In cellular and WLAN networks, the router interface IP address can be assigned statically or dynamically. A cellular port supports different modes of operation depending on whether the IP address must be assigned statically or dynamically. See PDN router interfaces for information about the supported modes of operation on the PDN router interface.
When the PDN router interface is operating in dynamic cellular interface IP mode, the dynamically changing interface IP address must be reflected in BGP advertisements. Neighbor peers that are originating services that rely on BGP routing information to reach the local node must use the IP address of the PDN router interface on the local node in order to reach it. The local address for BGP sessions from the local node to neighbor peers must therefore match the IP address of the PDN router interface at all times, even after the IP address changes.
To facilitate a dynamically changing router interface IP address, the BGP local-address command must be configured with the name of the loopback interface used by the unnumbered interface under the PDN router interface instead of a fixed IP address. Using the loopback interface name, the SR OS will automatically assign the current IP address of the PDN router interface as the BGP local-address when the PDN router interface comes up (for example, when the cellular PDN interface learns the IP address during the cellular attachment procedure). This means that the BGP local address will inherit the loopback interface's dynamic address information and when routes are being advertised to peers, those peers will be able to route traffic toward this router's PDN router interface.
Configuring the loopback interface name used by the PDN router interface as the local address is available in both the config>router>bgp>group context and the config>router>bgp>group>neighbor context. For complete command syntax, description, and parameter information, see "configure router bgp commands" in the 7450 ESS, 7750 SR, 7950 XRS, and VSR Classic CLI Command Reference Guide.
When BGP sessions are using the loopback interface name that is used PDN router interface as the local address, the remote neighbor peer must use the dynamic-neighbor command in order to accept BGP sessions from 7705 SAR-Hm nodes that have dynamically changing router interface IP addresses.
When dual SIM operation is required, there are two PDN interfaces, one per cellular port associated with each SIM. At a minimum, two BGP sessions are required, one for each PDN interface. Each BGP session must be configured with the local-address using the loopback interface associated with each PDN router interface.