2 update-source Loopback0  neighbor 2.2.2.2 route-reflector-client  neighbor 3.3.3.3 remote-as 65000  neighbor 3.3.3.3 update-source Loopback0  neighbor 3.3.3.3 route-reflector-client R2 router bgp 65000  bgp cluster-id 20  net 22.22.22.0 mask 255.255.255.0  neighbor 1.1.1.1 remote-as 65000  neighbor 1.1.1.1 update-source Loopback0  neighbor 4.4.4.4 remote-as 65000  neighbor 4.4.4.4 update-source Loopback0  neighbor 4.4.4.4 route-reflector-client R3 router bgp 65000  bgp cluster-id 30  net 33.33.33.0 mask 255.255.255.0  neighbor 1.1.1.1 remote-as 65000  neighbor 1.1.1.1 update-source Loopback0  neighbor 5.5.5.5 remote-as 65000  neighbor 5.5.5.5 update-source Loopback0  neighbor 5.5.5.5 route-reflector-client R4 router bgp 65000 ...

Figure 1.1 In this topology, I will use static route to connect the R1 and R2 networks. Static route can use the outgoing interface or next hop address, also can combine that two configure. R1 ip route 192.168.2.0 255.255.255.128 10.1.13.3 ip route 192.168.2.128 255.255.255.128 10.1.13.3 R2 ip route 192.168.1.0 255.255.255.0 10.1.23.3 R3 ip route 192.168.1.0 255.255.255.0 10.1.13.1 ip route 192.168.2.0 255.255.255.0 f1/0 At R3 we can use summary route for 192.168.2.0/25 and 192.168.2.128/25. R1#sh ip route      10.0.0.0/24 is subnetted, 2 subnets C       10.1.13.0 is directly connected, FastEthernet1/0 C       10.1.12.0 is directly connected, FastEthernet0/0 C    192.168.1.0/24 is directly connected, Loopback0      192.168.2.0/25 is subnetted, 2 subnets S       192.168.2.0 [1/0] via 10.1.13.3 S...