Troubleshooting guide

Additional BGP Configuration

Password

Use the password command to enable message digest 5 (MD5) password authentication on Transmission

Control Protocol (TCP) segments exchanged with the BGP peer.

(config-bgp-neighbor)#password <password>

<password> Specifies the password string to be used for authentication. The password is case sensitive

and must not exceed 80 characters.

Every BGP TCP segment sent is authenticated. Configuring authentication causes an existing session to be

torn down and re-established using the currently specified authentication.

Prefix List

Prefix lists are used in BGP configurations to define the routes that a router can advertise to or receive

from a neighbor. Common uses for prefix lists include:

• Preventing a network from becoming a transit for external traffic when multihoming

• Receiving only routes from remote virtual private network (VPN) sites

• Prohibiting the advertisement of a network

• Load balancing outbound traffic

IPv4 address, prefix length, or other attributes must be defined in a prefix list before it can be assigned to a

BGP neighbor.

First, use the ip prefix-list command to specify a prefix to be matched or a range of mask lengths:

(config)#ip prefix-list <name> seq <number> [deny | permit] <network ip/length>

(config)#ip prefix-list <name> seq <number> [deny | permit] <network ip/length> [ge| le] <value>

<name> Specifies the name of the list. Up to 80 characters are allowed in a name.

<number> Specifies the entry’s unique sequence number that determines the processing order. Lower

numbered entries are processed first. Range is 1 to 4294967294.

permit <network ipv4 address/length> Permits access to entries matching the specified network IPv4

address and the corresponding network prefix length (for example, 10.10.10.0/24).

An entry for the default route must appear in the IGP routing table in order for the previous

command to work in BGP. Refer to Example 7 on page 49

for an illustration using the

network command to enable transmission of the default route to the eBGP neighbor.

Authentication must be configured on both peers using the same password.

Refer to Example 3 on page 39 for an example using prefix lists to filter routes.