In the network shown in the figure, the BGP routing learning between the routers is normal, and the BGP routing attributes all take default values.
After the route 8.0.0.0 is imported into BGP on RTA, if the next hop of route 8.0.0.0 on RTD is to point to RTB, the following adjustment methods are feasible again.

As shown in the figure, EBGP runs between AS100, AS200, AS300, and AS400.
When AS100 receives the BGP route sent by AS400, the path sequence recorded in its AS-Path attribute is ______

Run the command and display information on the router as follows:

From the above information, we can know that RTB has ______ IS-IS Level-2 routes sent to neighbors through LSP.

In the IS-IS network as shown in the figure, the RTA, RTB, and RTC configurations are as follows:
RTA]isis
RTA-isis-1]is-level level-1
RTA-isis-1]network-entit01.1111.1111.1111.00
RTA-isis-1]interface serial 2/0
RTA-Serial2/0]isis enable 1
RTA-Serial2/0]isis cost 1
RTB]isis
RTB-isis-1]is-level level-1
RTB-isis-1]network-entit01.2222.2222.2222.00
RTB-isis-1]interface serial 1/0
RTB-Serial1/0]isis enable 1
RTB-Serial1/0]isis cost 1
RTB-Serial1/0]interface Ethernet 0/0
RTB-Ethernet0/0]isis enable 1
RTB-Ethernet0/0]isis cost 1
RTC]isis
RTC-isis-1]is-level level-1
RTC-isis-1]network-entit01.3333.3333.3333.00
RTC-isis-1]interface Ethernet 0/0
RTC-Ethernet0/0]isis enable 1
RTC-Ethernet0/0]isis cost 1
RTC-Ethernet0/0]interface Ethernet 0/1
RTC-Ethernet0/1]isis enable 1
RTC-Ethernet0/1]isis cost 1
After the routing of each router is stable, the cost of IS-IS routing 192.168.3.0/24 on RTA is ______.

In BGP protocol messages, unreachable routing information is advertised through ______.