TTL

By default, Stingray Service Gateway in L2 BRAS mode does not change TTL packets, that is, it is transparent for utilities, such as traceroute. This is not always convenient: for example, the subscriber will not see his subscriber gateway in the output of the traceroute command.

In SSG 10.1 an additional parameter bras_transparency in the fastdpi.conf configuration file was introduced:

# Transparent (1) or not (0) SSG in L2 BRAS mode
# In transparent mode, L2 BRAS does not control the TTL of the packet, it does not send ICMP Time Exceeded when the TTL is exhausted,
# therefore, for example, the traceroute utility will not recognize the subscriber gateway when tracing.
# In non-transparent mode (0) L2 BRAS corrects the TTL of the packet and sends ICMP Time Exceeded when exhausted.
# Default value: 1 (L2 BRAS is transparent)
# This parameter works only in the DPDK version of SSG
bras_transparency=1

In the case of bras_transparency=0, when the TTL is exhausted, the SSG will send ICMP Time Exceeded to the sender of the packet. In this case, in the IP header of the ICMP packet srcIP will be equal to the subscriber's gateway IP or bras_arp_ip if the subscriber's gateway is unknown to the SSG.

For IPv6, a virtual IPv6 address is always used bras_ipv6_address

In the router mode, the TTL is always corrected, regardless of the bras_transparency parameter.